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8/8/2019 Sampling of Dust DUST in MINES -5
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SAMPLING OF DUST
1
Suitable preventive and suppressive
measures for allaying dust in amine
it is essential to have a suitable device to
estimate orsample air-borne dust likelyto be breathed by miners.
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The following facts must be considered inthe choice of a sampling instrument:
Knowledge of the necessary dustconcentration in the dangerous size range.
Mass concentration of the respirablefraction of dust is widely acceptedrelevant standard.
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But MAC is not an agreed international
standard
as conclusive correlation betweendifferent dust parameters and incidence of
pneumoconiosis uniformly applicable to all
types of dusts is not established yet,
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particle concentration with particle-size-frequency distribution and surface areaconcentration still continue to be assessed
Also the sample should be able to give anassessment of the dangerous component of dustas regards composition.
This necessitates adequate volume of sample forchemical and mineralogical analysis.
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The duration of the sample should be suchas to give a true picture of dustiness ofany mining operation or place or work with
optimum number of samples to be taken. The sample should be continuous over at
least a working shift.
The sample should be representative ofthe dust cloud in the breathing zone of theworker.
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SAMPLING INSTRUMENTS
The sampling instrument should be easily
portable and
robust for rugged underground conditions and
require minimum maintenance
need minimum operational skill
self-contained power supply
render the measurement of dust in as little time,as possible without involving too much
accessory work.
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DURATION AND INTERVAL OF
SAMPLING Dust generation in any mining operation
varies a great deal with time
the variation is greatly irregular due toirregularities in normal cycles of operation.
the time and duration of sampling should
be very carefully chosen to give a fairly
accurate estimation of the dustiness of any
particular operation over a long period.
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A true estimate can however be obtained by acontinuous sampling device working over thewhole shift or even over several shifts.
From this a true shift mean or, in general terms,a true period mean can be obtained.
A mean close to the true period mean canbe obtained by taking a large number of short-
duration samples over the period, the deviation approaching zero as the number
approaches infinity but it is impracticable.
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Snap samples or samples taken over a short
duration are liable to deviate a great deal from
the true average at a source dust production
owing to large variations in the rate of dustproduction at the source and
to get a close approximation to the true mean,
a very large number of samples will be
necessary .
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Studies suggest that a sampling period of
ten minutes will reasonably represent the
true mean with the result falling within
20% or the average in two out of three
cases.
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POSITION OF SAMPLING
intermittent dust production in any mining
operation and
the irregular nature of air movementgoverning the dust distribution at the face,
it is difficult to select the correct part of the
face where a true representative sample
can be obtained
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Samples taken at different sections of theworking face near the source of production of
dust show wide variation .The variations gets gradually attenuated as thesampling points are receded farther along thereturn airway,
This is due to the longitudinal and lateral mixingof the dust in the air-stream,
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If the Reynolds number characterizing the air-flow is well above the critical number which isgenerally the case with any ventilation system,complete mixing takes place at a distance, of
five to ten roadway diameters It is possible to get a more average dust sample
beyond the mixing zone than near the source,
Any temporary variation in dust production at the
source tends to be attenuated with increasingdistance from the source,
Due regard of course is to be paid to anyadmixing with dust from other sources.
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With velocities prevalent in mine airways,gravitational settling of dust is slow,particularly so for the pathogenic size of
< 5 m and it is possible to get arepresentative sample at a distance of
6 to15 m away from the face.
uniformity or distribution in an air-streamis dependent on the uniformity of flow inthe airway.
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for getting a representative sample, it isnecessary to collect the sample in a straightposition on of the air way at least ten diameters
away from any bend or major obstruction.
Another factor which should be borne in mindwhen collecting a sample of dust from air is that
the sampling velocity should be as far aspracticable, equal and equidirectional to thevelocity of air from which the sample is beingcollected.
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this is essential for minimizing the loss ofparticles by inertial separation before entryinto the sampler.
The best sample having direct relevance tohealth hazard is one collected at the breathingpoint of the worker.
This has led to the development personal dustsamplers which collect a sample of therespirable dust fraction over a sufficiently longperiod, usually a shift
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METHODS OF SAMPLING
Based on the principle of operation,the present-
day air-borne dust sampling methods can be
classed as follows:
(a) filtration,
(b) sedimentation.
(c) inertial precipitation,
(d) thermal precipitation, (e) electrical precipitation ,and
(f) optical methods based on light scattering,
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Filtration has been the earliest method ofsampling.
Sugar tubes - had only 87 % filtrationefficiency by weight for silica dust and afair amount of fine dust of the respirable
size range escaped collection
(Later replaced by konimeters.)
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Other soluble filtering media like potassium
nitrate, salicylic acid etc. were tried to improve
filtration efficiency.
Non soluble filters tried were of cotton ,wool,flannel etc.,
synthetic membrane filters having pore size
down to. 0.01m have been developed. Paper thimbles are claimed to have 98 %
filtration efficiency.
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Attempts have been made to obtain the massconcentration in the respirable size range from dustsamples collected on soluble filters, by dissolving thefilters and separating the < 5m particles by
sedimentation. This does not give the picture of the mass concentration
of the respirable fraction in the air-borne dust since theaggregates present in the air-borne dust cloud which arenormally treated as single particles of the aggregate size
in the respiratory system, Dust particles break up in the process of solution and
separation and so an overestimation of the massconcentration of the respirable fraction is obtained.
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Particle concentration
Particle concentration and size distribution havebeen determined by spreading a certain amountof dust collected on the filter on a glass slide in a
solution of Canada balsam in xylol (the dust getsfixed in Canada balsam when the xylolevaporates ) and counting it under amicroscope.
But proper dispersal of dust on a microscope slide by
this method is difficult and errors creep in due to bothsmall particles flocculating and aggregates breaking up.
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Other Instruments based on the principle
of filtration include the Soxhlet and Gothe
filters which use paper thimbles,
Siter filter using a synthetic membrane
filter and the microsorban filter using a
low-resistance soluble filter.
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Filtration has become important in recent timesdue the adoption on of the mass concentrationof the respirable size fraction as the relevantdust parameter signifying health hazard.
The sampling instruments commonly known asgravimetric dust samplers
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Gravimetric dust samplers
use an elutriator of gravitational settling orcyclone type which separates out dust particlesabove respirable size range
the respirable size fraction is collected on amembrane or glass fibre-paper filter in a suitableholder.
The filter can be weighed before and after
sampling to get the mass of dust collected. Today matched-weight filters are available which
obviate double weighing.
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These are of two main types:
(a) those using a gravitational-settling type
of elutriator and
(b) those using cyclones for elutriation.
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Respirable dust monitorRDM-201
This is a gravimetric sampler
developed in the U.S.A. which gives anautomatic digital readout of the mass
concentration of the respirable dust by a
beta-ray absorption technique.
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The instrument comprises a cyclone elutriator whichseparates the coarse fraction and a filter which collectsthe respirable fraction.
The mass, of dust deposited on the filter perunit area is obtained from the absorption of
radiations from a carbon-14 source as measured by a
Geiger counter.
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Inertial Precipitation
Dust sampling instruments using inertial precipitation arebased on three principles:
impaction.
impingement and
centrifuging.
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impaction.
Certain volume of mine air is made to
impinge on a glass slide coated with a
suitable dust collecting adhesive.
The dust which gets deposited on the slide
is counted under a microscope which, in
most makes, forms an integral part of the
instrument
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A measured quantity of the dust-laden airfrom the nozzle impinges on the bottom ofthe flask and the dust is collected in the
collecting fluid. A portion of the dust-laden fluid is filledinto a
counting cell usually 1 mm deep which is
then covered with a cover slip. The cell is allowed to stand for about half
an hour to allow the dust particles to settleto the bottom or the cell and counted
under a microscope.32
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centrifuge is based on the principle of
centrifuging. This instrument collects a
size-graded spectrum of dust, but its high
speed of rotation confines it to the
laboratory only
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THERMAL PRECIPAITATION
This method utilizes the principle that when abody surrounded by dusty air is heated, a dust-free zone is produced around the hot body,
the extent of the dust-free zone depending onthe temperature gradient between the hot bodyand the surrounding air.
If such a zone is intercepted by two glass cover
slips and a current of dusty air allowed to enterthe space between them the dust in the air getsdeposited on the cover slips where it remainsattached by molecular attraction
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The dust particles collected on the cover slips is
are counted under the microscope
The thermal precipitator has a high collection
efficiency. The low velocity of sampling does notbreak up aggregates and a fairly representative
sample over a reasonably long duration is
obtained. .
Microscopic counting of the dust
particles is tedious and subject to personal error
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Automatic electronic devices have been
developed for counting and sizing of dust
samples collected by the thermal precipitator.
A diffraction size-frequency analyzer developed
has been claimed to give an accurate measure
of the size distribution of the dust sample as well
as the respirable surface area of the
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Electrical Precipitation
The electrical precipitator essentially consists ofa charging wire maintained at a high negativepotential of about 12000 volts and surrounded
by an earthed concentric cylinder Dust-laden air is drawn through the cylinder bya fan at a constant rate.
The dust particles when passing through the
instrument, get charged and are drawn to andprecipitated on the inner surface of the earthedcylinder..
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This instrument, like the filtration devices, has alarge sampling capacity and is suitable forcollecting large quantities of dust for chemical
analysis. The mass concentration can also be determinedby noting the difference in weight of the cylinderbefore and after collection of dust.
The instrument has a high collecting efficiency,but the high voltage used in it makes itunsuitable for use in coal .
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Gravimetric dust samplers utilizing
electrical precipitators have
been developed where the coarse fraction
of the air-borne dust is first removed in a
cyclone separator and the respirable
fraction is collected in the electrical
precipitator for estimation of massconcentration by differential weighing
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Optical Method
This method utilizes the property of scattering oflight by suspension of fine particles
For particles large enough compared to thewave length of light i.e., particles above 1m indiameter
the intensity of scattered light is roughly given bythe relation
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where
Is - intensity of scattered light,
1o intensity of incident light,
N number of particles per unit
volume,
D diameter of particles The equation shows that the intensity of
scattered light is proportional to the surface area
of the particles, a fact well borne out in practice.41
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OPTICAL
Holographic System
A different optical system which has been
developed for counting and size analysis of a
suspended cloud of aerosols consists of apulsed ruby laser illuminating the particles
The light diffracted from the cloud forms ring-like
wave interference patterns on a photographic
film which gives a permanent record of these
patterns called the hologram
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This can be magnified 300 times by aVidicon camera tube and televisionmonitor.
Individual particles can be focused anddisplayed on a screen for visual analysis.
However, the existing instruments are
capable of recording particles above3
msize only and hence are not suitable formine-dust analysis.
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Location of dust sampling
For fixed point 'area sampling' (also
sometimes referred to as static sampling),
the location is so chosen as to obtain
representative concentration
of respirable dust associated with dust
generating sources.
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The National Dust Prevention Committee
for coal mines in India by its
recommendation of 1993(21) gave
sampling locations for area sampling in
(a) board and pillar,
(b) long wall,
(c) drivage, and
(d) opencast workings and surface plants .
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Board and Pillar workings
(i) Intake airway of a working district within 20 to30 m of immediate out-bye ventilationconnection from the first working face
(ii) Return airway of a working district within 20to 30 m of immediate out bye ventilationconnection from the last working face
Where there is more than one return airway, all
the return airways are to be sampled. iii) Working place or face where dustconcentration is likely to be maximum .
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(iv) Resting place of workers.
(v) 3 to 5 m on the return side of other placeswhere substantial dust is generated, e.g.
loading/transfer point (vi) Main return airway of the mine.
Long wall workings 30 m out bye of the face inthe intake airway
(ii) at the centre of the face and (iii) 50 to 70 m out bye of the face in return
airway
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Drivages or long headings
Sampling shall be done in the drivages
when they are driven more than 30 m or
when the system of working is fully
established to enable collection of a
representative sample.
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Opencast workings / surface
plants-
(i) Samples should be taken at the working
places near workers.
(ii) In surface plants like coal handlingplant (CRP), crusher house, siding and
other places n processing plants where
dust is generated.
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In opencast workings, sampling bypersonal dust samplers is considered
more reliable than sampling by fixed-point
samplers.
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The frequency of respirable dust sampling
is specified as once at least every six
months in CMR 1957.
The maximum interval between sampling
is reduced to three or one month when the
dust concentration exceeds fifty or seventy
five percent respectively of the allowablevalue
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4.0 Sampling procedure
4.1 Places to be sampled
4.1.1 General:As required under Reg. 123(3)(a) & (c) of CMR 1957, all the workplaces in a
mine, where respirable dust is evolved, shall besampled to ascertain the air borne dustconcentration of that area and the dust exposureprofile for different categories of work personsworking in that area.
In view of the above Manager of every mineshall identify such places in the mine.
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Mine air of that area shall be sampled by'static sampling' method by a fixed typesampling instrument.
This shall be cross checked by measuringthe dust exposure of selected workers,whose exposure is deemed to berepresentative of their groups, by 'portal toportal personal monitoring' by usingpersonal dust samplers
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4 1 2 Fixed point or static sampling
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4.1.2 Fixed point or static sampling
for underground workings
The following places shall be sampled byfixed type samplers orstatic samplers:
(a) For B & P orR& P workings : Thesampler shall be positioned on the returnside of the point of dust generation (and
within 1m of the normal working positionof, but not behind the operator or worker)in the following places-
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Within 30m out bye of the first working
face in the intake side of the district.
30m out bye of the last working face in the
return side of the district
Working faces
Loading / Unloading / Transfer points
Bunkers / Chutes
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b) For Longwall working
30m outbye of the face in the intakeairway
10m from the intake end at the face
10m Inbye from the return end of the faceAt the- centre of the face
30m outbye of the face in the return
airway Loading / Unloading / Transfer points
Bunkers / Chutes58
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c) ForMechanized Long wall gate
roads
10m out bye of the face
30m out bye of the face
Loading / Unloading / Transfer points Bunkers / Chutes
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4.1.3 PersonalDust Sampling for
underground workings In addition to the static sampling as above, dust exposure to the
following categories of persons shall be determined by personaldustsampler
Loaders
Shotfirers and helpers
Drillers / Dressers
Operators and helpers of loading machines (SDL, LHD, Shuttle carsloaders or other mechanical loaders)
Operators of Cutter loaders ( Shearer, Plough, Continuous miners,
Road headers etc., Longwall face-crews
Conveyor. bunker or chute operators
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4.1.4 Fixed point or static sampling for Opencast
workings surface operations
Direction of air current should be kept in mindand the instrument be placed on the return sideof the 'point of dust generation.
It is suggested that more than one fixed-point
sampler shall be installed along thecircumference of concentric circles of 5m & 10mradius around the point of equipment generatingdust.
The sampler shall be installed along thedirection of wind or air current and the samplersmay change their location during sampling withchange of direction of the air current.
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However, in opencast workings, sampling
by personal samplers is considered more
reliable than sampling by fixed samplers.
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5PersonalDust Sampling for open
cast working and other operations
Dust exposure of the following categories
of persons shall be determined personal
dust sampler :.
Drillers & helpers
Operators of excavators/loaders (Shovel,
Back-hoe, Dragline, Front-end loaders)
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Dumper & Tipper operators & helpers
Dozer/Grader operators & helpers
CHP crews, crusher operators & helpers,wagon loaders
Any other machine operators.
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Sampling with MRE or its equivalent should be carriedout with air inlet of the Instrument facing the air currentkeeping the instrument in horizontal position,
if the velocity of air is less than 4 metre per second
and perpendicular to the air current, if the velocity of air is more than 4 metre per second. The
instrument shall be placed at about breathing levelwith normal posture of the operator and located centrally,and away from the side, as far as practicable.
Safety of the instrument and interference of the normalwork shall be kept in mind for positioning the instrument.
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4.3 Frequency of dust sampling
(a)As required under Reg. 123(3) (a) of CMR1957, every place as mentioned above shall be
sampled by fixed point samplers, at least oncein every six months.
Provided that if any measurement of any workplace shows the concentration in excess of 50%of the permissible limit, the subsequentmeasurement shall be carried out at intervalsnot exceeding 3 months.
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Provided that if any measurement of' anyworkplace shows the concentration in excess of75% of the permissible limit, the subsequentmeasurement shall be carried out at intervals
not exceeding one month (b) Such measurement shall also be carried out
immediately upon the commissioning of anyplant, equipment or machinery or upon theintroduction of any new work practice or upon
any alteration therein thatis likely to bring about any substantial change inthe levels of airborne respirable dust.
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(c) If respirable dust concentration exceeds the
permissible limit, then the area shall be sampled
again for 5 times in the next 7 successive normal
working shifts according to the provision of Reg.123(4) of CMR 1957 and continued till the
permissible concentration comes within limit
Once the respirable dust concentration falls to
permissible limit or below, the places shall besampled as required under clause 4.3(a).
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4.4 Sampling by personal dust sampler
shall also be carried on along with
fixed point sampling to have a cross
check.
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5.0 Recording of results
All results of measurements of air borne respirable dust and all therelevant particulars shall be recorded in accordance with Reg.123(3) (f) of CMR 1957.
5.1 A plan in a suitable scale should/be maintained showing clearlyall the places where sampling of air borne dust has been done.Date 0f last sampling should be indicated on toe plan,
5.2Results of air-borne dust survey shall be maintained in theformat per annexure I & II.
5.3Recordof respirable dust content and the Quartz content shallmaintained in the enclosed format and kept in a bound paged book
5.4Details of the weighing balance used shall be recorded alongsampling data.
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6 0 P ti f filt
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6.0 Preservation of filter papers
The filter papers used for a particular
sampling shall be preserved along with its
serial number and date of sampling.
These filter papers be. preserved at least
for a period of 3 years and shall be readily
available to the Inspector of Mines if so
desired
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7 0 R t Ai B D t
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7.0 Report on Air Borne Dust
Survey
After conducting the air borne dust survey as per
the above guide report shall be prepared giving
the following details
(i) Introduction (ii) Methodology of dust sampling
(a) Sampling strategy for static sampling
(b) Sampling strategy for personal sampling
(c) Instrumentation
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iii) Observation
(a) Tables showing the results of air bornedust survey for static and personal
sampling.(b) Maximum dust concentration and Time
Weighted Average( TWA ) for differentlocations.
iv) Conclusions.
v) Recomendations
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