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Table Of Contents

TElement B5: Hazardous substances -monitoring and maintenance of control measures6T ............. 4

T1.0 Measurement of airborne contaminants6T .................................................................................................... 4

1.1 Principles of Environmental Monitoring ...................................................................................................... 5

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T4.4 Biological Monitoring Techniques6T ............................................................................................................... 80

T4.5 Interpretation of Results6T ................................................................................................................................ 82 4.6 Case Studies ....................................................................................................................................................... 83

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Element B5: Hazardous substances -monitoring and maintenance of control measures

Learning outcomes

On completion of this element, candidates should be able to:

• Describe the strategies, methods, and equipment for the sampling and

measurement of airborne contaminants

• Outline the principles of biological monitoring

• Outline the statutory and other requirements for the monitoring and

maintenance of control measures for hazardous substances

Relevant Standards

• International Labour Office, Safety in the Use of Chemicals at Work, an ILO Code

of Practice, ILO, 1993. ISBN: 9221080064

Section 6: Operational control measures (see controls in S.6.5 – S6.9)

• International Labour Office, Ambient Factors in the Workplace, an ILO Code of

Practice, ILO, 2001. ISBN 922111628

Minimum hours of tuition 6 hours.

1.0 Measurement of airborne contaminants

So far we have examined the way that chemical agents can cause occupational ill-

health, the factors that influence the risk of harm to the individual and some examples

of substances and occupations that present a risk of harmful exposure. This enables us

to recognise when and where there is a risk of exposure to chemical agents. In this and

the following study unit we shall consider the next stage of the occupational health and

hygiene programme, which is to quantify the extent of the problem through

measurement.

Environmental monitoring, the work of the occupational hygienist, is a specialist

function that enables us to assess the risk of harm from exposure to chemical agents

by identifying and quantifying the level of exposure. To understand this important topic

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we must understand the principles of environmental monitoring, and then the actual

techniques used to sample, measure and analyse hazardous substances.

The subject of monitoring techniques and strategies for airborne contaminants is a

substantial topic in its own right and this unit is exclusively devoted to this component

of workplace monitoring for substances hazardous to health.

1.1 Principles of Environmental Monitoring

In our quest to prevent exposure to substances hazardous to health it is essential that

we are able, firstly, to recognise or identify hazardous agents, and then evaluate the

extent to which they represent a risk to health. Environmental monitoring techniques

are designed to enable chemical health hazards to be identified through qualitative

analytical techniques, then measured using quantitative techniques.

The health effects of exposure to chemical agents can be acute or chronic.

Consequently there are different types of measurement to account for this:

• Long-term measurements to assess average exposure over a given time

period

• Continuous measurements that can detect short-term acute exposure to high

concentrations of contaminants

• Spot readings to measure acute exposure if the exact point in time exposure is

known

We will be examining the different types of sampling procedures to enable these types

of measurement to be made later, but we begin by considering the range of analytical

techniques that are available to enable us to identify and quantify chemical agents.

Analytical Techniques

In simple terms, the analytical techniques that we will be studying in this section, with

the exception of gas chromatography, all generally involve subjecting the substance in

question to a burst of energy (heat, X-ray, infra red, light) and examining the way the

substance responds. The response is characteristic of the substance being examined

and therefore can be used as a “fingerprint” for the particular agent. This usually

involves comparison of the response with a database of known chemical agents to aid

identification. In addition, the magnitude of the response can be used to estimate how

much of the agent is present. We will see how this operates in practice as we examine

the following specific techniques.

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Gas Chromatography

Gas chromatography is a valuable technique for the separation, identification and

measurement of organic contaminants. It involves a moving stream of the contaminant

under study mixed with a carrier gas (an inert gas such as helium) which is passed

over a solid, or a liquid adhering to a solid, packed in a column. The technique relies on

the components of the gas mixture being attracted to different extents by the material

in the column. As the gas mixture passes through the column, substances in the

mixture are attracted differently to the stationary column packing and are therefore

separated. The time taken for the substance to pass through the column, the retention

time, is fixed and depends on the particular substance and can therefore be used to

identify the substance. In this way a mixture of substances can be separated, or a

single substance identified from its retention time.

At the end of the separation process the gas mixture passes over a detector which

registers the retention time and also measures how much of the component is present.

If the signal intensity and retention time are plotted on a chart recorder a

chromatogram, such as that shown in Figure 1.2, is produced.

This example is of a hexane mixture and shows clearly the four components of the

mixture and their relative concentrations.

Figure 1.1 Gas Chromatography

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Figure 1.2 - a chromatogram

If you examine the methods for the determination of hazardous substances listed in

Table 1.1 you can see the wide range of substances listed against techniques 1-3 for

which gas chromatography is used as an analytical method.

1. Charcoal pumped adsorption tubes and gas

chromatography

acrylonitrile,

carbon disulphide,

benzene, styrene

glycol ether,

glycol ether acetate

vinyl chloride

ethlene oxide

chlorinated hydrocarbons

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toluene

mixed hydrocarbons

2. Porous polymer adsorption tubes and gas

chromatography

acrylonitrile, benzene

glycol ether,

glycol ether acetate

styrene

dioctylphthalate

toluene

mixed hydrocarbons

3. Molecular sieve sorbent tubes and gas

chromatography

1,3-butadiene

4. Flame atomic absorption spectroscopy cadmium, lead

tetralkyl lead

5. X-ray fluorescence spectroscopy cadmium, chromium

6. Syringe injection technique organic vapours

7. Permeation tube method organic vapours

8. Colorimetric field method lead, formaldehyde

chromium,

9. Personal monitoring/filter method lead tetraethyl, beryllium

10. Gravimetric filtration respirable/inhalable dust

coal tar pitch volatiles

11. Adsorbent tube/cold vapour atomic absorption

spectroscopy mercury vapour

12. High performance liquid chromatography isocyanates

13. Diffusive sampler

14. Ion selective electrode fluorides hydrogen fluoride

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hydrogen cyanide

15. Infra red spectroscopy quartz

16. X-ray diffraction quartz

17. Phase contrast microscopy asbestos,

man-made mineral fibres

Table 1.1: Methods for the Determination of Hazardous Substances

1.1 Principles of Environmental Monitoring (Cont.)

Flame Atomic Absorption Spectroscopy

Flame atomic absorption spectroscopy is a useful technique for the identification and

measurement of metallic substances. The principle of operation is that if certain metals

are heated to high temperatures in a flame, electronic changes in the metal atom

cause a change in colour to the flame. A flame test is a simple way to identify an

element and a basic demonstration of this is the way that common salt (sodium

chloride) sprinkled into a flame will cause the flame to turn yellow. In contrast,

potassium gives a violet flame and lithium and strontium a red flame. Although the red

flames from lithium and strontium appear similar, the light from each can be resolved

by passing it through a prism into distinctly different colours. If the light resolved by

the prism is examined closely it can be seen to consist of a cluster of distinctive lines at

different parts of the spectrum. Each element has a characteristic line spectrum. It is

this particular fingerprint associated with the distinctive electronic changes that occur

when the metal atoms are subjected to high temperatures that is the basis of the

technique.

In practice an atomic absorption spectrometer is used for the analysis and the sample

in question is injected into an air-acetylene flame (to give a suitably high temperature)

and the resultant spectrum is analysed by the spectrometer. Since the resulting

spectrum is characteristic of the particular metal sample, both the identity and the

quantity of substance can be determined.

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Figure 1.3 - Flame Atomic Absorption Spectroscopy

Figure 1.4 - A diagram of a flame atomic absorption spectrometer

X-ray Fluorescence Spectroscopy

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X-ray fluorescence spectroscopy is another technique that will directly determine an

element from its characteristic spectrum. The basis of the technique is that if a beam of

X-rays impinges on a sample it will excite some of the atoms. The excited atoms are

unstable and undergo electronic rearrangement which causes emission of energy in the

form of X-rays whose frequencies are characteristic of the particular atom. Thus a well-

defined X-ray spectrum is emitted from the sample which can be used both to identify

the element and also estimate the quantity present.

Figure 1.5 - X-ray Fluorescence Spectroscopy

Infra Red Spectroscopy

Infra red spectroscopy is a widely used general chemical analytical technique. It is

based on the principle that the chemical bonds that connect atoms into molecules are

in a continuous state of vibration and the energy of this vibration falls within the infra

red wavelength range (2.5-15 µm). If infra red radiation is passed through a sample,

absorption of energy will take place at the characteristic wavelengths of the chemical

bonds in the molecule. Different substances will contain different bonds and therefore

the absorption spectrum gives a characteristic fingerprint of the substance. You can

see an example of an infra red spectrum in Figure 1.7. Again the infra red spectrum

provides both a means of identifying the substance and also quantifying how much is

present.

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Figure 1.6 - Infra Red Spectroscopy

Figure 1.7 - The infra-red spectrum for ethanoic acid

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X-ray Diffraction

Infra red spectroscopy can sometimes be used to analyse a sample directly on a filter

as a solid and X-ray diffraction is another non-destruvtive analytical technique that can

be used for solids. It is based on the principle that a beam of X-rays passed through a

solid crystal will be deflected and scattered (diffracted) in a characteristic fashion,

which depends on the crystal structure and the spacing between the atoms. A

spectrum of diffracted wavelengths provides a characteristic fingerprint for the

substance.

Automated X-ray diffractometers generate an X-ray beam which is diffracted by a

crystal of the substance being analysed. Both the crystal and an X-ray detector rotate

under computer control to record the angles and intensities of thousands of X-ray

reflection spots. After computer analysis of the data a molecular structure can be

determined to aid identification of the sample.

Figure 1.8 - X-ray Diffraction

Further information regarding X-ray Diffraction can be found at

http://www.utc.edu/Faculty/Jonathan-Mies/xrd/xrd.html this also includes movies of

the X-ray Diffraction unit in use.

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Optical Microscopy

The most widely used analytical technique for samples containing fibrous dust, such as

asbestos, is optical microscopy.

To determine the concentration of asbestos fibres, dust sampling is carried out and the

dust is collected on a membrane filter then counted under an optical microscope.

Before counting the membrane filter is rendered transparent by treating it with a

suitable liquid. Since the membrane filter is already marked with a grid pattern, the

number of fibres within any grid square can be counted. A minimum of 20 squares

chosen at random is generally used, or a sufficient number of squares to count at least

100 fibres.

Phase contrast microscopy is used for this purpose, to enhance the contrast between

the fibre on the filter and the background. From the sample of fibres counted, the total

number of fibres collected can be estimated. The volume of air sampled is known from

the sampling time and the flow rate, so the concentration of fibres per unit volume can

be calculated.

Where it is necessary to determine the type of asbestos present, polarised light

microscopy is used. With this technique different types of asbestos fibres show

characteristic colours under various conditions of polarised light, and can thus be

distinguished and identified.

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Figure 1.9 - Optical Microscopy

Information regarding Optical Microscopy and Specimen Preparation can be found at

http://www.doitpoms.ac.uk/tlplib/optical-microscopy/tmicroscope.php?printable=1

1.2 MDHS Guidance on Analysis

In Table 1.1, Methods for the Determination of Hazardous Substances, we noted

the range of substances that can be analysed using gas liquid chromatography. You will

also have noticed that some of the other techniques we have described are also listed

in Table 1.1. This list of techniques is a summary of “MDHS (Methods for the

Determination of Hazardous Substances) Guidance on Analysis” which is a series of

detailed descriptions of analytical methods which have been approved by the Health

and Safety Executive.

HSE link - Methods for the Determination of Hazardous Substances (MDHS) guidance

The MDHS series of guidance sets out approved analytical methods for most chemical

agents that are likely to be encountered in the workplace. They provide reliable and

consistent methods to ensure that accurate measurements of workplace chemical

agents can be made. The use of these standardised methods, in conjunction with the

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hygiene standards that we will consider, enables you, as a health and safety

practitioner, to demonstrate that adequate controls for chemical agents are in place.

The analytical methods we have considered give an indication of the sorts of

techniques that are available for the identification and analysis of workplace

contaminants. However, before we can carry out any measurements on chemical

agents we must first obtain a representative sample of the contaminant that we are

concerned about. Obtaining a relevant and accurate sample is as important as the

analysis itself and much of the MDHS Guidance on Analysis is concerned with

specifying methods of sampling.

1.2.1 Guidance on Analysis United States of America

The United States has five types of written methods:

1. Methods in the National Institute for Occupational Safety and Health (NIOSH)

Manual of Analytical Methods (NMAM). These methods are available in

downloadable files from the Internet at

http://www.cdc.gov/niosh/nmampub.html which also gives information on

obtaining the full printed version. The NIOSH site also links to MSHA, EPA,

ASTM, and ISO.

2. Methods developed by the Occupational Safety and Health Administration

(OSHA) Analytical Methods Manual. OSHA also has a list of partially validated

methods, in the IMIS series, which is available in paper form or CD-ROM. Both

sets of methods can be accessed on the Internet at http://www.osha-

slc.gov/SLTC/index.html. From this site, ⇒ OSHA Technical Manual selects OSHA

Sampling and Analytical Methods and ⇒ Chemical Sampling Information selects

the IMIS methods.

3. Methods developed by the Intersociety Committee (IC).

4. Methods developed by the U.S. Environmental Protection Agency (EPA). These

methods are written for ambient air applications, but many are applicable also to

the workplace. The methods are available on the Internet at

http://www.epa.gov/standards.html.

5. Methods developed by the American Society for the Testing of Materials (ASTM).

These are indexed under http://www.astm.org and selecting ⇒ ASTM store.

1.2.2 Guidance on Analysis International Standards Organisation (ISO)

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Internationally agreed standards (which do not necessarily conform to the CEN/TC137

performance requirements, but generally include precision data according to ISO 5725)

and published by the International Standards Organisation, Casa postale 56, CH-1211

Genève, Suisse. Many of these methods are translated into National Standards. The

web site is http://www.iso.ch. Selecting ⇒ ISO catalogue ⇒ international standards

(HTML) ⇒ ICS field 13 ⇒ ICS field 13.040.30 leads to workplace air quality standards.

1.2.3 Comparison of air-sampling methods for nickel in a refinery

Air sampling methods

United States

Air sampling for substances with time weighted average exposure limits, should be

conducted in terms of the correct sampling technique referred to in the National

Institute for Occupational Safety and Health (NIOSH) Manual of Analytical Methods

(MAM) (Plog 2002:505).

NIOSH method 7300, is commonly used for the detection of elements which includes

sampling for the total fraction of nickel dust (National Institute for Occupational Safety

and Health, Manual of Analytical Methods, Method 7300, 1997:1). Since 1998 the OEL

for nickel and nickel species were set for inhalable dust and NIOSH method 7300,

although still widely used, is not a suitable sampling method (American Conference of

Governmental Industrial Hygienists, 2003:43)

United Kingdom

EH 40 (1999:14-15) states that sampling methods that should be used in the United

Kingdom, can be found in the HSE’s sampling series “The MDHS”. The following

relevant sampling methods for nickel and nickel species are listed namely:

• MDHS 14/3 and,

• MDHS 42/2.

• MDHS 14/3 (2000:1) describes the general methods for sampling and

gravimetrical analysis of respirable and inhalable dust fractions.

• MDHS 14/3, measures particulate matter in accordance with the ISO’s, as well

asthe CEN’s, respirable dust convention.

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• MDHS 42/2 (1996:1) describes the measurement of nickel and inorganic

compounds of nickel in air.

South Africa

The Department of Minerals and Energy (DME) in South Africa, has guidelines for the

gravimetric sampling of airborne particulate matter. The guidelines provide for the

sampling of the total dust fraction and for the respirable dust fraction (Department of

Minerals and Energy, 1994:1). No other reference is made to sampling methods for the

measurement of specific hazardous chemical substances in the Hazardous Chemical

Substances Regulations (South Africa, 1995:5-6).

Discussion

The American, United Kingdom and the South African exposure limits, are set, based

upon obtaining, personal samples, which represents inhalable dust exposure

concentrations of the measured workers (American Conference of Governmental

Industrial Hygienists, 2003:8.; Environmental Hygiene 40, 1999:14.; South Africa,

1995:26).

It would appear that NIOSH method 7300 which has been set for the measurement of

total dust, using a Casette sampler is the least desirable sampling method to use to

determine compliance to the exposure limits as:

• occupational exposure limits are set for the inhalable dust fraction,

• the cassette sampler under estimates exposure concentrations,

• the cassette sampler collects the total dust fraction which is difficult to define

correctly

1.3 Sampling For Gases and Vapours

Sampling Methods

The two basic methods of collecting gaseous samples are:

• Grab sampling: −An actual sample is taken in a flask, bottle bag or other

suitable container −Samples are collected over a period of around a minute

−Useful for a peak concentration or when concentrations are relatively constant

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• Continuous or integrated sampling: −Gases or vapours are removed from the air

over a measured time period and concentrated by passage through a solid or

liquid sorbent

−The sample is collected by:

(i) Dissolving in a liquid

(ii) Reaction with a solution

(iii) Collection onto a solid sorbent

−Samples are collected over a period of up to several hours

−Useful if:

(i) The contaminant concentration varies with time

(ii) The contaminant concentration is low

(iii) A time weighted average exposure is required

−Sampling may be achieved:

(i) Actively (using a pump)

(ii) Diffusively (natural diffusion)

Grab Samplers

• Evacuated Flasks

A flask fitted with a valve at each end (Figure 1.10) is evacuated through one valve

whilst the other valve is kept closed. The open valve is then closed to seal the vacuum.

When the valve is opened a sample of the atmosphere under test is drawn into the

flask.

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Figure 1.10: Gas or Liquid Displacement Sampling Bottle

• Gas or Liquid Displacement Container

A flask similar to the one in Figure 3.3 can be connected to a pump and the vessel

filled with the test atmosphere through one valve by pumping out the air in the flask

through the other valve.

Another method is to fill the flask with water and then let the water drain out slowly

from one valve as the test atmosphere is sucked into the flask through the other valve.

Obviously this procedure cannot be used to collect water-soluble gases.

• Flexible Plastic Containers

Plastic bags can also be used as grab samplers. They have the advantage of being

light, non-breakable and simple to use.

• Hypodermic Syringes

Syringes of 10 to 50 ml volume can be used to draw a test atmosphere into the body

of the syringe as the plunger is extended. They are available in glass and disposable

plastic and are cheap, convenient and easy to use.

1.4 Continuous Sampling

Active Samplers

• Liquid Sorbents The four types of sampler using liquid sorbents to collect gases

and vapours are:

• Gas washing bottles :

(i) Suction applied to an outlet tube causes sample air to be drawn through an inlet

tube into the liquid contained in the sampler.

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(ii) Suitable for collecting non-reactive gases and vapours that are highly soluble in the

liquid sorbent, e.g. methanol and butanol in water; esters in alcohol.

(iii) Suitable for collecting gases and vapours that react rapidly with the reagent in the

sampling medium, e.g. ammonia neutralised by dilute sulphuric acid.

(iv) The midget impinger (Figure 1.11) is a commonly-used sampler with an air flow

rate of 1.01/min and 10 ml of liquid sorbent.

The impinger is connected to a pump and can be attached to the worker’s clothing.

(i)Used for collecting gas samples that are only moderately soluble in, or are slow in

reacting with, the reagents in the collecting vessel.

(ii) The spiral or helical structures in the collection vessel provide a higher collection

efficiency by allowing a longer residence time of the contaminant with the reagent for

slower acting and less soluble substances.

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Figure 1.11: The midget impinger

− Fritted bubblers:

(i) Used for collecting gas samples that are less soluble in the collecting medium.

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(ii) Design is similar to the impinger but the collection vessel contains sintered or

fritted glass, or multi-perforated plates at the bottom of the collection tube. Air drawn

into these devices is broken up into very small bubbles and the froth that develops

increases the contact between gas and liquid.

− Glass-bead columns:

(i)Used for special situations where a concentrated solution is needed.

(ii) Glass beads wetted with the liquid sorbent provide a large surface area for the

collection of the sample. However, the rate of sampling is necessarily low.

• Cold Traps

Cold traps are used where it is difficult to use any other method of collection. Vapour is

separated from air by passing it through a coil immersed in a cooling system such as

dry ice (solid carbon dioxide) and acetone, liquid air or liquid nitrogen. The

disadvantage is that water is condensed along with the organic materials being

sampled.

• Plastic Sampling Bags

Plastic bags, as used in grab sampling, can be used to collect air samples over periods

of a shift or longer in conjunction with a pump.

• Solid Sorbents

Absorbent solids can also be used to collect airborne contaminants. The two principal

materials in use are:

− Charcoal

Activated charcoal is an excellent sorbent for most organic vapours. The most common

procedure is to use activated charcoal sampling tubes of the type shown in Figure 1.12.

A glass tube with flame-sealed ends contains two sections of activated charcoal

separated by 2 mm portions of polyurethane foam. Immediately before sampling the

ends of the tube are broken and the tube is connected to a calibrated pump to draw

the atmosphere under test through the tube.

The duration of the sampling may be several minutes up to 7-8 hours, depending on

the tube’s capacity. The air flow should be checked with a flow meter from time to time

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while the sampling is in progress. At the end of the sampling period the tube is capped

at each end ready for analysis.

The first step in the analysis procedure is to remove the sample from the charcoal,

usually using solvent desorption with carbon disulphide. Although this does not remove

all the sample it is possible to apply a correction to take account of the efficiency of

desorption.

Once the sample has been desorbed from the charcoal it can be analysed using one of

the techniques described later.

Figure 1.12: Charcoal sampling tube

− Silica Gel

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Silica gel is another effective sorbent for collecting gases and vapours. The method of

use is similar to that of charcoal, involving sample tubes and a desorption solvent

which, in the case of silica gel, is usually water or methanol.

The advantages of silica gel over charcoal include:

(i) Many contaminants can be removed from the sorbent by using common solvents

such as water or methanol.

(ii) Certain substances such as amines, nitro compounds and some inorganic

compounds are unsuitable for absorption on charcoal.

(iii) Avoidance of the use of carbon disulphide (a highly flammable and toxic solvent)

for desorption.

The disadvantage is that silica gel has a high affinity for absorbing water and, if there

is much moisture in the air being sampled, the water will displace any absorbed

organic solvents from the silica gel surface. This limits the quantity of humid air that

can be passed through a silica gel absorption tube.

• Thermal Desorption

Another method of desorbing the collected sample is to heat the sample tube and drive

off the substance that has been absorbed. This avoids the use of hazardous solvents

such as carbon disulphide and provides a less laborious method. In general this is not a

practical method with charcoal sorbents because the high temperature needed to drive

off the sample would result in its decomposition. Consequently this method uses

carbon molecular sieves or porous polymer sorbents.

The thermal desorption procedure uses larger tubes than previously described,

desorption can be made fully automatic and analysis can be carried out using gas

chromatography.

1.5 Sampling Equipment

We have seen from the descriptions given above that the continuous sampling

procedure involves a sampling device (either liquid or solid sorbent) connected to a

sampling pump and an air metering device. This enables the contaminated air to be

pulled through the sampling device at a known flow rate. From this both the amount of

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contaminant and the total volume of air can be determined to enable the average

concentration of airborne contaminant to be calculated.

• Pump

The pump should have an adjustable flow rate and be able to operate continuously for

a period of up to 8 hours. For personal sampling the pump should be able to be worn

by an operator whilst carrying out their normal duties.

• Flow Measurement

Flow measurement is important in enabling an accurate estimation of the total volume

of air that has been sampled. An external flow meter with a known level of accuracy

should be used rather than relying on any flow meter built into the pump. These are

useful as a guide to the operating flow rate and indicate that the pump is working, but

are not accurate enough unless calibrated in some way during the sampling process.

One method of measuring flow is to use a bubble flow meter. This consists of a

calibrated tube with a soap film that is drawn along the tube by the pump under test.

The passage of the film is timed between two marks on the tube which represents a

known volume. From these measurements the flow rate for the pump in terms of

volume per unit time can be calculated.

• Analysis of Gases and Vapours

The description of the various methods for continuous sampling given above shows the

range of sample collection methods available. Table 1.2 lists a range of gases or

vapours that can be sampled by absorption on charcoal. Table 1.3 gives examples of

types of sampler used for the collection of airborne contaminants, the sorbent used and

the analytical method used to determine the quantity of substance collected.

Gas or Vapour Desorption

Acrylonitrile Carbon disulphide

Benzene Carbon disulphide

Carbon tetrachloride Carbon disulphide

Chlorobenzene Carbon disulphide

Chloroform Carbon disulphide

1,2-Dichlorobenzene Carbon disulphide

Dichloromethane Carbon disulphide

1,2-Dichloropropane 15% acetone in cyclohexane

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2-Ethoxyethanol 5% methanol in dichloromethane

Ethylene oxide Carbon disulphide

2-Methoxyethanol 5% methanol in dichloromethane

2-Methoxyethyl acetate 5% methanol in dichloromethane

Styrene Carbon disulphide

Tetrachloroethylene Carbon disulphide

1,1,1-Trichloroethane Carbon disulphide

Trichloroethylene Carbon disulphide

Vinyl chloride Carbon disulphide

Table 1.2: Examples of Gases and Vapours that can be Sampled by Absorption

on to Charcoal and the Desorption Medium

Gas or

Vapour Sampler Sorbent Analysis

Acetaldehyde Bubbler Water Iodoform reaction

Acetic acid Wash

bottle

Glycerol/water pH change

Acetonitrile Syringe Permanganate Colour change

Amines Bubbler HCl in isopropanol Ninhydrin/spectrophotometry

Ammonia Bubbler Dil H2SO4 Phenol/hypochlorite/

spectrophotometry

Aniline Bubbler Dil H2SO4 Spectrophotometry

Benzene U-tube Silica gel Spectrophotometry

Butanol Bubbler Water Chromate oxidation

Carbon

disulphide

Glass

beads Copper/diethylamine Colour reaction

Chlorine Bubbler Methyl orange Spectrophotometry

Ethanol Impinger Water Chromate oxidation

Formaldehyde Impinger Bisulphite Iodine titration

Hydrogen

sulphide

Bubbler Iodine soln Iodine oxidation

Methanol Impinger Water Fuchsin/formaldehyde

Nitrobenzene Bubbler Ethanol Spectrophotometry

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Nitrogen

dioxide

Bubbler Naphthylethylenediamine Colour reaction

Ozone Impinger KI Titration

Phenol Impinger Ethanol Spectrophotometry

Sulphur

dioxide

Impinger Tetrachloromercurate Spectrophotometry

Toluene U-tube Silica gel Spectrophotometry

Toluene Impinger Acid Diazotation/coupling/

diisocyanate spectrophotometry

Table 1.3: Examples of Samplers, Sorbents and Analytical Methods for

Common Gas and Vapour Contaminants

• Calculation of Result

As indicated above the collected sample is analysed either directly if a gas sample or

liquid sorbent, or after desorption if collected on a solid sorbent. Gas samples will be

expressed directly as a concentration in ppm. Samples absorbed in another medium

will initially be expressed as a concentration which can be converted to a mass by

multiplying by the sample volume. In calculating the actual average concentration of

airborne contaminant, factors such as the sampling efficiency of the collector (i.e. what

percentage of sample dissolves in the collecting medium) and the desorption efficiency

(i.e. how much of the sample is recovered from the sorbent after desorption) need to

be included in the calculation. These factors are usually determined by using samples

of known concentration as a control.

1.6 Diffusive Samplers

We have seen how continuous sampling can be carried out by pumping contaminated

air through a collection device to trap and measure the quantity of airborne

contaminant. This is termed “active sampling” since the process involves the active

movement of air through the sampler.

Another important method of sampling involves the use of a “passive” sampler or

diffusive sampler. This is a device which takes samples of gas or vapour from the

atmosphere under test by a physical process such as diffusion, but does not involve the

forced movement of air through the sampler.

•Method of Operation

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Pollutants are removed from the atmosphere at a rate controlled by diffusion through a

static layer or permeation through a membrane. The mass uptake by the diffusive

sampler depends on the concentration gradient (i.e. the concentration of contaminant

in the atmosphere compared to the concentration of contaminant in the sampler), the

time of exposure, and the area of sampler exposed to the atmosphere. Complications

to the process include fluctuating concentrations, sorbent saturation, wind velocity and

turbulence at the sampler surface, temperature and pressure.

The two principal types of design are shown in Figure 1.12. In Figure 1.12 (a) you can

see a badge-type sampler which has a flat permeable membrane supported over a

shallow layer of sorbent. Figure 1.12 (b) shows the tube-type sampler which has a

smaller permeable membrane supported over a deep metal tube filled with sorbent.

There are diffusive equivalents of most of the active systems, such as a liquid-filled

badge equivalent to the impinger, a charcoal badge equivalent of the charcoal tube and

also a thermal desorption badge. It is accepted that active and diffusive sampling are

complementary approaches with each having useful areas of applicability, and that

there seems to be no significant difference between accuracy and precision of diffusive

sampling and active pumped sampling.

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Figure 1.12 (a) Badge Sampler & Figure 1.12 (b) Tube Sampler

• Factors Affecting Performance

−Temperature and pressure

Mass uptake is independent of pressure but depends on the square root of absolute

temperature. In practice this means that temperature dependence at ambient

temperatures can generally be ignored but increased temperatures may adversely

affect the capacity of the sorbent.

− Humidity

High humidity can adversely affect the absorption by charcoal badges.

− Concentration variations

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It is possible that a sudden rapid fluctuation in contaminant concentration may be

“missed” before it has a chance to diffuse into the sampler. Since the time taken for

diffusion into the sampler varies between 1 and 10 seconds the sampling time will

usually be well in excess of this and therefore this effect will not present a significant

problem.

− Sorbent efficiency

Diffusive samplers rely on the sorbent having a high affinity for the contaminant being

sampled and therefore a suitable sorbent being selected for the contaminant in

question.

− Face velocity

This is an important parameter: if there is insufficient air movement over the face of

the sampler, transport of pollutant to the membrane will be limited and the effective

sampling rate will be reduced.

If there are high air velocities inducing turbulence in the sampler body the diffusion

path length will be reduced and the sampling rate increased. The geometry and design

of samplers should be such that sampling rates should be constant within the range of

air velocities likely to be encountered in the workplace, but badge-type samplers used

in static positions may experience air flows below the critical value for this type of

sampler.

• Calculation of Result

The method of calculation of the result is similar to that for active samplers in that the

collected sample is analysed and the total mass of the sample determined; the total

sample volume is calculated from the effective sampling rate (which depends on the

diffusion coefficient of the substance and the length and area of the diffusion path [the

geometry of the sampler]) and the time of exposure (sampling time). This gives an

average concentration in mg/m3.

1.7 Sampling Procedures

In the previous two sections we examined the range of techniques available to analyse

and measure workplace contaminants and also the different methods available to

sample gases and vapours.

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We are now ready to move on to consider how these occupational hygiene techniques

are used in practice. We shall now examine the different types of sampling procedure

that are available for use in the workplace, and look at some examples of actual

measuring instruments that may be used for environmental monitoring.

Before we look at sampling procedures we must remind ourselves what the purpose of

environmental measurements are:

• To give a qualitative analysis of an atmosphere, i.e. to indicate the presence of,

and identify, contaminants.

• To provide a quantitative analysis, i.e. to determine exact concentrations and

assess compliance with hygiene standards, or to assess exposure.

• To indicate the development of a potentially hazardous concentration, i.e. to

act as an alarm system.

Before any analysis of a workplace atmosphere is carried out, it is vitally important that

a representative sample of the environment under test is obtained. Any analysis,

however sophisticated, is useless in terms of the data produced unless the sample

analysed is representative of the particular hazard being monitored.

Types of Sample

You will remember that there are in general three forms of sample:

• The “spot” or “grab” sample, taken at one point (or in a limited area); it is

representative of the sample area at that point in time.

• The time averaged sample, taken over a period of time and after analysis the

results will give the total contamination. A time average can be deduced by

dividing total concentration by the time. This is sometimes termed continuous

sampling.

• The continuous monitored sample, continually taken and analysed during the

monitoring. At the end of any period of time a record of the variation in hazard

level in the vicinity of the sample point is obtained. This system is used on vinyl

chloride (chloroethene) plants. Continuous sampling systems can be used in

conjunction with alarm systems; when a set level is exceeded the alarm is

activated.

Sample Positioning

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The equipment used to carry out an analysis may vary, depending upon the type of

sample taken and the sampling location. There are three general positions for samples

to be taken:

• In the general working atmosphere, e.g. ozone monitoring in a welding shop or

oxygen deficiency in a closed vessel (grab sample).

• In the operator’s breathing zone, e.g. dust collectors (time averaged sample).

• At a position close to the contaminant generation, e.g. where beryllium metal is

being machined (continuous monitored sample).

Sampling Frequency

Some sampling procedures are laid down in guidance notes in conjunction with

Statutory Regulations. For asbestos fibres, sampling should take place for 4 hours to

conform with the requirement of the control limit. This may be altered to take into

consideration factors that might upset the taking of a viable sample, e.g. if the

collected fibre density was low, then extra time would be used to provide the required

conditions. In this case the fibre concentration would be adjusted to give the corrected

time requirement.

Sampling frequency will depend to some extent upon the risk level of the contaminant

being analysed. When entering a confined space for inspection purposes, an initial

sampling of the atmosphere would be satisfactory, provided the environment was safe.

If welding is to be carried out, regular grab samples may be satisfactory. If there is a

likelihood of excess fume generation then continuous monitoring would be more

appropriate.

In processes where lead is a hazard, bi-monthly sampling is recommended, provided

conditions remain satisfactory. More frequent samples are required if stable conditions

cannot be achieved.

Measurement Procedures

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There are basically two general procedures used for making environmental

measurements:

• The first procedure involves taking the sample, then carrying out an analysis in

separate equipment, often away from the sampling position, i.e. in a laboratory.

Measurements of dust concentration, e.g. asbestos fibres, are carried out in this

way.

• In the second procedure, sampling and analysis takes place in the same

instrument. One of the most commonly used instruments is the stain tube

detector for gaseous contaminants.

Methods of Sampling

There are two main ways that an airborne contaminant can be sampled: diffusion

sampling and mechanical sampling, which we looked at earlier in this unit.

• In diffusion sampling the contaminant passes over the sampling system in natural

air currents and diffuses into a chamber containing an absorbent material. At the end

of a given period of time, usually an 8-hour shift, the sampler is sent off to a laboratory

where the contaminants can be desorbed and analysed. The system is sometimes

described as passive sampling.

An example of such a system is the Draeger ORSA (ORganic SAmple) personal gas

measuring unit illustrated in Figure 1.13. The small glass tube containing the special

absorbent activated charcoal, is supported in a clip that can be worn in the breathing

zone of the person at risk. The mass of contaminant absorbed on the charcoal depends

upon its concentration in the air, the time of exposure and its diffusion characteristic

(i.e. some materials will diffuse quicker than others and therefore more mass will be

absorbed).

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Figure 1.13

With a knowledge of the diffusion characteristics, time of exposure and mass absorbed

(from analysis), the time averaged concentration can be calculated.

• The mechanical sampling system uses a pump to provide air flow through the

sampling device or analysing instrument. The use of reciprocating diaphragm pumps or

peristaltic pumps enables volume or air flow measurements to be monitored as each

stroke of the diaphragm or rotation of the compressor delivers a measured quantity.

This is sometimes called active sampling.

1.8 Analytical Mechanisms

There are three basic analytical mechanisms used in environmental measuring

instruments: chemical, electrical and physical. They can be used separately but are

more generally used in combination, depending upon the particular analysis involved:

− Chemical reactions are usually designed to produce a coloured product which

enables a qualitative analysis to be made, i.e. simple detection of a contaminant.

Quantitative analysis is done by measuring the depth of colour produced; or, in stain

tubes, the amount of reactant used in the detection reaction.

− Electrical detection is usually arranged in conjunction with chemical or

electrochemical processes, e.g. combustion on a resistance wire or current generation

between electrodes.

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− Physical methods may involve the use of ultraviolet or infra red radiation. The

absorption of the radiation by the gaseous contaminant is proportional to its

concentration, e.g. some mercury vapour analysers use ultraviolet radiation systems.

Other physical processes are visual microscopic analysis, e.g. asbestos fibres,

gravimetric analysis, size classification, using cyclone separators.

2.0 Measuring Instruments

For examination purposes you need to be able to describe the principles of operation

and methods of use of selected types of instruments. The information presented here

will not make you a competent analyst. To use hygiene equipment you will have to

receive practical training and develop a technique.

Stain Tube Detectors - “The Draeger”

Stain tube detectors provide a convenient method of analysing gaseous contamination

of the workplace air.

The principle of operation is very simple: a known volume of air is drawn over a

chemical reagent supported in a glass tube. The contaminant reacts with the reagent

and a coloured product, a stain, is produced.

The technology behind the manufacture of commercially viable stain tubes and their

accurate functioning is extremely complex. It has taken many years to develop since

the idea was first put into operation in about 1920, when carbon monoxide in mines

was detected and analysed by this technique.

Stain tube detectors are now made to allow grab sampling or long-term sampling,

operated by hand bellows, hand pistons or motorised pumps. The ubiquitous

breathalyser is a stain tube detector system, but the contaminated air is blown through

the tube to provide a volume of sample controlled by the bag.

•Draeger Multi-gas Detector

“The Draeger” as it is more generally known, is a common instrument used for

environmental testing. The unit consists of two main parts, the bellows pump and the

Draeger tube, selected to suit the particular measurement to be carried out:

−Bellows Pump

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The bellows hand pump is shown in Figure 2.1. and the basic structure of the bellows

hand pump is shown in Figure 2.2.

The pump is designed to draw in 100 cm3 of air with one stroke. To achieve this, the

bellows must be fully compressed before it opens automatically by the spring to its

maximum volume, controlled by the limiting chain. This mode of operation is

comparable to a dosage pump. The time taken for the bellows to open fully from the

closed position gives one pump stroke. The stroke time will depend upon the type of

Draeger tube being used and can vary from three seconds to 40 seconds.

Owing to the time involved and the number of strokes required for a particular

measurement, it is very important to have a stroke counter fitted to the unit.

Never rely on memory!

Figure 2.1: The bellows hand pump

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Figure 2.2: The basic structure of the bellows hand pump

−Detector Tubes

The detector tubes contain a reagent which reacts with the contaminant in the air flow

passing through it to cause a coloured reaction.

The method of controlling the colour developed is either by drawing a fixed volume of

air through the tube using a specified number of strokes, or by counting the strokes

required to produce a colour change.

In the first method the tubes are marked with a graduated scale; the longer the stain

produced the higher is the concentration of contaminant. This is the most commonly

used system: they are sometimes called scale tubes.

In the second method, used less frequently, the greater the number of strokes taken,

i.e. the greater the volume of air sampled, the smaller is the concentration of the

contaminant, e.g. for the olefine 0.5% detector tube, five strokes indicate 500 ppm,

while ten strokes indicate 200 ppm.

2.0 Measuring Instruments (Cont.)

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A scale tube is illustrated in Figure 2.3.

Note the colouration on the used tube indicating a concentration of 50 ppm carbon

monoxide, the stroke number n = 10, and the arrow showing the end to be inserted

into the pump.

The formation of the colour shows just how precisely the indicating material has to be

made. The reagent has to be evenly distributed through the carrier material and

accessible to the contaminant so that it reacts quickly enough to give the colouration

within the scale markings.

As the reagent is used up by the contaminant, the contaminant is able to passs further

along the tube to react, and a higher concentration is indicated.

• Automatic Multigas Detector

A refinement on the bellows type pump is the automatic multigas detector. This is an

electrically operated bellow pump model which can be set to switch off when the

selected number of strokes for the particular tube is complete. It is useful where an

operator has to be free during testing and where the measurements require a high

number of strokes.

• Polytest Tubes

Some tubes, called polytest tubes, are designed to make qualitative measurements to

determine only the presence of potentially harmful substances. Varying colour and

stain length sometimes give an indication of the possible contaminant.

2.1 General Method of Operation of Stain Tube Detectors

Select the appropriate tube for the measurement being made, taking note of any

possible cross-sensitivity.

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− Break the end off the tube to be inserted into the pump. Use the tube end-breaker

provided.

− Insert the tube into the pump and exhaust the bellows by fully depressing the front

plate.

− Allow the system to remain in this state for a few minutes and check for possible

leaks.

− If there are no leaks, break off remaining tip in an uncontaminated atmosphere.

Cover end with rubber cap provided.

− Select the sampling position, remove rubber cap and proceed to carry out the

sampling procedure, e.g. the given number of strokes for a scale tube and the time

allowed for the colour to develop fully.

− Note the reading and record the result and sample position.

− Remove the stain tube, cover both ends with a rubber cap and dispose of it

according to the manufacturer’s instructions.

Problems with Stain Tube Detectors

You should be aware of some of the problems related to the use of stain tube detectors

as their control will help to make fuller and more effective use of the stain tube

system:

− The rate of flow of air is important, so the stain tube should have the ends removed

properly.

− The accuracy of the sampled volume is critical, therefore the bellows action must be

fully operated for every stroke. The number of strokes must be recorded accurately,

hence the need for an effective counter. Leaks must be eliminated.

− There may be the possibility of cross-sensitivity of tube reagents to other substances

than the one being analysed. This will be indicated on the data sheet accompanying the

particular stain tube.

− There may be problems caused by variations in temperature and pressure. Stain

tubes are designed to operate at about 20°C and one atmosphere pressure. Variation

in atmospheric pressure will probably be within the limits of accuracy of the system,

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although changes in altitude could cause problems. Normal variations in temperature

may be problematic; remember, a change of 10°C can cause a reaction rate to be

doubled or halved. With ambient temperature ranging between 0°C and 30°C, the

potential for error is considerable.

− Because of the complexity of the indicating reagent, tubes have a shelf life, so care

must be taken to turn over stock and only to use currently operative tubes.

− Reagent complexity also causes a variation between each tube; hence, judgments

cannot be made on one grab sample.

− Hand-operated stain tube systems are capable only of a “point in time” or grab

sample. Long-term tubes have now overcome this problem.

2.2 Long-term Stain Tubes - Draeger Polymeter

To overcome the problem of point-in-time analysis, long-term tubes have been

developed. The Draeger Polymeter long-term testing system consists of a battery

powered peristaltic pump, providing 15 cm3/minute air flow rate, with a built-in counter

and a special long-term stain tube. The whole unit is small enough to be carried, with

the stain tube in an extension section, or to be easily positioned for static operation.

The stain tubes are marked in µ (equivalent to ppm). They are similar to the normal

stain tube except that the time-weighted average concentration is not indicated but

has to be worked out.

The average values are calculated using the following equation:

The air volume is calculated by multiplying the number of revolutions of the peristaltic

pump by the amount of air displaced from the pump tube during one revolution of the

tube compressor.

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Vapour Absorption Tubes

As we saw earlier vapour absorption tubes with an identical appearance to stain tubes

have now been developed to collect samples of organic vapour on activated charcoal or

silica gel. They can be attached to a long-term polymeter or an automatic multigas

detector. After an appropriate sampling time, the tubes can be sealed and sent off to a

laboratory for more sophisticated chemical analysis, e.g. gas chromatography.

2.3 Oxygen Monitor

Analysis of a working environment to monitor or determine the concentration of

oxygen is very important. For concentrations below 20% oxygen, the possibility of

death or brain damage from simple anoxia has to be considered. For concentrations

above 20% enhanced fire risk is the problem, with the possibility of horrific burns to

operators and excessive fire damage to property.

In the operating condition the oxygen in the air sample monitored diffuses into the

sensor through a special membrane. It then passes into the electrochemical measuring

system, where the resultant electrochemical process produces electric current directly

proportional to the oxygen concentration.

The principles of operation of the sensor probe are illustrated in Figure 2.4.

The signal produced by the electrochemical reaction is transmitted to a direct readout

gauge giving the oxygen concentration in percentage oxygen. The instrument can be

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pre-set to a given oxygen concentration which activates an alarm system. The

electrochemical sensor is not directly attached to the main instrument but is connected

by a lead, thus enabling more flexibility in use when it is carried by the operator. The

instrument is also suitable for static monitoring in workplaces, especially where

compressed or liquid oxygen is being used.

Electronic Gas Detectors

Flammable gases and vapours may be detected and their concentration analysed at

low levels by causing a reaction to occur on an electric filament. The main type of

reaction is oxidation of the substance, i.e. combustion with oxygen from the air. The

heat generated upsets the balance of the wheatstone bridge system. A schematic

diagram of a combustible gas detector is given in Figure 2.5.

The system can be used to detect and monitor toxic gaseous substances at low

concentrations as well as flammable substances below their explosive limits.

There are a few problems related to the use of gas detectors:

• They give incorrect readings if the level of oxygen in the air is reduced.

• In high concentrations of flammable substances, i.e. above their upper explosive

limits, readings can fall back to zero, so a false sense of safety can occur.

Portable units have been designed for operator use; large portable units for general

plant maintenance work, e.g. welding, where there is a hydrogen gas risk; and also

fixed installations which are connected to alarm section consoles in control rooms, e.g.

an oil production well site.

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Photoionisation Detectors

In this type of analyser the contaminant is drawn into a cell and is ionised by UV

radiation. Most inorganic or organic substances present will dissociate into charged

ions. The major components of air such as oxygen, nitrogen, carbon dioxide and water

vapour will remain unchanged. Consequently ions are generated only from the

contaminant. These ions generate a current between two electrodes mounted in the

sample chamber. The current is proportional to the concentration of contaminant

present. Although the technique is non-selective and does not distinguish between

ionisable contaminant species, it is useful in measuring concentrations of airborne

contaminants generally. It has the advantage of being highly sensitive and gives a fast

response. It can be used for continuous monitoring or general screening of a

workplace.

Semiconductor Detectors

The metal oxide semiconductor is another electronic device which is widely used, but

again is nonspecific. Absorptions of a contaminant gas on the surface of a

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semiconductor will change its surface conductivity. This principle can be used to

measure the concentration of contaminant gas present in a particular atmosphere

down to levels less than 1 ppm. However, most organic gases will cause a response

and therefore the results may need careful interpretation.

Infra Red Analysis

These instruments rely on the specific infra red (IR) absorption characteristics of

certain chemicals; we studied their principle of operation earlier. IR radiation is passed

through a cell containing the contaminant and the absorption of radiation at specific

wavelengths is proportional to the concentration.

This provides an accurate measurement of concentration for a wide range of gases

provided they exhibit absorption in the IR range. Modern instruments are precalibrated

for an extensive range of gases and vapours. However, the equipment tends to be

complex and bulky, and very expensive. If its use is likely to be specific and infrequent

it is usually more cost-effective to hire this type of detector.

Ultraviolet Analysis

These analysers are similar in principle to the IR analyser but use radiation in the

ultraviolet/visible wavelength range. Again the absorption of radiation at wavelengths

specific to the particular contaminant is proportional to the concentration of the

contaminant. The equipment can only be used to monitor gases which absorb radiation

in the UV/visible wavelength range, but gives a highly accurate measurement. As with

IR analysers the instruments tend to be costly and complex in use.

Portable Gas Chromatographs

Gas chromatography is one of the ultimate techniques for accurate analysis of chemical

mixtures as we found in our earlier study of analytical principles. A sample of

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contaminant atmosphere is injected into a carrier gas stream which passes through a

tube containing a stationary phase.

Each chemical in the mixture will distribute itself between the gas phase and the

moving phase. This has the effect of slowing down the rate of travel of the components

in the mixture to varying degrees, thus separating them. Each component in the

sample travels down the tube at a different rate and can be measured separately by a

detector at the end of the tube, which makes the technique accurate and sensitive for

contaminant mixtures. However it cannot be used for continuous sampling and the

analysis time is quite lengthy. The technique also requires some skill. It is likely that

developments in sensor technology and microprocessor control will increase the use of

gas chromatography as an environmental monitoring technique.

2.4 Dust Sampling

Dust sampling is a common technique used to assess the exposure of operators to a

potential dust hazard or to determine the amount of dust generated by a particular

process. To satisfy these requirements, both personal and static sampling systems

have been designed.

The simplest general method used to carry out dust sampling is the use of a porous

filter which is able to retain the dust particles contained in an air flow. When sampling

is complete, the filter media is removed together with the “captured” dust. The dust

collected is determined gravimetrically, i.e. by weighing. Generally, the filter media is

weighed before and after the sampling and the mass of dust collected is given by

weight difference.

A simple dust filtering sampling device is illustrated in Figure 2.7.

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Such a simple system does not provide the hygienist with information about the all-

important respirable mass of dust in a sample. To overcome this problem, systems

have been developed which only retain the respirable fraction of airborne dust. Dust

samplers have been designed with cyclone elutriators which remove large particles and

only allow the respirable fraction to pass to the filter. Contaminated air is fed to the

sampler by a special pulsation-damped pump which gives a smooth air flow in the

cyclone to ensure efficient separation. The samplers are small enough to be worn

comfortably on a lapel or on a safety helmet.

The basic principles of operation are shown in Figure 2.8.

Where more detailed information is required about the size range of the airborne dust,

more sophisticated cyclone systems are used. The cyclones are arranged in series and

are so constructed that specific size ranges of dust are deposited in each section of the

unit.

Air flow is arranged by using a damped flow suction pump. After the instrument has

been run for a given time, each cyclone can be opened and the dust which has adhered

on a disc covered with a non-drying sticky film can be removed and sent for analysis.

Analysis of the cyclone disks involves counting deposited particles under a microscope.

As the cyclones have been graded for size retention, then the percentage of each

particle range in the dust sample can be calculated.

For “simple” particulate dust, microscopic analysis is accurate. Unfortunately, where

asbestos fibres are analysed, the technique provides variable answers even with

experienced operators. Variations of the order of 50% have been quoted. For some

dust samples, e.g. silica-containing dusts, chemical analysis is sometimes carried out

to assess toxic potential.

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2.5 Summary

In this study unit we have looked at the principles on which environmental monitoring

is based. We have considered various analytical techniques, most of which involve

subjecting the substance in question to a burst of energy and examining the way in

which the substance responds. The techniques we have studied include gas

chromatography, flame atomic absorption spectroscopy, X-ray fluorescence

spectroscopy, infra red spectroscopy, X-ray diffraction and optical microscopy. Specific

characteristics of the substances are identified and the danger they represent to health

is evaluated.

The MDHS Guidance on Analysis is important as it provides reliable and consistent

methods to ensure that accurate measurement and assessment of chemical agents can

take place.

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When sampling for gases and vapours there are two basic methods: grab sampling and

continuous or integrated sampling. We examined both methods in some detail, also

spending some time on diffusive samplers.

Various aspects which must be taken into account when sampling, such as sample

positioning, methods of sampling, and analytical mechanisms were discussed before we

concluded with an assessment of various measuring instruments ranging from stain

tube detectors, to photoionisation detectors, to infra red and ultraviolet analysis and

dust sampling.

3.0. Monitoring Strategies for Airborne Contaminants

Introduction

In the previous study unit we introduced the topic of environmental monitoring and

demonstrated how it is a specialist function that enables us to assess the risk of harm

from exposure to chemical agents through identification and measurement techniques.

You will remember from your Part 1 studies that exposure to airborne contaminants is

an important route of entry of chemical agents. In the previous study unit we began

this topic by examining the methods and equipment employed to analyse and measure

vapours and gases; now we move on to examine how we sample particulate

contaminants (or aerosols).

3.1 Solid Particulate Sampling

We have just examined the various methods available for the sampling, analysis and

measurement of gases and vapours. Here we shall consider how we sample and

measure the wide range of solid particulates that we are likely to encounter in the

workplace. These include mineral and metallic particles produced by the manufacturing

industries, fumes produced during welding and smelting, fibrous particles such as

asbestos or man-made mineral fibre, and bioaerosols produced in the agricultural or

food industries.

We have already considered the aerodynamic behaviour of aerosols generally in

previous units and the variation in size and shape that is encountered in airborne

contaminants. We shall build on these ideas by relating the sampling of aerosols to

sampling criteria based on the likely health effects of inhalation. In simple terms this

relates the “size” of the particle to the area of the respiratory tract that it is able to

penetrate, and consequently cause occupational ill-health.

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Sampling Criteria Based on Health Effects

The original classification of aerosols based on health effects recognised two sizes of

particle. Coarse particles were those able to be deposited in the upper respiratory

tract, associated with toxic effects; finer particles were those able to penetrate

further into the respiratory tract to be deposited in the gas exchange regions of the

lungs. These particles were those associated with the pneumoconiosis found in the

mining industries.

As a consequence the associated sampling procedures were based on these

distinctions. Sampling of coarse particles for health-related purposes was based on

“total” aerosol, which in theory is the complete range of all sized particles, but in

practice varies with the type of sampling device. (Certain designs of sampler might

“miss” particular size ranges of particles.) The sampling of finer particles was based on

the “respirable” fraction which is that fraction of particle size that penetrates the gas

exchange regions of the lung.

In an attempt to provide consistent and internationally acceptable definitions for

health-related aerosol fractions, recent reviews of these concepts have agreed the

following three relevant fractions for occupational use: inhalable, thoracic and

respirable.

•The Inhalable Fraction

This is defined as the mass fraction of total airborne particles which is inhaled through

the nose

and mouth. It replaces the concept of “total” aerosol.

This fraction is relevant, for example, for biologically active particles such as bacteria,

fungi and allergens. Exposure to these particulates can lead to inflammation of

sensitive membranes in the upper respiratory tract, nose and throat (i.e. hay fever,

rhinitis). In addition, particles such as nickel or wood dust can cause serious local

conditions such as ulceration or nasal cancer.

•The Thoracic Fraction

This fraction has an aerodynamic diameter of less than 30 µm. It is defined as the

mass fraction of inhaled particles that are able to penetrate the respiratory system

beyond the larynx.

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This fraction is relevant for particles that may provoke a response in the tracheo-

bronchial region such as chronic bronchitis or bronchial carcinoma (e.g. chromium).

•The Respirable Fraction

This fraction has an aerodynamic diameter less than 10 µm. It is defined as the mass

fraction of inhaled particles that is able to penetrate the respiratory system as far as

the alveolar region.

This fraction is relevant for particles that are able to deposit in the alveolar region and

lead to pneumoconiosis, alveolitis and pulmonary carcinoma (e.g. antimony, iron or tin

dust).

Fibrous Particles

In the case of fibrous particles, the definition of respirable is based on shape as well as

aerodynamic diameter. For asbestos, long, thin particles are thought to be more

hazardous than short, fat ones. This is due to the fact that long, thin fibres are capable

of penetrating deep into the alveolar region of the lung and, having become lodged

there, they are difficult to remove by the lung’s defence mechanisms.

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For this type of solid particulate, selection of the respirable fraction is carried out after

the fibres have been collected on a filter. They are first sized against the following

criteria:

−length/diameter ratio of 3:1

−length >5 µm

−diameter <3 µm

then counted under a microscope using the method described in Study Unit 3. We shall

look at the actual method for asbestos fibre determination in more detail later in this

study unit.

3.2 Aerosol Sampling Systems

Principal Components

Having looked at sampling criteria and sampling strategies we must now examine the

principal components of an aerosol sampling system, i.e. the sampling head; the

particle size selector (if required); the collecting or sensing device; the system for flow

monitoring and control (including calibration of flow); and the pump.

•Sampling Head

There is a wide range of sampling heads available with different sampling efficiencies.

It is important that the sampling device represents the way that the airborne

contaminant penetrates the respiratory system and arrives at its point of deposition in

order to ensure that a representative sample of the appropriate aerosol fraction is

taken.

•Size Selection

The inhalable fraction, as we have seen, represents the total size range of the aerosol,

but in order to sample the thoracic or respirable fractions some form of particle size

selection is necessary.

Generally, in sampling systems particles are selected using aerodynamic methods,

which mimic those involved in the deposition of particles in the respiratory system.

Consequently elutriators use gravity to separate particles of different mass; cyclones

employ centrifugal force; impactors rely on the inertia of moving particles (i.e. their

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tendency to continue in their original direction of travel and impact on obstructions

rather than follow the air stream around them).

•Filters

Filters are commonly used to collect the aerosol prior to assessment, which might

involve weighing the sample or counting fibres using a microscope. As you might

expect there is a range of filters available and the choice depends on the application.

Membrane filters made out of cellulose acetate are good at retaining particles on their

surface and are therefore used for collecting particles such as asbestos fibres that will

be counted by microscopy.

Fibrous filters made out of glass, for example, allow particles to penetrate into the

filter, thus providing greater capacity for deposition and a larger sample size which is

good for gravimetric determinations.

Pore sizes range from 0.1 µm-10 µm and need to be matched to the particle size being

collected. The selection of the filter also needs to take into account any direct

measurements that may be made on the collected sample, (e.g. infra red

spectrometry, X-ray fluorescence), the need to make the filter transparent for asbestos

fibre counting, or extraction from the filter for subsequent chemical analysis.

•Pumps

The type of pump required depends on whether static or personal sampling is being

carried out.

For personal sampling the pump must be light enough to be worn on the body without

inconvenience to the wearer. The flow rate required for personal sampling is usually

relatively low in the range 1-41/min.

For static sampling larger flow rates up to 1001/min may be employed and the

equipment does not have the limitation of needing to be portable.

Internal flow meters are usually incorporated into pumps but, as we have stressed

before, it is important to ensure that calibration is carried out against a reliable

standard. If filters become blocked during the sampling period this will cause the flow

rate to drop significantly.

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Static Samplers

Static samplers designed to sample coarse aerosols in the workplace comprise a

complete assembly that includes the filter, pump and flow meter. They are designed to

be left unattended in an appropriate position in the workplace for the duration of the

sampling period. Their purpose is to provide an average measure of the level of

airborne particles but few static samplers meet the inhalability criteria we discussed

earlier.

Static samplers for the respirable fraction are of a similar design but use elutriators or

cyclones to remove the larger particles before collecting those in the respirable size

range.

Personal Samplers

Personal sampling is the preferred technique for monitoring airborne contaminants

since the result represents more closely the intake of airborne particulates by the

worker and can be used to make direct comparisons with occupational exposure limits.

In general, personal samplers tend to meet the inhalability criteria under limited

environmental conditions but tend to under-sample at high windspeeds.

Where personal sampling is necessary for the respirable fraction and particle

separation is necessary, elutriators tend to be too bulky for use in personal samplers.

Consequently for the respirable fraction personal samplers tend to use cyclones to

remove the larger particles.

3.3 Direct Reading Instruments

The instruments described so far all rely on particles being collected on a filter or other

substrata before being assessed separately at the end of the sampling period. This

technique is satisfactory if a time-averaged measurement is required, but on some

occasions a short-term measurement may be required. Under these circumstances a

direct reading instrument is needed to give an instantaneous result. Direct reading

instruments rely on the following basic principles:

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•Light scattering

This technique is based on the principle that light passing through a suspension of

airborne particle will be scattered in proportion to the concentration of the airborne

contaminant. The advantage of this technique is that measurement can be made

without disturbing the aerosol.

The disadvantage is that scattering is strongly dependent on particle size and type

which makes accurate determination of the result difficult. This type of equipment can

be fitted with a cyclone to enable the respirable fraction to be measured.

Solder fume viewed under ambient View using dust lamp, under reduced View from

small angle of the dust

light and with flash light, without flash lamp, under reduced light

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Light scattering is commonly know as use of a Tyndall beamas a Tyndall dust lamp is

any lamp with a reasonably parallel beam of light which can reveal the presence of fine

dust by the scattering process first identified by Professor Tyndall.

•Beta particle attenuation

If beta particles are passed through the layer of particulate material deposited on a

filter, the reduction in intensity of the beta particle stream gives an instantaneous

indication of the rate at which particles are collecting on the filter and hence the

concentration of the sampled aerosol.

•Oscillating micro-balance

The frequency of mechanical oscillation of a quartz crystal is directly proportional to the

mass of the crystal. If airborne particles are allowed to deposit on the surface of the

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crystal, this causes a change in its resonant frequency which can be used as a measure

of the amount of material deposited and hence the airborne concentration.

3.4 MDHS Method for the Gravimetric Determination of Dusts

In element B3 we discussed the significance of MDHS Guidance which sets out HSE

approved methods for the analysis and measurement of chemical agents.

Freely downloadable from www.hse.gov.uk/pubns/mdhs/pdfs/mdhs14-3.pdf

MDHS 14 (General Methods for the Gravimetric Determination of Respirable and Total

Inhalable Dust) prescribes in considerable detail the general methods used by the HSE

for the collection and gravimetric determination of aerosol samples. Outlined below is a

summary of the method to give you some indication of how such measurements are

carried out in practice.

Scope of the Method

The methods described in the guidance are deemed suitable for the determination of

the gravimetric concentrations of most particulate dust and fume in the workplace. The

lower limit of detection of the method is determined by the length of time of the

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sampling period (how large a sample can be collected) and the sensitivity of the

balance used to weigh the collected sample (how accurately the deposited sample can

be weighed).

Principle

A measured volume of air is drawn through a membrane filter mounted in a sampler,

and the mass of dust collected is determined by weighing the filter before and after

sampling.

For total inhalable dust the sampler collects the fraction of airborne material that

approximates to that which enters the nose and mouth during breathing.

For respirable dust an appropriate size selection device is used to ensure that the filter

collects the fraction of airborne material that approximates to that penetrating to the

gas exchange region of the lung.

Apparatus

The apparatus specified is similar to that already described in this section, i.e. a filter

(on which to collect the sample), a pump, a filter holder and size selection device and

an air flow measuring device. The flow is controlled by a flow stabilised pump or by

frequent adjustments of the flow rate. Remember that it is important to maintain a

known and steady flow rate to enable the total volume of air sampled to be measured.

Personal Sampling - Total Inhalable Dust

The personal sampler for total inhalable dust has a sampling head with seven

equispaced inlet holes of 4 mm diameter and a pump unit capable of maintaining a

smooth flow rate of 2.0 l/min.

Background Sampling - Total Inhalable Dust

Either an open-faced filter holder or similar equipment to that described above for

personal sampling may be used. High volume samplers using flow rates up to 50 l/min

may be used to collect samples in shorter periods. Aerodynamic effects resulting from

the use of higher flow rates or different sampling head design change the collection

characteristics and hence only allow a rough estimate of the total inhalable dust.

Personal Sampling - Respirable Dust

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The respirable fraction is collected in the same manner as the total inhalable fraction

but the equipment has a cyclone pre-selector.

Background Sampling - Respirable Dust

Again the personal sampling equipment described above can be used or a special

sampler using a parallel plate elutriator as a preselector.

Filters

Glass fibre filters are suitable if no analysis is required and only gravimetric

measurements are to be made. If analysis of the collected material is required then the

appropriate MDHS guidance will specify the type of filter to be used for that particular

contaminant.

3.5 Procedure

MDHS 14 describes the sampling procedure in practical detail. Key points to consider

are:

• Use of fully charged battery-operated portable sampling pump fitted with air flow

smoothing device.

• Run the pump for 15 minutes to stabilise the air flow at the required rate.

• Fit the sampling head with a clean, preweighed filter.

• Attach the sampling head to the operator, not more than 30 cm away from the

nose-mouth region.

• Record the time at the start of the sampling period and check, record and

readjust the flow rate as necessary at the end of each hour.

• At the end of the sampling period note the time and remove the filter for

reweighing.

• Background (static) samplers should be positioned carefully to obtain a

representative sample of dust.

Calculation of Dust Concentration

The volume of air passing through the filter is calculated by multiplying the flow rate

(cubic metres per minute) by the sampling time (minutes). The weight gain (mg) of

the filter, divided by the volume sampled, gives the average dust concentration in

milligrammes per cubic metre of air (mg/m3).

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MDHS Method for Sampling Asbestos Fibres in Air

MDHS 39/4 (Asbestos Fibres in Air, Sampling and Evaluation by Phase Contrast

Microscopy (PCM) Under the Control of Asbestos at Work Regulations) gives

details of the method to be used to sample and measure the concentration of asbestos

fibres in air. The basic method is similar to that described above for general dusts but

the key differences are as follows:

• An open-faced filter holder exposing a circular area of filter of at least 20 mm in

diameter is used.

• The filter is protected with a downward-facing cylindrical cowl.

• Membrane filters of pore size 0.8 - 1.2 µm are used which can be rendered

transparent to allow fibre counting by phase contrast microscopy.

• For personal sampling in relation to the 4 hour control limit the flow rate must

be 1.0 l/min and the period of measurement must be representative of worker

exposure over a 4 hour period.

• Fibres are counted on the membrane filter in accordance with the protocol we

have already discussed and the airborne concentration is calculated by dividing

the total number of fibres collected on the filter (determined by extrapolation

from a proportional sample for large fibre counts) by the total volume of air to

give an airborne asbestos concentration in fibres per millilitre (f/ml).

3.6 Monitoring Strategies for Airborne Particulates

We have considered the way that procedures for sampling airborne contaminants need

to be related to the likely health effects. Here we examine the way that sampling

procedures are employed as part of an overall strategy to investigate the nature,

extent and control of exposure to hazardous substances which may be present in the

workplace air.

Guidance on “Monitoring Strategies for Toxic Substances” is contained in the HSE

Guidance Note HS(G) 173 which replaces a previous Guidance Note EH42 of the same

title.

HS(G) 173

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Recommended Reading

HSG 173

Basic philosophy

"do not measure unless you know what you are measuring and what you will do with

the results“.

3.6 Monitoring Strategies for Airborne Particulates (Cont.)

We shall now look at the different types of monitoring that can be carried out, then

consider a range of sampling strategies that can be employed depending on the

information required and the nature of the workplace activities.

Monitoring Methods

Static Sampling and Personal Sampling

Depending on the objectives of the monitoring strategy, static or personal sampling

can be used to evaluate airborne contamination.

•Static Sampling

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If the primary aim is to assess the effectiveness of engineering controls of plant

emissions into the working environment, then static sampling may be the best

technique to employ. Sampling aims to identify likely sources of contamination from

machines or processes. The sampling equipment is placed at fixed positions which are

selected to provide the most useful information regarding the principal contaminants

emitted into the workroom air. The activities of the workforce or whether employees

regularly work there at all is secondary to the monitoring strategy.

Sampling test intervals are designed to smooth out short-term fluctuations but should

be frequent enough to correlate changes in contamination levels with plant activities.

In this way changes in the degree of engineering control (i.e. changes in airborne

contamination levels) can be related to plant or process activities. Often the data will

be shown graphically on a control chart, with warning and action contamination. When

the action line is exceeded it indicates the engineering controls of plant emissions are

not operating satisfactorily and the plant manager must take action to rectify the

situation, which could involve plant shutdown.

Remember that the control levels are an indication of the performance of the plant

engineering control measures for possible airborne contaminants and not individual

personal exposures.

Hence levels should be set well below personal exposure limits to give ample margins

of safety.

•Personal Sampling

In the case of personal sampling the principal aim is to assess individual exposure to

airborne contaminants in relation to the set occupational exposure limits. Sampling

concentrates on individuals and their specific work locations to determine personal

levels of exposure to the contaminants present in the working environment. However,

it is possible to divide sampling amongst employees performing similar jobs and to

obtain generic sampling data for a particular work activity.

3.7 Exposure Limits

It is important to avoid or control any long or short-term exposure leading to ill-health.

Exposure limits are expressed in concentrations averaged over specified reference

periods. Long-term exposure limits (LTELs) are usually averaged over a period of 8

hours (based on a working day), whereas short-term exposure limits (STELs) are for a

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15-minute reference period. Thus, LTELs equate to chronic effects and STELs to acute

effects.

Factors Influencing Airborne Concentrations

A number of variables affect the airborne concentrations of contaminants in the

workplace. Some of the most important include:

− Number of sources from which the contaminant is released.

− Rates of release from each source.

− Type and position of each source.

− Dispersion or mixing of contaminants in the air of a workroom, influenced by

ventilation and random movements or turbulence in the air.

− Ambient conditions, particularly for outside operations, where factors such as wind

speed, direction and air temperature are important.

Because plant and process conditions and general ventilation characteristics are likely

to vary from day to day or to display a seasonal pattern, it is not advisable to make

assumptions about long-term exposure patterns based on a single estimate of

contaminant concentrations at one point of time.

The situation becomes more complex when the actions and behaviour of employees is

considered. At its simplest, an individual’s exposure will depend on how close he is to

the various sources of contamination and the length of time spent in a contaminated

area. This may vary from day to day and from one worker to another, although their

jobs may appear to be similar in nature. When an employee has a direct influence on

the process or the number or nature of contaminant sources, the situation is further

complicated as, for example, in manual handling operations such as scooping dusty

materials or pouring liquids.

The main variations which need to be considered include:

• Within shifts : variations in the concentration of the contaminant in the

breathing zone of each person over the period of each shift;

• Between shifts : variations in the shift pattern and average exposure of each

worker;

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• Between processes : variations due to the nature of the process as influenced

by operating procedures, product composition, ambient conditions, etc;

• Between individuals : variations between individuals in their exposure levels,

even between people doing the same kind of work in the same place on the

same shift.

The choice of sampling strategy also has a major influence on variations in the results

obtained. Figure 3.1 shows an exposure pattern obtained over an 8-hour period using

continuous sampling, while Figure 3.2 shows five shift averages (8-hour TWAs).

Figure 3.1: An exposure pattern for a working shift (8 hours)

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Figure 3.2: Daily variations in shift averages (8-hour time-weighted averages)

First consider the data on the basis of personal sampling in the worker’s breathing

zone. Since workers move around, do their work at different rates, handle materials in

different amounts and in a different order, etc., it follows that each will have different

exposures data. Also see how continuous sampling/measurement produces a much

more accurate picture of the exposure characteristics. In general terms, increasing the

duration of sampling reduces variation in sampling results.

If we were relying on environmental “spot” sampling, then we would only know the

concentration variability at the sampling head and, if it were static, in a particular

location. It would tell us very little about personal exposure.

3.8 Devising a Sampling Strategy for Airborne Contaminants (HSG 173)

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Before carrying out a sampling strategy for airborne contaminants, an initial appraisal

should lead to a decision on whether a quantitative air sampling study is required; and

if so, the form it should take. It is vital that this initial appraisal is performed by a fully

competent, experienced and knowledgeable person, especially if the conclusion is that

no further action is necessary.

Information needed for the initial appraisal will include:

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− The substances which occur at the workplace (raw materials, intermediates,

products, contaminants, etc);

− The airborne nature of the substances (dust, mist, fume, aerosol, vapour, etc; and if

a dust, whether it is fibrous, inhalable or respirable);

− How hazardous the substances are and whether the effect of mixing or combining

substances is hazardous, even if the individual substances are not;

− Whether there is a real possibility of exposure by inhalation, ingestion or skin

absorption;

− During which processes or operations exposure is likely to occur;

− Which groups or individuals are most likely to be exposed;

− The likely pattern and duration of exposure;

− Whether there is any information on exposures measured elsewhere to which

reference can be made (such as typical or worst cases, industry-based reports, etc.).

A great deal of information about individual substances can be obtained from labels on

containers and from data sheets provided by manufacturers, suppliers or importers,

from HSE Guidance Notes and from other sources such as trade associations, technical

literature and from previous operating experience.

The initial appraisal may be supported by simple, qualitative tests using the following

equipment, which provide guidance on the nature and extent of the emission problem:

• Dust lamps, which allow very fine airborne dust particles (invisible under normal

lights) to be seen;

• Smoke tubes, which enable air currents to be studied and an easy assessment

made of the effectiveness of ventilation equipment;

• Stain detector tubes, which may be used to assess the nature and extent of

contamination.

However, the appraisal cannot be completed without first-hand evidence of exposure

levels, and so full quantitative sampling for contaminants will have to be arranged.

Basic Survey

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A basic survey should provide some detailed quantitative information about the

efficiency and effectiveness of engineering control measures and likely exposure of

workpeople to airborne contaminants. Everyone likely to be exposed should have been

identified, including those likely to be intermittently exposed, such as maintenance

workers, cleaners, transport and crane drivers, laboratory staff and utility workers. This

exercise will require a detailed knowledge of the processes, substances used, and the

workers involved.

Although static or background surveys may assist in the evaluation of exposure

patterns and provide additional information on the performance of engineering

controls, it will usually be necessary to carry out personal sampling since exposure

limits are based on personal exposure.

Detailed Survey

If the extent and pattern of exposure cannot be reliably appraised by a brief basic

survey, a more exhaustive investigation will be necessary so that accurate and reliable

information about the risks can be obtained. A detailed survey will be appropriate

when:

− The results of the basic survey show that exposure is variable;

− Large numbers of people may be at risk from excessive exposure;

− Personal sampling results are close to the appropriate exposure limits and the cost of

additional control measures cannot be justified without further evidence about the

extent of the risk.

Detailed surveys may also be called for when:

− Starting up a new process;

− A new Workplace exposure limit (WEL) is set;

− There has been a substantial change in the process, operations, or control

measures;

− Unusual, intermittent or infrequent operations or processes are to be conducted.

Routine Monitoring

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Once the basic or detailed surveys have been carried out, it may be necessary to

initiate further routine monitoring to ensure that the control measures remain effective

and compliance with the exposure limit is likely to be maintained in the future.

The frequency of routine monitoring will depend on the level and frequency of

exposure, so that for a substance with a low WEL and with a high risk of exposure, it

may be necessary to carry out routine monitoring once a week or once a month.

However, for substances considered to be well controlled, it may only be necessary to

test once a year. Frequency of monitoring will also depend on the variability of results,

reliability of the control measures, and the nature of the substance.

3.9 Sampling Strategies

The strategy selected depends largely on the purpose of the survey and, as with

surveys, there are three strategies that can be adopted:

• First-level strategy for basic surveys

• Second-level strategy for most detailed surveys

• Third-level strategy for surveys needing a high degree of sophistication

First-level Strategies

These are used wherever relatively crude quantitative information is required so that

decisions can be taken as to whether an exposure problem actually exists, prior to

conducting a detailed survey.

The best approach is to divide the exposed population into homogeneous groups in

relation to the type of work or degree of exposure. Those groups with the highest

suspected exposure and a pattern of work which relates to suspected sources of

contaminant release can be studied in detail. This will involve a great deal of personal

sampling, especially at “peak periods”, although static sampling can be useful in

verifying the existence, sources and spread of contaminant release and in assessing

the effectiveness of the control strategies. Once the high-risk group has been

adequately monitored, it will be necessary to carry out additional surveys to ensure

that other groups are not at the same, or higher, risk.

Second-level Strategies

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A second-level strategy will be appropriate for most detailed surveys and for routine

monitoring. The emphasis should be on the accurate measurement of average

exposures and relating them to the WEL reference periods (LTEL and STEL).

Whenever possible the entire period of an individual’s exposure should be covered,

either by one or preferably by several consecutive samples. Where several samples are

taken to cover the period of exposure and sampling is not continuous, care must be

taken to ensure that any periods of known high exposure are thoroughly covered. This

is best achieved by concentrating more samples into likely periods of maximum

exposure to give a more accurate time-weighted average. To enable compliance,

periods of maximum exposure should be tested against both LTELs and STELs on

different days or shifts to cover the range of expected exposure conditions.

Third-level Strategies

Occasionally a high degree of sophistication is required in a sampling programme. For

example, if all reasonably practicable measures have been taken and personal

exposures remain close to the WEL, it may be necessary to increase the accuracy of

the sampling programme to verify compliance. It may also be necessary to have a high

degree of sophistication in routine monitoring programmes where it is necessary to

identify accurately small changes in exposure, especially when the contaminant has a

very low WEL; a change of only one or two parts per million is much more significant if

the WEL is, say, 10 ppm than if it is 1,000 ppm.

In all such cases, it will be necessary to collect data according to strict protocols to

ensure that any subsequent analysis is both repeatable and amenable to detailed

analysis.

Interpretation of Results

The results of any exposure measurements should be carefully interpreted when

considering compliance with exposure limits. Factors to be considered include:

• The precision and accuracy of the sampling and analytical techniques used,

together with the sampling strategy applied. Thus, if a relatively simple first-

level strategy has been used, it would be unwise to place too much confidence in

the results, especially when they are found to be close to the exposure limit.

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However, if the results are significantly above the exposure limit, a more reliable

appraisal of the extent of the problem can usually be made.

• The qualitative evidence. For example, one figure relating to the exposure of one

worker at one process on a specific day is unlikely to be a reliable indicator of

the exposure of other workers on other days or on other processes.

In considering the results of exposure surveys:

• The results for each worker should be considered individually, but where a group

of workers do similar work the results should be considered in the context of the

group so that it may be determined if all members of the group are likely to be

adequately represented.

• Any discovery of high exposure should result in investigation, remedial action

and further surveys.

• In routine monitoring, due allowance should be made for the variability of

successive results; the greater the variability the greater should be:

− The efforts towards improved control;

− The margin that is maintained between the mean level of exposure and the exposure

limit; and

− The frequency of monitoring.

Consideration of the results of exposure measurements should always lead to answers

to five important questions:

• Is immediate action necessary to eliminate or reduce exposure?

• Is immediate action necessary to re-establish adequate control?

• Is a programme of planned improvements necessary?

• Is a more detailed survey required?

• Should routine monitoring be implemented or continued?

3.10 Summary

In this study unit we have looked at the sampling of particulate contaminants

(aerosols). We noted the current classification of health effects of aerosols which

consists of the three relevant fractions for occupational use: the inhalable, thoracic and

respirable fractions, before moving on to review the techniques and equipment

available for sampling and measuring aerosols.

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The principal components of an aerosol sampling system are the sampling head; the

particle size selector; the collecting or sensing device; the system for flow monitoring

and control; and the pump.

We considered the use of static samplers, personal samplers and direct reading

instruments, before describing the method recommended by the HSE for the collection

and gravimetric determination of aerosol samples, and the sampling and measurement

of the concentration of asbestos fibres in air.

Finally, we examined the types of monitoring strategies that can be used to make best

use of these methods. Factors influencing airborne concentration, how to devise a

sampling strategy, alternative sampling strategies and the interpretation of results are

all topics that we discussed.

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4.0 Biological monitoring

Definition

The COSHH Approved Code of Practice defines this as:

"The measurement and assessment of workplace agents or their metabolites

(substances formed when the body converts the chemical) in exposed workers.

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Measurements are made either on samples of breath, urine or blood, or any

combination of these. "

For example, the detection of significant levels of lead in blood indicates the presence

of potentially harmful levels of absorbed lead. The concentration of bromide in blood is

an indicator of methyl bromide exposure and the concentration of mandelic acid in

urine is an indicator of styrene exposure.

Biological monitoring is a technique that complements environmental monitoring as a

method of measuring and evaluating the risk to health of exposure to chemical agents.

Its purpose is to assess the extent of exposure, uptake and metabolism of chemicals in

the workplace. It involves the analysis of biological samples to provide an index of

exposure, thereby giving an indication of the possible risks to health.

Terminology

It is possible to distinguish three different types of procedure:

• Biological monitoring: this involves the measurement and assessment of

workplace agents (or metabolites) in tissues, secretions, excretions or expired air to

evaluate exposure and health risk compared to an appropriate standard.

For example, the detection of significant levels of lead in blood indicates the presence

of potentially harmful levels of absorbed lead.

• Biological effect monitoring: this involves the measurement and assessment of

early biological effects that are not harmful in themselves but are an indication of a

workplace agent causing detectable (and, therefore, presumably unwanted)

biochemical alterations.

For example, the detection of free erythrocyte proto-porphyrin (from the breakdown of

haemoglobin) is not an indication of biological harm but does indicate excessive

exposure to, and absorption of, inorganic lead.

• Health effects monitoring (health surveillance): this involves the periodic

physiological (e.g. audiometry [noise] or spirometry [lung function]) or clinical (e.g.

skin examination for dermatitis) examination of exposed workers in order to detect and

prevent occupationally related ill-health and disease

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For the purposes of this study unit it is useful to examine this last category of health

effects monitoring first, and to consider health screening and health surveillance

procedures before we study the various techniques and standards available for

biological monitoring.

4.1 Pre-employment Health Screening

Many employers now recognise the desirability of carrying out pre-employment health

screening on prospective employees, particularly where dust, fumes and chemicals are

likely to result in ill-health of some sort. Pre-employment screening also ensures that

employees are fully fit at the outset and able to perform the work efficiently.

HSE Guidance sets out four main objectives to pre-employment health screening:

• To ensure that the person is fit to carry out the work, or other work which he (or

she) might be expected to be transferred or promoted to, without being a hazard

to themselves or to others.

• To ensure that any chronic ill-health or disability will not be likely to result in

repeated absenteeism or early retirement.

• To ensure that appropriate advice, support or training is given in the light of the

applicant’s state of health.

• To see if there is any scope for job restructuring, flexible working hours, or

assistance under any of the government’s training initiatives (e.g. Special Aids to

Employment, Adaptations to Premises and Equipment, etc.).

The balance between protecting workers by excluding those with health problems from

jobs which might exacerbate those problems and ensuring that those with health

problems are not excluded from jobs which they could competently perform, is a

recurring problem in health screening.

HSE Recommendations

In general, the HSE recommends that routine medical examination for all workers prior

to employment involves an uneconomic use of available medical manpower and is

neither necessary nor desirable. Instead, the HSE recommends that pre-employment

health screening should be undertaken in the following circumstances:

− For new employees, or those being transferred from one type of work to another, if

it is considered that the work is hazardous to health;

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− Where the worker has to enter an environment to which he or she has not previously

been exposed, such as the presence of toxic substances or sensitising agents; working

in compressed air; deep-sea diving; work involving possible exposure to ionising

radiations; or to lead, mercury or isocyanates;

− Where there is a high risk of accidents to themselves or others, such as in transport,

e.g. drivers of heavy goods or public service vehicles, pilots of aircraft, train drivers,

and those involved with transport at sea;

− Where there is a risk of endangering others through transmission of infection;

− Where the work entails high standards of physical or mental fitness because of its

arduousness or degree of responsibility involved.

4.2 Types of Screening

The content and type of pre-employment health screening depends on the type of work

being carried out. Tests and procedures should relate to the physical and mental

demands of the work and the potential hazards it presents:

• Near vision acuity is important for dangerous machine operations.

• Far vision is essential for crane operators, train drivers, etc.

• Colour perception is essential for train drivers, airline pilots, ships’ officers and

electricians (10% of males are colour defective at the red end of the spectrum).

• Depth perception is important for crane drivers, vehicle drivers, airline pilots,

etc.

• Hearing should be tested (audiometry), particularly if there is likely to be

exposure to high noise levels resulting in noise-induced hearing loss.

• Lung function should be tested if work is likely to impair respiratory functions

(dusts, fumes, etc.).

• Conditions such as epilepsy should never automatically disbar someone from

employment, but caution should be observed if the work involves working at a

height from which the worker could fall; working around unprotected machinery

during maintenance; where there is a risk of severe burning; or where the safety

of others could be jeopardised.

Most of these tests do not have to be administered by a fully qualified medical

practitioner; most can be performed by occupational health nurses. It is also accepted

that some tests can be carried out by other than qualified personnel.

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Records of pre-employment health screening will provide a base line measurement of

an individual’s health which can be used as a comparison for any subsequent health

testing.

4.3 Health Surveillance by Routine Procedures

The purpose of routine health surveillance is to identify, at as early a stage as possible,

any variations in the health of employees which may be related to working conditions.

HSE’s Levels of Health Surveillance The HSE recognise three levels of health

surveillance:

• Where hazards are low and the likelihood of occupational disease remote

there may be no need for a system of regular health checks. Nevertheless it is

recommended that basic personal records should be kept for all employees along the

following lines:

− Basic records, which as a minimum should include the employee’s name, address,

place and date of birth, sex, National Insurance number and NHS number, together

with a historical record of jobs performed. Such jobs should be classified by occupation

and not by pay scales or position.

− Detailed records, which should be kept for all employees exposed to toxic

substances, biological hazards, harmful physical agents, or for those whose work is

subject to particular physical stress, e.g. firemen, rescue personnel and steel erectors.

They should also be kept for those whose work could affect public safety, such as

public service vehicle drivers, train drivers, pilots, etc.

These records should include, in addition to that collected for the basic records, periods

of exposure to harmful agents and details of absence due to sickness or injury,

detailing cause and duration of absence.

In cases where there is an “in-house” occupational health department or access to a

group practice, details should be kept of all attendances, treatment, and referrals

made, including dates of medical examinations, screening tests, etc.

To facilitate meaningful statistical and epidemiological studies at some later date, it

would be advantageous to record the following additional data regarding any exposure

to toxic substances and/or contaminants:

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− Details of the sampling protocol.

− Information linking a sample to an individual.

− An accurate, narrow description of the activity sampled.

− An estimate of the time-weighted average exposure of the employee.

− Details of any protective clothing worn.

− Comments on any other possible exposure (skin, absorption, ingestion, etc.).

− Relevant comments of the industrial hygienist which may assist in the implication of

particular work activities on the incidence of disease and help to identify a possible

careless worker.

• Where hazards are low but there is known to be the possibility of

occupational disease leading to easily recognisable symptoms, checks need not be

made by a doctor provided that medical advice is sought immediately a suspicious

condition is noticed. There are three types of “low-level” checks: self-checks; special

checks by a responsible person; and checks by a suitably trained person.

− Self-checks are appropriate in cases where an occupational disease produces easily

recognisable symptoms clearly related to a specific agent or process, e.g. workers in

contact with substances which provoke a dermatitic response, such as oil, pitch and

tar. Illustrated cards have been produced by EMAS to assist in making these checks.

− Checks by a responsible person are required for persons working on processes

where there is a risk of chrome ulceration.

Regular inspection of the workers’ hands and forearms must be made by a responsible

person and the results must be recorded in the prescribed register. The responsible

person is normally a nurse, first-aider or supervisor, and a poster detailing the effects

of chrome ulceration must be displayed in the workplace.

− Checks by a suitably trained person are appropriate in cases where an

occupational exposure gives rise to signs or symptoms amenable to assessment by a

suitably trained person such as a nurse or first-aider. This method could be used for

checking for dermatitis, but care needs to be exercised as the condition may not be of

occupational origin. All suspect cases should be referred to a doctor for further advice.

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• Where there appears to be a higher level of risk an assessment of the level of

surveillance needed should be made with the assistance of an occupational physician.

These “higher-level” checks may include screening tests, review of records, medical

examination and medical reviews.

− Screening tests are aimed at identifying specific biological or physiological changes

in workers known to be at risk from specific substances or agents. They may include

tests for absorption (e.g. blood lead or urinary mercury); lung function tests; or

audiometry; and they should be carried out by a nurse, occupational hygienist or

technician.

This type of test is exemplified by the statutory requirements of the Control of Lead

at Work Regulations 2002, wherein four different levels of blood lead concentration

are defined as requiring different courses of action on the part of the examiner; this

action to be modified in the light of the employee’s medical and occupational history

and the type of work performed.

− Review of records involves a regular review of the employee’s absences in

conjunction with records of attendance for medical examination and exposure levels. A

correlation could provide useful evidence of variations in health which might pass

unnoticed if no regular checks were made, as absence might not necessarily follow

immediately after exposure.

− Specific medical examination might include both initial and periodical medical

examinations by a doctor:

(i) Initial examinations may be used to establish fitness to undertake a particular

occupation, e.g. diving or working in compressed air, or to establish a base-line against

which to monitor the results of subsequent tests, e.g. workers exposed to lead,

mercury or ionising radiations.

(ii) Periodic examinations may be given when other tests indicate that medical

intervention may be necessary, or because the review of records shows it to be

desirable.

Medical Review of Employees Temporarily Removed from Specific Employment

This type of review ensures that no employee returns to the work from which he or she

was removed. In the case of statutory removal, it is illegal for the person to return

until certified by the Employment Medical Adviser or the Appointed Doctor and this has

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been recorded in the appropriate Health Register. It also ensures that the return to

work and normal health are monitored.

4.4 Biological Monitoring Techniques

Biological monitoring is a valuable assessment technique in the following

circumstances:

• Absorption is likely to be through skin and ingestion rather than inhalation

therefore air monitoring is a less useful indicator of potential uptake;

• There are valid laboratory methods available for the detection of the chemical or

its metabolites;

• There are reference values available for the interpretation of the results

obtained.

Types of Tests

Biological tests are used for the early detection of occupational disease and its

precursors and include periodic examinations of blood or urine samples to detect

excessive absorption of potentially toxic substances; analysis of gases and vapours in

exhaled breath; chest X-rays; liver function; renal function and nerve condition.

Biological monitoring takes into account routes of absorption, effects of workload, and

exposure outside the workplace. This can sometimes be a more reliable indication of

health risks than environmental measurements. The most commonly used techniques

are described briefly below.

•Blood Sampling

Occupational hazards may affect the production of red cells, white cells and platelets.

Red and white cell counts can be measured by automatic analysis techniques. Cell

sedimentation techniques can be used for the early detection of anaemias and

leukaemias.

In certain industries there are known hazards to the blood-forming organs from

haemolytic agents such as lead and benzene. In these cases pre-employment

examinations should include a full blood count and haemoglobin estimation. This

provides a base-line measurement before any exposure occurs and allows early

detection of damage to enable preventative action to be taken.

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•Urine Tests and Renal System

The kidneys play a central role in dealing with toxins in the body. Consequently urinary

concentrations of certain metals such as lead, cadmium and mercury, and also the

metabolites of certain organic compounds, can be used to assess exposure and

absorption of those substances.

Screening for occupational renal disease is of two types. The first involves

measurement of toxic substances or metabolites in body fluids (particularly urine) to

assess exposure; the second involves monitoring renal function by screening for

protein and sugar. Heavy proteinuria (high concentration of protein in urine) is a sign

of major renal failure which can be caused by prolonged exposure to industrial toxins.

•Chest X-rays

The principal use for chest radiography is for screening people in dusty occupations

where there is a risk of pneumoconiosis. Although mass radiography has been used to

advantage in identifying tuberculosis in the population at large, pneumoconioses

require X-ray films of greater definition to establish accurate classification of the stages

of the disease. It also has uses in investigating symptoms which become apparent in

the upper respiratory system. However, the use of X-ray techniques for screening for

lung cancer (either occupational or nonoccupational) is more doubtful.

•Liver Function

The liver is another organ which plays a central role in metabolic processes and is

susceptible to the effect of absorbed toxic substances, especially if they are fat soluble.

Damage can be to the liver cells themselves or to the transport mechanisms to and

from the liver. There is a considerable list of occupational hepatotoxins (toxins which

can damage the liver), including organic compounds (alcohol included), antimony,

arsenic and yellow phosphorus; and infective agents such as serum hepatitis.

Screening techniques for occupational liver disease also fall into two groups: those

related to the measurement of exposure to and absorption of hepatotoxins; and those

which monitor general liver function. Tests involve monitoring levels of specific

metabolites such as bilirubin and gamma-glutamyl transferase to assess liver function.

•Nervous System

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Toxic damage to the nervous system may affect the peripheral nervous system (motor

and sensory function) or central nervous system (brain function and impairment of

consciousness). There is a range of neurotoxins which produces peripheral neuropathy

(arsenic, lead and mercury) or behavioural changes (carbon disulphide, methylene

chloride, toluene); thus there is a need for medical and environmental control of

persons working with known neurotoxins, including regular biological monitoring. Tests

include visual testing, nerve transmission tests (electomyography, neuromuscular

transmissions) and assessment of intelligence, personal and psychological tendencies.

4.5 Interpretation of Results

The interpretation of results obtained from biological monitoring may need to take

account of the following factors:

•Individual factors such as:

−Age

−Sex

−Size

−Metabolism

−Pregnancy

−Lifestyle

−Pre-existing conditions

•Exposure factors such as:

−Timing

−Mixed exposures

−Route of exposure

•The chemical itself:

−Half-life residence in the body

−How it is metabolised

−How it is excreted

Another important issue is that of reference values for biological monitoring. We have

already studied EH40 which gives standards (workplace exposure limits) for airborne

contaminants. This document is published by the HSE and updated annually, providing

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a valid set of reference standards against which exposure to airborne contaminants can

be compared.

However, standards for biological monitoring are less well prescribed. The American

Conference of Governmental Industrial Hygienists (ACGIH) publishes an annual list of

biological exposure indices (BEI). These attempt to link biological monitoring and

environmental monitoring (threshold limit values) by expressing the level of chemical

or metabolite that is likely to be detected in a worker who has been exposed to the

TLV. Note that exposure to the BEI does not necessarily represent biological harm but

does provide a reference standard.

In addition, the HSE publishes guidance notes in their medical series. These set out

procedures for techniques such as lung function measurement and audiometry. They

also give information on medical surveillance for a range of substances, including

mercury and isocyanates which we shall now examine as a case study.

4.6 Case Studies

Case Study 1: Medical Surveillance of Workers Exposed to Isocyanates

Isocyanate splashes may cause severe chemical conjunctivitis. They are mild irritants

and skin sensitisation sometimes occurs. In high enough concentrations, isocyanates

have a primary effect on the respiratory tract, producing a dry sore throat and a

cough. This frequently leads to asthma. Sensitised workers may develop asthma at

atmospheric levels well below the WEL.

•Medical Supervision

Workers likely to be exposed to airborne isocyanates should undergo the following

tests:

−Pre-employment medical examinations

−Routine periodic examinations

−Re-examination following return to work after illness

−Instruction in the treatment of accidents

•Pre-employment Examination

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The aim is to identify individuals with respiratory disease and to establish base-lines of

fitness.

The examination should include a history-taking based on the MRC respiratory

questionnaire, together with lung function tests (spirometry), and a physical

examination of the respiratory system. Where appropriate, a chest X-ray might be

included.

People with the following conditions should not be exposed to further hazard: hay

fever, recurrent acute bronchitis, pulmonary tuberculosis, asthma, chronic bronchitis,

interstitial pulmonary fibrosis, atopic eczema, occupational chest disease, and impaired

lung function.

•Periodic Examination

It is believed that a significant proportion of subjects who become sensitised do so in

the first two months of exposure. Tests of ventilatory capacity of the lungs should be

carried out two weeks, six weeks and six months after engagement, and subsequently

at six-monthly intervals.

Any significant departure from normal should lead to suspension and reconsideration of

environmental hygiene and control facilities (as, for example, under the COSHH

Regulations).

In the absence of significant sickness, the MRC respiratory questionnaire should be

repeated annually.

• Sickness Absence

It would be prudent for a doctor to examine the worker by questioning, examination

and spirometry to find out if there has been any significant departure from previous

values, following any absence through ill-health.

4.7 Case Study 2: Medical Surveillance of Workers Exposed to Mercury

Mercury is used in the electrical industry for the manufacture of scientific instruments,

in the chemical industry, in metallurgical processes, in the clothing industry, in diving

operations, and in dentistry. Industrial absorption is mainly by inhalation of mercury

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vapour, or dust of mercury compounds. It is absorbed through the skin and ingestion

may rarely happen.

• Acute Poisoning

This is not often encountered in industry but when it occurs it affects the whole of the

digestive tract (gingivitis, stomatitis, and diarrhoea), causes kidney damage and even

respiratory problems if large concentrations of the vapour are inhaled in an enclosed

space.

• Chronic Poisoning

Onset is insidious with headache, lassitude, anorexia, dyspepsia and pallor being

usually described. Symptoms include:

− Mercurialentis: a permanent grey, granular discolouration of the lens of the eye

which does not interfere with vision. This is an early sign of mercury absorption and

may be present with or without mercury poisoning.

− Erethism: manifested by excessive shyness, self-consciousness, indecision,

irritability and resentment of criticism. Eventually there may be a loss of memory and

intellectual deterioration, leading to a personality change.

− Tremors: fine tremors of eyelids, lips, protruded tongue and outstretched fingers.

Later it interferes with voluntary movements, e.g. shaky, irregular handwriting,

becoming more and more illegible.

− Gingivitis: a history of repeated dental treatments and extractions. Rarely a blue

line on the gums may be observed.

− Nephrosis: after long or excessive exposure, mercury is localised in high

concentrations in the kidneys. Early removal from further exposure usually results in

full recovery, but deaths have occurred.

• Notification

Mercury poisoning is reportable under the Reporting of Injuries, Diseases and

Dangerous Occurrences Regulations 1995 and is a prescribed disease under the

Social Security (Industrial Injuries) (Prescribed Diseases) Regulations 1985

as variously amended.

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• Screening

New employees with poor standards of dental hygiene should be encouraged to have

dental treatment before starting work. A history of previous kidney disease or disease

of the peripheral nervous system are indications that a person should not be exposed

to mercury or its compounds. Special attention should be paid to nail-biters.

• Surveillance

For currently exposed workers, the following regime is desirable.

−Monthly estimation of urinary mercury

−Monthly test for proteinurea

−Monitoring of workplace atmosphere at regular intervals

4.8 Advantages and disadvantages of biological monitoring

The advantages and disadvantages of both air and biological monitoring have been

discussed (Droz et al., 1991). Briefly, air monitoring can introduce a large bias into the

estimation of the true target tissue dose. This is because variation in factors such as

physical workload, skin absorption, the partition coefficient of the chemical of interest,

and individual anatomical, physiological and biochemical differences which determine

chemical pharmacokinetics, ensure that air monitoring is, more often than not, a crude

indicator of the biologically active dose.

Wide variations in exposure concentrations due to changes in work procedures and

practices occurring over periods of months to years can be taken into account during

air monitoring. Whereas, short-term within-day and day to day fluctuations are best

captured by BM.

Biological monitoring uses measurements of chemicals or their metabolites, known as

‘biomarkers’, in biological media such as blood, urine, breath, saliva, and sometimes

tissues, as an index of absorption of chemical. The strength of BM lies in the potential

to define total absorbed dose from all routes of exposure i.e. inhalation, dermal

penetration and ingestion. The results are affected by biological variability and in the

past the biological basis of this variability has been characterised as problematic. More

recently however, the introduction of biologically based mathematical modelling

techniques, has demonstrated the potential to enhance the value of BM by

quantitatively accounting for and explaining the underlying biological variability.

Advantages and Disadvantages of Biological Monitoring

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The advantages of biological monitoring include the following:

• It can help to demonstrate whether personal protective equipment (e.g. gloves,

masks) and engineering controls (e.g. extraction systems) are effective in

controlling exposure.

• It measures individual exposure to a chemical by all routes of entry-

• It identifies what has been absorbed by the body (unlike airborne monitoring).

• It shows how effective improvements in control measures have been in reducing

exposure.

• It gives reassurance to workers that their individual exposure is being

monitored.

The disadvantages of biological monitoring include the following:

• Sampling may require blood to be taken which would require a physician or

nurse.

• Measurements relate to individuals, so confidentiality and data protection issues

need to be addressed. -

• As with all standards BMGVs aim to protect the majority of the exposed

population. An individual may suffer adverse changes at concentrations below

the published BMGV.

There are relatively few accepted BMGVs.

The HSE have derived Biological Monitoring Guidance Values (BMGVs) for interpreting

biological monitoring measurements. There are, however, very few listed in EH40

although several more are under consideration.

4.9 Summary

In this study unit we have looked at the general philosophy and application of the

various types of hygiene standards relevant to chemical substances. The methodology

for developing a standard and the criteria for establishing a standard were obvious

starting points.

We explained the concept of occupational exposure limits and their use, before moving

on to consider biological monitoring. Here we looked at three types of procedure and

technique: biological monitoring (measurement of chemicals or metabolites); biological

effect monitoring (measurement of early biological effects); and health effects

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monitoring (health surveillance). When interpreting the results of such monitoring,

individual factors, exposure factors and the chemical itself must be taken into account.

5.0 Monitoring and maintenance of control measures

Control of Substances Hazardous to Health Regulations 2002 - Regulation 9

(Maintenance of Control Measures), paragraph (1) has been amended to read:-

Every employer who provides any control measure to meet the requirements of

regulation 7 shall ensure that—

a) in the case of plant and equipment, including engineering controls and personal

protective equipment, it is maintained in an efficient state, in efficient working order, in

good repair and in a clean condition; and

b) in the case of the provision of systems of work and supervision and of any other

measure, it is reviewed at suitable intervals and revised if necessary.

Carrying on from unit B4 it must be noted that - All LEV systems need to be subject to

commissioning to ensure that they are capable of meeting their design specifications.

Under the Control of Substances Hazardous to Health Regulations 2002 all control

measures need to be maintained in an efficient state, in efficient working order and in

good repair.

Maintenance procedures need to include information on: how frequently maintenance

needs to be carried out for each component of the system, what maintenance tasks are

necessary and how defects are to be detected and remedied, and who is to be

responsible for the maintenance.

The maintenance procedures should cover the full range of maintenance activities from

simple visual checks to detect obvious defects, to major overhauls for preventative and

remedial purposes. In addition to effective preventative maintenance, the Control of

Substances Hazardous to Health Regulations 2002 and other regulations contain

statutory requirements for the undertaking of formal examination and testing of LEV

systems.

It may be prudent for these examinations to be carried out by persons not normally

responsible for the system maintenance in order that an independent second opinion

can be obtained. For effective examination and testing comprehensive information on

the system and its design specification needs to be provided. The Control of

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Substances Hazardous to Health Regulations 2002 requires the thorough examination

and testing of LEV systems at least once every 14 months.

However, more frequent thorough examination and testing is required in the following

processes:

Process Frequency

(Minimum)

Where blasting is carried out in or incidental to the cleaning

of metal castings in connection with their manufacture 1 month

Jute cloth manufacture 1 month

Processes, other than wet processes, in which metal articles

(other than gold, platinum or iridium) are ground, abraded

or polished using mechanical power, in any room for more

than 12 hours per week

6 months

Processes giving off dust or fume in which non-ferrous metal

castings are produced 6 months

When deciding the frequency of thorough examination and testing: treat parts of

equipment such as the machine casing and guards as LEV if they are directly ventilated

and if one of their functions is to control emissions, regard make-up air systems that

replace exhausted air as LEV if they are an integral part of an exhaust system, treat

flues from furnaces, ovens etc as LEV where the draught created by the flue is

necessary to control the release of hazardous substances, and only treat vacuum

cleaners as LEV if they are connected to a portable machine or tool.

The Control of Substances Hazardous to Health Regulations 2002 specify that records

be kept of the results of the tests including details of any repairs carried out as a result

of the examination and tests, these records being kept for a minimum of 5 years.

Maintenance and thorough examination and testing need to be planned together in 3

stages:

1- Initial appraisal.

2- Regular maintenance including frequent visual inspections, perhaps daily, weekly or

monthly.

3- Thorough examination and testing.

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The information required while carrying out the above includes:

Component Detail

Enclosures and Hoods

Maximum number in use at one time. Location and

position. Static pressure behind each hood or

extraction point. Face velocity.

Ducting Dimensions. Transport velocity. Volume flow rate.

Filter and Collector Specifications. Volume flow rate. Static pressure at

inlet, outlet and across the filter.

Fan or Air Mover Specifications. Volume flow rate. Static pressure at

inlet. Direction of rotation of fan.

Systems which return

exhaust air to the

workplace

Filter efficiency. Concentration of contaminant in

the returned air.

Regular inspection and checking of LEV is not the same as the thorough examination

and testing.

The aim of inspection and testing being to identify potential problems so that they can

be rectified before performance deteriorates. Weekly visual checks should be carried

out to identify any obvious defects, although these may need to be more frequent

where certain hazardous substances are involved.

The inspection and checking should cover: ensuring that the LEV is always running

when hazardous substances are either being emitted or are likely to be emitted,

observing the condition of the suction inlet such as the hood or booth to see if it has

moved or been damaged, observing the condition of any visible ductwork etc,

observing any evidence of control failure such as unusual dust deposits or stronger

odours than usual, observing any local instrument fitted to the LEV to indicate its

performance and undertaking any minor servicing such as the emptying of filter bins

etc. A formal system for dealing with verbal reports from employees should be in place

in order that details can be recorded into maintenance reports.

Thorough examination and resting of a LEV system represents a regular audit of the

performance of the system and should reveal whether or not the plant is performing

correctly and effectively, although it may not reveal the precise cause of the

unsatisfactory performance that has been identified. The thorough examination and

testing will comprise of: a visual check, a measurement of plant performance and an

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assessment of control, and an assessment of the performance of the air cleaner or

filter where the air is re-circulated.

The most common categories of instruments and techniques used for the examination

and testing of LEV systems are: direct measurement of emissions through air

monitoring (in the breathing zone close to the source), measurement of plant

performance (static pressure and air velocity) and visualisation techniques (smoke

generators and dust lamps).

The type of information kept in the record for a thorough examination and test should

include: the conditions of the LEV system at the time, the intended performance of the

LEV system and the way it should be used, methods used to judge the performance of

the LEV system and whether it achieves the intended performance, results of routine

ventilation measurements, results of tests of the concentration of airborne material and

request for remedial action with details of repair or modifications needed. The record

should be kept for 5 years with a copy being available at the workplace in which the

LEV is located.

5.1 Capture velocity

The capture velocity is the air-flow rate necessary to draw the contaminant into the

inlet of the LEV system. The HSE recommends different capture velocities for different

situations and different contaminants. These range from 0.26 m s-1 for the solvent

vapours released by paint drying, up to 10 m s- for dust released from fast machines

with small collection inlets. However, the capture velocity required depends greatly on

the design of the hood or enclosure forming the inlet aperture.

Duct velocity

The duct velocity required to keep the contaminant flowing in the duct also depends on

the nature of the contaminant: dusts and particles drop out of the flow unless it is

sufficiently fast. Gases, vapours, smoke and fume require only low velocities while

dusts require at least 15 m s-, with heavy particles requiring at least 25 m s. For a

given capture velocity and inlet size, the smaller the duct diameter, the greater the

duct velocity but the resistance to the flow is also increased. The small duct diameters

(75 to150 mm) usually used in education require relatively powerful fans which may

increase the noise generated.

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Flexible ducting is frequently used in LEV systems in educational establishments. How-

ever, it has a relatively high flow resistance because of its corrugations, so long lengths

and bends should be avoided.

Other regulations

The Control of Asbestos at Work Regulations also call for local exhaust ventilation

under certain circumstances. The Control of Lead at Work Regulations apply to certain

processes, such as casting lead and its alloys. Soldering with small irons and

temperature-controlled systems does not give rise to significant levels of lead vapour

(which would be covered by these Regulations) but does produce fumes from fluxes

which is a matter for the COSHH Regulations.

Quantitative and semi-quantitative tests

For some substances, there are instruments which draw in samples of the inhaled air,

automatically measure the concentration of the particular substance and show the

result as figures on some kind of display.

Such figures may look very impressive but the user must be aware of the limitations of

the instrument; some results could be inaccurate because of the presence of another

contaminant, others could need a calibration correction, etc. These tests are

particularly useful for measuring the concentration of fine dusts which are invisible

under normal lighting.

In other tests, samples of the air are drawn through a tube containing chemicals and

an indicator changes colour according to the concentration of the substance being

tested for. These systems are often referred to as ‘semi-quantitative’ because the

precise extent of the colour change in the tube may be difficult to determine, the

volume of air drawn in may be uncertain, etc. Nevertheless, these uncertainties are

often no greater than those introduced by the choice of sampling position or test

conditions.

Such measurements may be as accurate as those from a much more sophisticated

instrument. They are useful, for example, for monitoring levels of solvent vapours.

Suitable detection kits are available from Anachem Ltd (for Gastec products) or

Draeger Ltd.

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Qualitative tests for dust

In the case of dust or any other particulate contaminant, the Tyndall Dust Lamp can

provide a qualitative indication of whether or not a problem exists. When a strong

beam of light is passed through a cloud of dust particles, the light is scattered by the

particles, making them visible. The effect is particularly strong for the fine particles

which make up ‘respirable dust’ that is responsible for certain health conditions.

The lamp does not have to be a special one if it produces a confined beam of strong

light. A stage spotlight or slide projector may be suitable. If photography is being used

to record the result, the beam should be ‘photographically white’ but, for visual

observation, the colour is not important. The required arrangement is illustrated above.

In use, the lamp is usually set up to produce a beam in front of the face of the user of

a machine or process. The observer stands facing the lamp but with a screen to protect

the eyes from the direct beam. It is often useful to reduce other lighting in the area

and then the dust cloud, if there is one, will become visible. The presence of a visible

cloud means that the dust-control system is not adequate to protect the user.

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Attempts to use this technique have shown that, in practice in many workshops, space

is so tight that a dust lamp is very difficult to use. Machines are often so close to walls

that it is impossible for either the lamp or the observer to be positioned to achieve the

required angles. A further problem arises because many educational workshops have

good natural lighting and no blinds. The most practicable way of doing the test is to

wait until the winter and use the lamp at dusk, which may present problems for staff.

5.2 Measurements of air flow

Irrespective of the contaminant which the system is designed to remove, the

performance of LEV could be monitored by measurements of the flow of air through the

system. It may be necessary to confirm the direction of flow, since the measuring

instruments are often insensitive to it, by the use of a smoke pencil which will also help

to indicate the presence of eddies or other turbulence.

Measurements of flow rate

There are two types of anemometer which are used for measuring air-flow rates.

A vane anemometer has a very lightweight vane (which looks

like a fan). This is held perpendicular to the flow so that the air

pushes the vane around at a rate dependent on the speed of

the air. The instrument is calibrated to give a direct reading of

the flow rate. In order to give an accurate reading, the flowing

air column must be of a diameter larger than that of the vane.

If measurements are made of the air-flow rate into a large

aperture hood, several readings must be taken at different

places so that an average rate can be calculated. The calibration

of the anemometer should also be checked periodically.

A probe (thermal) anemometer has a small probe, a few

millimetres across, containing an electrical component (perhaps

a wire filament) which is heated by an electric current flowing

through it. If this is placed in an air current, the component is

cooled by the air flow, its electrical resistance changes and the

voltage across the component then depends on the flow rate.

Again, the instrument is calibrated to give a direct reading of

the flow rate. In this case, an accurate reading can be obtained

for the air flowing through a narrow slot, provided the

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calibration is checked periodically. Again, if this instrument is

used to measure the flow rate into a large aperture hood, an

average rate should be worked out by taking several readings

at different places.

Hoods vary enormously from the very small aperture of a pipe attached to an

individual soldering iron to the mesh wall of a spray booth. Vane anemometers will only

give accurate, mean flow rates for large apertures (e.g., fume cupboards in a science

department).

Their use is not appropriate for very small apertures, i.e., those for which the inlet area

of the hood is much smaller than the area of the vane. An instrument with a small

probe is therefore required for many flow-rate measurements in design & technology.

It is not very helpful to measure flow rates unless the whole system is set up for use

because the flow rate will change. If a small probe is introduced into a duct, it may be

possible to make a measurement without changing the duct velocity. In many systems,

it is only practicable to measure the flow rate with an anemometer at either inlet hoods

or outlets. Since outlets are usually at or above roof level, measurements at these

points require safe access to heights. As a regular procedure, therefore, flow rates can

often only be measured at the inlets.

A relatively low flow rate (e.g., 2 m s-1) is sufficient to capture most contaminants and

sweep them into the duct but a higher flow rate (e.g., 20 m s-1) is required to keep a

solid contaminant suspended in the air flowing along a duct.

This is normally achieved by having a hood with a larger area of cross section than that

of the duct, with a tapered section to join the hood to the duct. To achieve a ten-fold

increase in linear velocity, the area of cross section of the duct must be one tenth of

that of the aperture of the inlet hood. If the flow rate is measured at the inlet, the flow

rate in the duct can be estimated by multiplying the measurement by the ratio of the

areas.

In practice, once the system has been installed, it should be given a commissioning

test by the installer, using a dust monitor or other contaminant test, as appropriate, to

show its effectiveness. The values of the input flow rates should be recorded. In any

future tests, a 10% change from these initial readings would indicate a problem in the

system. If the system fails the commissioning test or initial appraisal, its design must

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be re-evaluated and the necessary modifications made so that the system is proved to

be effective.

Measurements of pressure in ducts

The measurement of the static pressure in ducts can locate a restriction to air flow and

indicate the nature of any problem. The instrument used to measure the pressure

should be calibrated in pascal and a range of 0-5000 Pa may be required, with the

ability to discriminate changes of 5 Pa between 500 Pa and 5000 Pa. Test points could

be provided close to each inlet, either side of any dust collector or filter unit and before

and after the fan. If the pressures are recorded when the system is performing

satisfactorily, changes will indicate blockages or faulty fans.

Problems with this method arise when the ducting etc is in an inaccessible location.

However, test points can be permanently attached by the use of inexpensive plastic

tubing to provide connections at more accessible positions for regular monitoring.

Pressure measurements can also be used to determine the air velocity in ducts if a

pitot-static tube can be introduced without disturbing the air flow significantly. This,

however, requires a sensitive (and expensive) manometer and some specialist skill in

operating it.

5.3 Manufacturers or suppliers of LEV

Dust- and fume-control equipment is available through different routes: general

suppliers of equipment or specialist manufacturers and installers. While general

suppliers may offer components at keen prices, they are unlikely to be able to design

or recommend the best system for a customer’s needs and provide a proper

commissioning test after installation.

Specialist companies offering a full service should be asked to provide:

• a commissioning test with air sampling to demonstrate the effectiveness of their

equipment, whether designed to cope with fumes or dust;

• a manual covering the equipment’s maintenance and use, and/or

• training in the equipment’s use.

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As with any other contractor, the client should attempt to determine the competence of

the company to be contracted to do the work. The HSE has provided a list of suggested

questions but the most significant ones to ask here are given below.

• What experience does the company have in the type of work required?

• What qualifications and skills does its employees have?

• Does the company have any independent assessment of its competence?

• Is the company a member of a relevant trade or professional body?

Recommended test methods

It is not sufficient just to test a dust-collection system with measurements of pressure

or air-flow rate; measurements of dust concentration must be made under standard

conditions of use. Once the system has been shown to be controlling the dust

adequately, airflow measurements can be used to confirm that it is still doing so,

provided the materials used, and the process, have not changed substantially.

It is important to keep copies of all test reports, whoever has done the tests.

Test methods for systems controlling wood dust

First-choice procedure: Dust-level measurements

An aerosol dust monitor, used by a trained operator to measure the dust concentration

in the breathing zone of the operator, will give the most confidence that the dust levels

are well below the WEL. If this is done at 14 month intervals (as required by the

COSHH Regulations) while the dustiest process is being carried out, the employer can

demonstrate that the requirement is being met to ‘keep the dust levels as far below

the WEL as is reasonably practicable’. This test method will ensure that the control is

maintained even if the process or materials change. Records of the air-flow rates at

inlets (and in the ducts by calculation), and static pressures before and after filters, will

be useful in future tests to identify faults or blockages.

However, the instruments used to perform these tests are expensive to buy and, when

necessary, to maintain. Trained personnel are needed and the tests take a long time.

Consequently, measuring dust levels will be expensive. It is impracticable for most

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individual establishments to reduce costs by doing dust-level tests themselves but

some businesses do operate such a system.

Second-choice procedure: Flow-rate / pressure measurements

If a dust-control system is given a thorough test as above once, either on installation

or as soon as it has been brought into full use or on introduction of a proper testing

regime, the records of the air-flow rates and/or static pressures can be used in future

tests. As long as the processes and materials used remain the same, future tests of the

flow rates and/or pressures will indicate whether or not the system is still behaving in

the same way. Such tests could be carried out by a competent person, with

appropriate equipment.

Test methods for systems dealing with fume extraction

As with dust control, ideally, the system should be shown to deal with the fumes by

testing the concentration of the contaminant in the air being breathed. In many

situations, the major contaminant is carbon dioxide which has a relatively high WEL

value, i.e., the concentration can be allowed to rise considerably before it becomes

hazardous. If all the carbon dioxide is not being collected by the fume-extraction

system, the general ventilation of the workplace will dilute it and help to sweep it

away. Measuring the carbon dioxide level is therefore not a very effective test of the

collection system.

The other contaminants are released either at low concentrations (e.g., carbon

monoxide) or in short bursts (from the degassing tablets used when melting

aluminium). Consequently, tests of the effectiveness of fume-control systems have

usually been done using special tracer gases such as sulphur hexafluoride (which can

be detected at low concentrations).

Nevertheless, research being done by the HSE using this technique should enable a

simpler testing method to be devised. This has not yet been published so most testing

companies adopt a rule-of-thumb, minimum duct flow rate of 10 m s-1. The air-flow

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rate at an inlet will depend on the ratio of the cross-sectional areas of the duct and

inlet. It is the inlet air-flow rate that is easiest to measure.

Temporary procedure

The only practical way of testing a fume-extraction system is to compare the flow rates

and/or pressures at the various inlets with those recorded during a commissioning test

after installation. However, it is important to ensure that the settings of controls (fan

speed, if a regulator is fitted, and dampers to control air flow) are the same during

there test as they were during the commissioning test. (It is unusual for wind speed to

affect the results unless the workshop is in a very exposed location. In such cases, it is

important to check that the discharge is via a vertical stack.)

Maintenance requirements

The requirement to control dust concentrations in the air applies just as much to

maintaining the LEV system as to the normal uses of the dust-producing equipment.

Emptying dust containers

Problems

Some systems collect dust in ‘drawers’ at the bottom of the extraction unit. When the

drawer is opened for emptying, the dust can easily be disturbed by air currents and

carried up towards the breathing zone of the person emptying the dust.

In other systems, the dust is collected in plastic bags so that, when they are

sufficiently full, the top of the bag can be released and sealed with minimum risk of

disturbance to the collected dust. Clearly, this is much safer than handling a drawer

containing dust.

Further potential problems occur once the bag or drawer has been removed. The

practice of emptying the drawer into a dust bin or into a wheel-barrow which is then

taken outside the workplace and tipped into an open skip is unacceptable for three

reasons.

1. There is a high risk of releasing the dust back into the workplace.

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2. There is a high risk of the person carrying out the task inhaling the dust.

3. Dust blowing out of the open skip presents a hazard to passers-by or to neighbours.

Where plastic bags are used, they should be heavy-duty to minimise the risk of tearing

and releasing dust. However, these are relatively expensive and, to save money, some

businesses have been known to take full bags outside and empty them into skips or

other containers so that the bags can be reused. This is unacceptable for reasons 2 and

3 above.

Some establishments have offered collected dust to those keeping small mammals, as

litter for their cages. Although some dust-collection systems claim to separate fine dust

from shavings and coarse dust, such separation is not perfect. Animal litter should be

‘dust free’ and not contaminated with chemicals used to preserve wood or potentially

harmful hardwood particles. It is impracticable for workshops to meet these conditions

and litter for keeping animals should be obtained from a supplier who can guarantee

they are met.

5.4 Solutions

It is therefore important that all of the following control measures are adopted when

emptying dust containers. Because of the risk of dust being released into the air,

anyone not involved in the emptying process should be kept well away from the area.

1. The person handling loose, collected dust is provided with a toxic dust mask (also

referred to as a ‘disposable respirator’). Type FFP2 is suitable for ordinary sawdust but

type FFP3 should be used if there is a lot of fine dust present, e.g., from MDF.

2. The procedure used minimises the dispersion of the dust, e.g., the drawer can be

placed in a large plastic bag before inversion to empty it.

3. The wood dust is sealed in plastic bags for waste collection.

Filter maintenance

Dust filters

Most dust-collection systems incorporate filters which should be shaken periodically to

dislodge dust and maintain filter efficiency. The largest systems have motor-driven

shakers that operate automatically. More typical systems used in business have

manually-operated shakers; the supplier normally specifies the intervals at which these

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should be used. These specifications often give a time in hours, e.g., ‘every 8 hours’ or

‘once a week’ which would probably be appropriate where the extraction is running

throughout the working day.

If a particular system has been in heavy use, it might be necessary to shake the filter

at the end of the day to prevent its performance deteriorating, otherwise once a month

could be sufficient.

Disposable paper filters are used in some systems to collect the finest dust. These

should be changed at the frequency suggested by the supplier. If this is not done, the

performance of the system can fall off significantly such that replacement of the filter

produces an obvious improvement in the air-flow rate. Similarly, washable, fabric

filters or bags should be washed at such intervals that the clean one produces a change

in air flow that is only just detectable without instruments.

Fume filters

Systems to control fumes usually exhaust to the open air without a filter. An exception

is the type used to deal with fumes from soldering with rosin-based fluxes in

electronics work. These use filters which should be changed at intervals as specified by

the supplier.

Tips for good housekeeping

It is important to realise that dust can deposit within inlet hoods, inside band or

circular saws or on bends in ducting (especially flexible ones) and seriously affect the

performance of the system. If the problem points can be identified, regular brushing

can eliminate the problem, if there is an access port. (If this releases dust into the

workshop, the person doing the brushing must use a vacuum cleaner suitable for the

collection of dusts or wear a toxic dust mask.)

Where systems incorporate dampers to maximise the air flow at the inlet in use, it is

essential that the user appreciates how the controls must be used.

This implies a need for training in the use of the system, together with a guidance

sheet or users’ manual and reminder notices near machines.

Cleaning

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Even with an excellent dust-collection system at each machine, there will still be dust

produced, for example, by hand sanding or dust which escapes at each machine.

Cleaning in a workshop is not a trivial activity, because wood dust has been given a

WEL and the employer is required to keep its concentration as low as reasonably

practicable to protect the health of all employees.

Some establishments do not include wood-preparation and storage areas in cleaning

schedules. If regular cleaning staff do not service these areas, other arrangements

must be made to ensure that the health of the person doing the cleaning is

safeguarded.

Traditional cleaning methods using brushes and mops are unlikely to be suitable, even

if cleaning staff are required to wear appropriate dust masks. The only effective

method is to use a vacuum cleaner which is designed to collect wood dust; (a

‘general-purpose’ vacuum cleaner is unlikely to be suitable because it will not trap the

fine dust involved). Since the waste is likely to include small off cuts as well as dust, a

variety of nozzles will be needed. Cleaning staff will need training and supervision to

ensure that the specialist vacuum cleaner is used where it is required and that it is

emptied according to the manufacturer’s instructions, using similar care to that

described above.