Thermal Lecture

THERMAL ENVIRONMENT AND

COMFORT ZONE

Adapted from ISO 7726: 1998 (E), 7730, 9920, ANSI / ASHRAE, 55 – 1992, ANSI / ASHRAE, 55a – 1995,NIOSH

Introduction Introduction Operations involving high air temperatures, radiant

heat sources, high humidity, direct physical contact with hot objects, or strenuous physical activities have a high potential for inducing heat stress in employees engaged in such operations. Such places include: iron and steel foundries, nonferrous foundries, brick-firing and ceramic plants, glass products facilities, rubber products factories, electrical utilities (particularly boiler rooms), bakeries, confectioneries, commercial kitchens, laundries, food canneries, chemical plants, mining sites, smelters, and steam tunnels.

Outdoor operations conducted in hot weather, such as construction, refining, asbestos removal, and hazardous waste site activities, especially those that require workers to wear semipermeable or impermeable protective clothing, are also likely to cause heat stress among exposed workers.

Definitions

Heat stress is defined as the physical and physiological reactions of the worker to temperatures that fall outside of the worker’s normal comfort zone.

Comfort Zone: the condition of mind that expresses satisfaction with the thermal environment

The American Conference of Governmental Industrial Hygienists (1992) states that workers should not be permitted to work when their deep body temperature exceeds 38°C (100.4°F).

Reference; Accepted method of determining comfort zones is through

the use of an ASHRAE chart. ASHRAE –American Society of Refrigeration, Heating, and Air-

Conditioning Engineers. ASHRAE standard 55-1992/95 outlines human com ort zones

based on temperature and humidity.

DefinitionsIn physics, stress is the internal distribution of forces within a body that balance and react to the loads applied to it. Stress is a tensor quantity with nine terms, but which can be described fully by six terms due to symmetry. Simplifying assumptions are often used to represent stress as a vector for engineering calculations.

Stress (roughly the opposite of relaxation) is a medical term for a wide range of strong external stimuli, both physiological and psychological, which can cause a physiological response

Any real or perceived adverse stimulus, physical or psychological, that tends to disturb an individual¹s homeostasis.

A condition in which the organism is subjected to unfavorable or unfamiliar environmental conditions, resulting in some alteration in normal physical functioning. Short-term stress can often be overcome. Long-term stress can reduce resistance to disease and parasites, inhibit self-healing processes, and reduce life-span.

Definitions

Strain is the geometrical expression of deformation caused by the action of stress on a physical body Strain is defined as the amount of deformation an object experiences compared to its original size and shape. For example, if a block 10 cm on a side is deformed so that it becomes 9 cm long, the strain is (10-9)/10 or 0.1 (sometimes expressed in percent, in this case 10 percent.) Note that strain is dimensionless.

Muscle strain or muscle pull or even a muscle tear implies damage to a muscle or its attaching tendons. You can put undue pressure on muscles during the course of normal daily activities, with sudden, quick heavy lifting, during sports, or while performing work tasks.

Definitions

Heat is a measure of energy in terms of quantity.

A calorie is the amount of heat required to raise 1 gram of water 1°C (based on a standard temperature of 16.5 °C

to 17.5°C).

Conduction (K) is the transfer of heat between materials that contact each other. Heat passes from the warmer material to the cooler material. For example, a worker's skin can transfer heat to a contacting surface if that surface is cooler, and vice versa.

Convection (C) is the transfer of heat in a moving fluid. Air flowing past the body can cool the body if the air temperature is cool. On the other hand, air that exceeds 35°C (95°F) can increase the heat load on the body.

Reference

BS EN 12515 : 1997

OSHA Technical Document

Definitions

Evaporative (E) cooling takes place when sweat evaporates from the skin. High humidity reduces the rate of evaporation and thus reduces the effectiveness of the body's primary cooling mechanism.

Radiation (R) is the transfer of heat energy through space. A worker whose body temperature is greater than the temperature of the surrounding surfaces radiates heat to these surfaces. Hot surfaces and infrared light sources radiate heat that can increase the body's heat load.

Metabolic heat (M) is a by-product of the body's activity.

Reference

BS EN 12515 : 1997


Heat Balance Equation

M – W = Cres + Eres + K + C ± R + E = S

Conduction (K)

Convection (C)

Radiation (R)

Evaporation (E)

Metabolic Heat (M)

Net Heat Storage (S)

Respiratory Conduction (Cres)

Respiratory Evaporation (Eres)

Mechanical power (W)

External Heat

Source

Cooling

Internal Heat Source (Muscular Activity)

Reference

BS EN 12515 : 1997


Body heat productionBody heat production

Body heat produced by cell metabolism Energy ingested as food is changed into other forms of energy. 75-85% is released as heat energy The rate at which heat is produced by the body is a measure of the

rate at which energy is released from foods This is called the metabolic rate Exergonic reactions:

Oxidation and ATP use. Most heat generated by brain, heart, liver and glands at

rest. Skeletal muscles 20-30% at rest. Can increase 30-40

times during exercise

Cooling the bodyCooling the body

The body’s cooling system can be likened to the cooling system of a car.

Coolant (blood) is circulated through a pump (heart) and moves heat from the hot inner core to a radiator (skin surface).

Thermoregulatory CenterThermoregulatory Center

Hypothalamus:– Preoptic area neurons: hypothalamic

thermostat:Heat-losing centerHeat-promoting center

Monitors temperature of blood and receives signals from peripheral thermoreceptors.

Negative feedback loops


Heat-losing center: Activates heat losing mechanisms:

– Dilation of dermal arterioles: increase blood flow to skin.

– Sweating.– Increased respiration through mouth.– Behavioral: remove clothing.

Inhibits heat-promoting center.

Heat losing mechanisms: blood flowHeat losing mechanisms: blood flow

0

2

4

6

8

10

12

14

cool hot

blo

od fl

ow

(L/m

in)

skincoremusclesorgans


Heat-promoting center: Activates heat generating mechanisms:

– SNS: Vasoconstriction of dermal arterioles: decrease

blood flow to skin Stimulates arrector pili muscles: hair stands on

end Shivering thermogenesis: spinal reflex of

alternating contractions in antagonistic muscles– Nonshivering thermogenesis:

Long-term mechanism stimulating thyroid hormone release T3 and T4.

– Inhibits heat-loss center.


Hypothalamus:Hypothalamus: Peripheral thermoreceptors:Peripheral thermoreceptors:

– Temperature of skin.Temperature of skin. Central thermoreceptors:Central thermoreceptors:

– Temperature of core.Temperature of core.– Most important located in Most important located in

hypothalamus.hypothalamus.

ThermoregulationThermoregulation

Hypothalamus receives inputfrom thermoreceptors.

Too hot, results in vasodilationto allow conduction of heat fromskin (and evaporation).

Too cold, results in vaso-constriction of skin vessels andreduces sweating to reduceconduction of heat from skin.

Convection and radiation arealso involved in heat loss.

E.g.. Cool breeze moves warm air around body away.

Factors Affect Heat Stress

• Physical factors

• Physiological factors

Physical Factor of Heat StressAmbient temperature

Humidity

Radiant heat load

Air velocity

Insulation

Physiological Factors: Metabolic heat Mechanical work Physical fitness Fatigue Dehydration Previous heat illness Poor physical condition Heart disease High blood pressure Diabetes Skin disease Liver, kidney, and lung problems

Predisposing Physiological Factors of Heat Stress

Predisposing Factors, cont’d: Age Gender Nutrition Pregnancy Drugs Alcohol , caffeine, nicotine intake

Predisposing Physiological Factors of Heat Stress, Cont.

Heat Factor, cont’d - Drugs

Drugs that interfere with body’s thermo-regulation:

Heat production:– Thyroid hormone– Amphetamines – LSD

Decrease thirst:– Haldol

Decrease sweating:– Antihistamine – Anticholinergics – Phenothiazines – Benztropine

What are the health effects of

heat stress/strain?

Effects of heat on performanceEffects of heat on performance

Physical Work Capacity A two- to three-percent loss decreases

aerobic capacity by more than 10 percent. Dehydration that reduces body weight by 4.3 percent also will decrease performance by 22 percent

Mental Work Capacity Mental performance can be affected with an

increase in body temperature of 2oF above normal.

Moderate heat stress is believed to affect mental performance by lowering levels of arousal. Conscious effort can counteract this effect

Heat Effects and People

Heat affects people in different ways. People come in all different sizes, shapes, and tolerances for heat.

Some people can work comfortably in high temperatures, while others will develop sickness from heat stress/strain

health effects of heat stress/strain

Heat Rash (prickly heat, miliaria rubra):

Cause: Heat buildup in the skin due to clogged pores and sweat ducts. Prolonged skin wetness from sweating.

Symptoms: Area becomes reddened and may itch or hurt. Skin eruptions.

First Aid: Practice good personal hygiene; keep the skin clean and the pores unclogged, allow skin to dry, wear loose clothing, see doctor if rash persists.

Seriousness: Relatively minor.


Radiation Burns (Sunburn)

Cause: UV radiation is absorbed by the skin.

Symptoms: Water molecules within skin are disrupted, leading to drying-out of tissues. Extreme cases involve blisters, ruptures, and deep-tissue damage.

First Aid: Covering of exposed skin. Use of protective creams (sunscreen). Bandaging of serious burns.

Seriousness: Minor to relatively serious.


Transient Heat Fatigue:

Cause: Loss of fluids reduces circulatory efficiency

Symptoms: General feeling of tiredness or fatigue

First Aid: Fluid replacement and restSeriousness: No long-term adverse effects

% Weight loss

Fluid loss

Time* Effect & symptoms (* timing may vary based on

intensity of work and heat/humidity)

1% 0.75 L 1 hr unnoticed (at 1.5% weight loss you are considered dehydrated)

2% 1.5 L 2-3 hrs loss of endurance, start to feel thirsty, feel hot, uncomfortable

3% 2.25 L 3-4 hrs loss of strength, loss of energy, moderate discomfort

4% 3 L 4-5 hrs cramps, headaches, extreme discomfort

5-6% 3.5-4 L 5-6 hrs heat exhaustion, nausea, faint

7+% 5+ L 7+ hrs heat stroke, collapse, unconsciousness

Percentage of Weight Loss/Fluid Loss with Respect to Time


Heat Cramps:Cause: Loss of important electrolytes in the

blood and muscle tissues due to excessive amounts of “salts” being lost in the victim’s sweat.

Symptoms: Cramping of either voluntary (skeletal) muscles or involuntary (principally abdominal) muscles (or both).

First Aid: Replenish electrolytes through drinking of fluids constituted for this purpose such as Gator-Aide. Rest in a cool environment.

Seriousness: May debilitate the victim for several days. Full recovery is necessary before going back into heat stress conditions.


Heat Exhaustion:Cause: Depressed condition of the circulatory

system due for the most part to a lack of adequate fluid replacement (dehydration). Blood vessels dilate and blood flow is seriously reduced (clinical condition of shock has occurred). A victim may be able to take actions that will alleviate the condition, if the symptoms are recognized early enough.

Symptoms: Nausea, dizziness, weakness, headache, blurred vision, profuse sweating, cold/wet (clammy) grayish skin, unconsciousness, coma and death.


Heat Exhaustion (continued):

First Aid: Place victim in a face down position in a cool location, administer fluids if the victim is conscious. If unconscious, seek medical care or transport to a medical emergency room.

Seriousness: Shock is a serious medical condition regardless of the cause of its onset. Victims may require several days or even weeks to recover. Even longer periods may be necessary before the victim can resume working in heat stress conditions.


Heat Stroke:Cause: The body’s temperature regulation

mechanism, located in the hypothalamus, fails and sweating stops. Core body temperature rises dramatically and the victim’s condition becomes a serious medical emergency. The victim is unlikely to be able to reverse the condition without assistance or medical intervention.

Symptoms: Chills, restlessness, irritability, euphoria, red face and skin, disorientation, hot/dry skin (not always), collapse, unconsciousness, convulsions and death.

Heat Stroke (continued):First Aid: Immediate, aggressive cooling of the

victim’s body using wet cloths, immersion into cool water or using alcohol wipes. Transport to emergency medical facility.

Seriousness: Heat Stroke is a MEDICAL EMERGENCY. Without outside intervention, the victim will die. By the time the victim realizes s/he is in trouble, it is usually too late to employ effective self-intervention procedures that can reverse the thermo-regulatory failure and reduce core temperatures. Recovery times from heat stroke are generally the longest of any heat-related disorder.


Human Response to HeatHuman Response to Heat

Thermal IndicesThermal Indices Direct Indices

– Dry Bulb Temperature

– Wet Bulb Temperature

Rational Indices

– Operative Temperature

– Belding Hatch Heat Stress Index

– Skin Wettedness (%SWA)

Empirical Indices

– Effective Temperature (ET, CET & ET*)

– Wet Bulb Globe Temperature (WBGT)

Other Index

– Predicted 4-hour sweat rate (P4SR)

Thermal IndicesThermal Indices Direct Indices

Purpose Drawbacks

Dry Bulb Temperature (ta)

Estimating comfort conditions for sedentary people wearing conventional indoor clothing. For every 29watt or 25 cal reduce ta by 1.7oC

Use is not justified when the temperature is above comfort zone

Wet Bulb Temperature (twb)

Assessing heat stress and predicting heat strain under conditions where radiant temperature and air velocity are not large factors and where Twb approximate ta. Twb 30oC is the upper limit for unimpaired activity of sedentary tasks and 28oC is the upper limit for moderate physical work..

Use is not justified where radiant temperature and air velocity are present

Thermal IndicesThermal Indices Rational Indices

Purpose Drawbacks

Operative Temperature (to)

Heat exchange between a worker and the environment by radiation & convection in a uniform environment as it would occur in an actual industrial environment can be obtained.

For convective heat exchanges measure of air velocity is needed

Important factors of humidity and metabolic heat production are omitted

Belding Hatch Heat Stress Index

It is the ratio of the amount of body heat that is required to be lost to the environment by evaporation for thermal equilibrium (Ereq) divided by the maximum amount of sweat evaporation allowed through the clothing system that can be accepted by the environment (Emax). Assumes: a sweat rate of 1l/hr over 8 hr day for an average healthy worker without harmful effects

Not applicable at very high heat stress conditionsIt does not identify the heat stress differences resulting from hot, dry and hot, humid conditions. Strain from metabolic vs. environmental heat is not differentiatedEreq /Emax is a ratio, their absolute values are not addressed to.Measurements are difficult limiting the use of the index in field situation.

Thermal IndicesThermal Indices Rational Indices

Purpose Drawbacks

Skin Wettednes

s (%SWA)

Skin Wettedness considers the variables basic to heat balance (air temperature, humidity, air movement, radiative heat metabolic heat & clothing characteristics) and require that these variables be measured or calculated for each industrial situation. It is an indicator of strain under high humidity and low air movement where evaporation is restricted.The index is satisfactory as a basis for calculating the magnitude of thermal stress and strain and for recommending engineering & work practice controls.

The measurement requirements are time consuming

Wind speed at worksite is difficult to measure with any degree of reliability; it can only be an approximation.

For routine environmental monitoring, they are too complicated, require considerable recording equipment.

Thermal IndicesThermal Indices Empirical Indices

Purpose Drawbacks

Effective Temperature (ET, CET & ET*)

The ET combines dry bulb & wet bulb temperatures and air velocity. In a later version of ET, Corrected Effective Temperature (CET), the black globe temperature is considered. The index values for ET & CET were derived from subjective impressions of equivalent heat loads between a reference chamber at 100% humidity and low air motion and an exposed chamber where temperature and air motion were higher and humidity lower. They are used in studies of physical, psychomotor and mental performance changes as a result of heat stress. The recently developed new Effective Temperature (ET*) uses 50% reference relative humidity. It is useful in calculating ventilation or air conditioning requirements for maintaining acceptable conditions in buildings.

ET & CET seem to over estimate the effects of high humidity and under estimate the effects of air motion and thus tend to over estimate heat stress


Purpose Drawbacks

Wet Bulb Globe Temperature (WBGT)

The WBGT combines the effect of humidity and air movement (in tnwb), air temperature and radiation (in tg), and air temperature (ta) as a factor in outdoor situations in the presence of sunshine. While for indoors, only the natural wet bulb and the black globe temperatures are needed. The application of WBGT index was used for training schedules for military recruits during summer season resulted in a striking reduction of heat casualties.

When impermeable clothing is worn, the WBGT will not be a relevant index as evaporative cooling (wet bulb temperature) will be limited. Psychometric wet bulb temperature is independent of radiation & prevailing wind and so can’t be used in computing WBGT index


Purpose Drawbacks

Wet Globe Temperature (WGT / Botsball)

The heat exchange by convection, radiation and evaporation is integrated into a single instrument reading. WGT is used in many laboratory studies and field situations. When compared to WBGT, in general the correlation is high, however it is not he same for all combinations of environmental factors. A simple approximation of relationship is WBGT=WGT+20C for conditions of moderate radiant heat and humidity

The WGT is good for screening and monitoring, but it does not yield data for solving the equations for heat exchange between the worker and the industrial environment.

Thermal IndicesThermal Indices

Predicted 4-hour sweat rate (PSR)Predicted 4-hour sweat rate (PSR)

The PSR is an old physiological Heat Stress Index based on amount of sweat that would be produced in 4 hours, knowing the air movement, dry and wet temperature and globe temperature in case of radiant heat. Workload & clothing has to be taken into account.

Heat stress indices

WBGT (Wet Bulb Globe Temperature) is the accepted method for determining true temperature

–Accounts for air currents, relative humidity, solar load

Thermal Heat Stress

Reference

Yaglou and Minard 1957.

ISO 7243,1989.

WBGT Formulae

For indoor or shaded environments:

WBGT = 0.7 X Tnwb + 0.3 X Tg

Tnwb = natural wet-bulb temperature

Tg = globe temperature

For direct sunlight exposure:

WBGT = 0.7 X Tnwb + 0.2 X Tg + 0.1 X Tdb

Tdb = dry-bulb temperature

TLV Workload Definitions

Light: Standing with light work at machine/bench using mostly arms; using table saw

Moderate: Walking about with moderate lifting or pushing; scrubbing in a standing position

Heavy: Shoveling dry sand; cutting with a hand saw

Very Heavy: Shoveling wet sand

TLV’s

Estimating Energy Cost of Work by Task Analysis(TLV-1)

A. Body Position and Movement Kcal/min Sitting 0.3

Standing 0.6

Walking 2.0 - 3.0

Walking uphill add 0.8/meter rise

B. Type of work Average Kcal/min Range Kcal/min Hand work

Light 0.4 0.2 – 1.2

Heavy 0.9

Work one arm Light 1.0 0.7– 2.5 Heavy 1.8 Work both arms Light 1.5 1.0 – 3.5 Heavy 2.5 Work whole body Light 3.5 2.5 – 9.0 Moderate 5.0 Heavy 7.0 Very Heavy 9.0

C. Basal metabolism 1.0

D. Sample Calculation Average Kcal/min

Assembling work with heavy hand tools

1. Standing 0.6

2. Two-arm work 3.5

3. Basal metabolism 1.0

Total 5.1 Kcal/min

For standard worker of 70 kg body Wt.and 1.8 m2 body surface.

Recommended Heat- Stress Exposure Limits Heat –Acclimatized/Unacclimatized Workers

Clo: a unit to express the thermal insulation provided by garments and clothing ensembles,

Where 1 clo=0.155 m2. 0C/W

One clo will maintain a sitting resting man , whose metabolism is 50 kcal/m2.h (58W/m2) indefinitely comfortable in an environment of 21oC, relative humidity 50% and air movement 20ft/min (0.01m/s)

Reference

ANSI/ASHRAE 55-1992

Thermal insulation for typical combination of garments

Clo Insulation Units for Individual Items of Clothing

TLV WBGT Correction Factors with Clo Value

The following correction factors for the WBGT should be used:

Effective Temperature (ET)Effective Temperature (ET)

Effective temperature is defined as that temperature of still, saturated air which produces the same feeling of warmth as the given environment, with its particular combinations of air temperature, humidity and air movement.

Effect of additional radiation is not considered, so

that the ET scales are applicable when the mean radiant temperature of the surroundings and, therefore, the black globe temperature do not differ significantly from the dry bulb temperature.

Effective Temperature (ET)Effective Temperature (ET)

Two scales for ET were introduced, both meant for sedentary subjects:

The basic scale of ET; applicable to subjects striped top the waist

The normal scale of ET; applicable to subjects in ordinary indoor clothing (about 1 clo).

Effective Effective Temperature (ET)Temperature (ET)

Basic ScaleBasic Scale

Effective Effective Temperature (ET)Temperature (ET)

Normal ScaleNormal Scale

Effective Temperature (ET)Effective Temperature (ET)Certain features

ET gives one numerical measure of the combined effect of air temperature, humidity and air movement.

The two scales, basic and normal make allowance for only two levels of clothing.

ET scales do not make provision for different levels of activity

The ET scales are based on subjective feelings of comfort / discomfort, these are found to correlate very well with physiological responses, though within a limited range of thermal severity of the environment.

The ET scale is best suited in the middle range of thermal stress i.e. covering the comfort zone and the zone of the evaporative regulation within which a man could maintain his thermal balance through evaporative cooling of sweat.


The PSR is a physiological Heat Stress Index based on amount of sweat that would be produced in 4 hours, knowing the air movement, dry and wet temperature and globe temperature in case of radiant heat. Workload & clothing has to be taken into account.

P4SR of 4.5 l for a limit where no incapacitation of any fit, acclimatized young men occurred.

McArdle et al. (1947)




Heat Stress IndexHeat Stress Index

The Heat Stress Index is the ratio of evaporation The Heat Stress Index is the ratio of evaporation required to maintain heat balance (Ereq) to the required to maintain heat balance (Ereq) to the maximum evaporation that could be achieved in the maximum evaporation that could be achieved in the environment (Emax), expressed as a percentage environment (Emax), expressed as a percentage (Belding and Hatch 1955) (Belding and Hatch 1955)

Heat Stress Index (HSI)Heat Stress Index (HSI)

Operative temperatureOperative temperature

tO =ta •√ 10 • Va + tr

1+√ 10 • Va

Operative Temperature

Va air velocity, in meters per sec.

tr mean radiant temp in 0C

tr=[ ( tg + 273)4 + 2.5 X 108 X va 0.6 (tg – ta)] ¼ - 273

FORCED CONVECTION

STANDARD GLOBE

tr= [(tg + 273)4 +

1.1 X 108 X va0.6

gx D 0.4(tg – ta)]1/4 - 273

Heat Stress Index (HSI) Heat Stress Index (HSI) Interpretation of Heat Stress Index (HSI) values

HSI Effect of eight hour exposure

-20 Mild cold strain (e.g. recovery from heat exposure).

0 No thermal strain

10-30 Mild to moderate heat strain. Little effect on physical work but possible effect on skilled work

40-60 Severe heat strain, involving threat to health unless physically fit. Acclimatization required

70-90 Very severe heat strain. Personnel should be selected by medical examination. Ensure adequate water and salt intake

100 Maximum strain tolerated daily by fit acclimatized young men

Over 100

Exposure time limited by rise in deep body temperature

Hot Environment Dangers

Hot environments - analytical Hot environments - analytical determination and interpretation determination and interpretation of thermal stress using of thermal stress using calculation of required sweat calculation of required sweat rate rate

BS EN 12515 : BS EN 12515 : 19971997

Heat balance equation

Reference

BS EN 12515 : 1997

Heat balance equation M – W = Cres + Eres +K + C + R + E + S

M = W/m2…………………………………(1) M: Metabolic Heat

W = 0 W: skin wettedness

Cres = 0.0014M (Eres – ta) …………(2) Cres : Respiratory Conduction

= 0.0014M 350C – ta)

Erec = 0.0173M (Pex– Pn) Erec : Respiratory Evaporation

= 0.0173M (5.624KPn – Pn)…..(3)

C = hc . Fcl (tsk – ta) C : Convection

hc = 3.5 + 5.2 Var < 1m/s hc : Convective heat transfer

= 8.7 Var0.6 1m/s Coefficients

( Force convection)…..(4) Fcl : heat changes

2.38(tsk – ta)0.25 tsk: mean skin temp.

(Normal convection)…..(5) ta : Ambient temp.

Var = Va + 0.0052 (M – 58) Var : relative air velocity

Fcl = 1/[(hc + hr)/Icl + 1/fcl] Va : air velocity

fcl = 1+1.97 Icl Icl : thermal insulation of the

R = hr Fcl(tsk – tr) clothing in square meter kelvin per watt

E = wEmax E : Evaporation at skin surface

S = algebraic sum of heat flows S : Heat Storage

Heat flow by respiratory convection Heat flow by respiratory convection (C(Cresres))

Heat flow by respiratory convection Heat flow by respiratory convection can be estimated from metabolic can be estimated from metabolic power Mpower M

Cres = 0.0014M (tex – ta) W/m2

= 0.0014M (350C – ta) W/m2

Where tex is the temperature of the expired air and can be approximated to 35oC

Heat Transfer Heat Transfer Evaporation (E) Evaporation (E) W/m2

The heat flow by respiratory evaporation (Eres) can be stated algebraically.

Eres = 0.0173 M(pex – pa ) W/m2

Where the saturated water vapour pressure of the expired air Pex is equal to 5.624 kPa for expired air temperature (tex) of 350C

TemTempp

(oc)(oc)

00 11 22 33 44 55 66 77 88 99

1010 1.231.23 1.31.311

1.41.400

1.501.50 1.61.600

1.71.700

1.821.82 1.91.944

2.062.06 2.22.200

2020 2.342.34 2.42.499

2.62.644

2.812.81 2.92.988

3.13.177

3.363.36 3.53.566

3.783.78 4.04.000

3030 4.244.24 4.44.499

4.74.755

5.035.03 5.35.322

5.65.622

5.945.94 6.26.255

-- --

Pressure of saturated water vapour pressure (kPa) between 10o C to 37oC shown in steps 1oC

Heat TransferHeat Transfer

Convection Convection W/m2

Convection (C) is the transfer of heat in a moving fluid. Air flowing past the body can cool the body if the air temperature is cool. On the other hand, air that exceeds 35°C (95°F) can increase the heat load on the body.

Heat Transfer Heat Transfer Convection ( C ) Convection ( C ) W/m2

The rate of heat exchange between a person and an ambient air can be stated algebraically, where hc represents convective heat transfer coefficient






(Normal convection)…..(2) ta : Ambient temp.Var is defined as the ratio of the air velocity relative to the ground and the speed of the body or the parts of the body relative to the ground. If body movement is due to muscular work, Var can be calculated by the following equation:Var = Va + 0.0052 (M – 58) Va : air velocity in ms -1

Fcl = 1/[(hc + hr)/Icl + 1/fcl] Icl : thermal insulation of the fcl = 1+1.97 Icl clothing in square meter kelvin per watt

The reduction factor for loss of sensible heat exchange due to the wearing of clothes (Fcl) can be calculated by the following equation :

Key1 Forehead 2 Neck 3 Right scapula 4 Left upper chest 5 Right arm -upper location 6 Left arm -lower location 7 Left hand 8 Right abdomen9 Left paravertebral10 Right anterior thigh11 Left posterior thigh12 Right shin13 Left calf14 Right instep

Figure -Position of measuring sites

Mean Skin TemperatureMean Skin Temperature

Mean Skin TemperatureMean Skin Temperature

MeasuriMeasuring siteng site

Weighting coefficient Weighting coefficient

4 4 pointpointss

8 8 points points

14 14 pointspoints

11 0.070.07 1/141/14

22 0.280.28 1/141/14

33 0.280.28 0.1750.175 1/141/14

44 0.1750.175 1/141/14

55 0.070.07 1/141/14

66 0.070.07 1/141/14

77 0.160.16 0.050.05 1/141/14

88 1/141/14

99 1/141/14

1010 1/141/14

1111 1/141/14

1212 0.280.28 1/141/14

1313 0.20.2 1/141/14

1414 1/141/14

Mean skin temperature

tsk = ∑kitski

The mean skin The mean skin temperature is temperature is calculated by weighting calculated by weighting each of the local each of the local temperatures with a temperatures with a coefficient coefficient corresponding to the corresponding to the relative surface of the relative surface of the body area that each body area that each measuring point measuring point represents.represents.

Factors that influence hc are Parts of body, Air speed & direction, Clothing and Values of tsk vary depending on the method used

for the measurements, the number and location of measuring points on the body and values used for weighting temperatures measured at the different location

Heat Transfer Heat Transfer Convection ( C ) W/mConvection ( C ) W/m22

Radiation (R) W/m2

Radiation (R) is the transfer of heat energy through space. A worker whose body temperature is greater than the temperature of the surrounding surfaces radiates heat to these surfaces. Hot surfaces and infrared light sources radiate heat that can increase the body's heat load.

Heat TransferHeat Transfer

Radiation (R) W/mRadiation (R) W/m22

The rate of radiant heat exchange between a person and the surrounding solid objects can be stated algebraically.

hr (W/m2) = radiant heat transfer coefficient hr= σЄsk Ar / ADu [(tsk + 273)4 – (tr + 273)4]/(tsk – tr)Fcl = reduction of the sensible heat exchange due to clothing Fcl= 1/((hc+hr)Icl+1/fcl)fcl = 1+1.97Iclwhereσ = the Stefan-Boltzmann constant, equal to 5.67 x 10-8 w/(m2 x k4)

Єsk = skin emissivity (0.97);

R = hr Fcl (tsk – tr) W/m2

Ar / ADu = the fraction of skin surface involved in heat exchange by radiation.

This fraction is equal to 0.67 for a crouching subject, 0.70 for a seated subject, and 0.77 for a standing subject.

tr = mean radiant temperatureBSA(m²) = Wt(kg)0.425 x Ht(cm)0.725 x 0.007184

Heat Transfer Heat Transfer

Value of hr will depend on body position of the exposed worker and on the emissivity of the skin and clothing, as well as on insulation of clothing

Body position will determine the total body surface actually exposed to radiation.

Emissivity of the skin and clothing will determine the radiant energy absorbed on those surfaces.

Insulation of clothing will determine the amount of radiant heat transferred by the garments to the skin

Radiation (R) SI Units Radiation (R) SI Units

Evaporation Evaporation

Evaporative (E) cooling takes place when sweat evaporates from the skin. High humidity reduces the rate of evaporation and thus reduces the effectiveness of the body's primary cooling mechanism.

Heat Transfer Heat Transfer Evaporation (E) SI UnitsEvaporation (E) SI Units

The maximum evaporation rate Emax ( W/m2) is that which can be achieved in hypothetical case of the skin being completely wetted.

Emax = (psk.s – pa)/RT

Where

Psk.s = saturated vapour pressure at skin temperature (kilopascals)

Pa = partial water vapour pressure in the working environment (kilopascals)

RT = evaporative resistance of the limiting layer of air and clothing (kilopascals)2

For partially wetted skin, the evaporation rate E (W/m2)

E = wEmax where, w is skin wettedness

TemTempp

(oc)(oc)

00 11 22 33 44 55 66 77 88 99

1010 1.231.23 1.31.311

1.41.400

1.501.50 1.61.600

1.71.700

1.821.82 1.91.944

2.062.06 2.22.200

2020 2.342.34 2.42.499

2.62.644

2.812.81 2.92.988

3.13.177

3.363.36 3.53.566

3.783.78 4.04.000

3030 4.244.24 4.44.499

4.74.755

5.035.03 5.35.322

5.65.622

5.945.94 6.26.255

-- --

Pressure of saturated water vapour pressure (kPa) between 10o C to 37oC shown in steps 1oC

Emax = (Psk,s – Pa)/RT

Equation

RT = 1/(he . Fpcl)

he = 16.7 hc

Fpcl = 1/(1 + 2.22 . hc (Icl – (1 – 1/fcl)/(he + hr)) RT:is the total evaporative resistance of the limiting layer of air and

clothing, in square meter kilopascals per watt.

Fpcl: reduction factor for latent heat exchange

he : evaporative heat transfer coefficient

Icl : thermal insulation of the clothing in square meter Kelvin per watt

hr : heat transfer coefficient

A few major limitations to the maximum amount of sweat evaporated from skin (Emax) are

Human sweating capacityMaximum vapour uptake capacity of the ambient air The resistance of the clothing to evaporation

Heat Transfer Heat Transfer Evaporation (E) W/mEvaporation (E) W/m22

Required Evaporation Rate (Ereq, in watts per square meter)

Ereq = M – W – Cres – Eres – C - REvaporation rate required for the maintenance of the thermal equilibrium of the body and, for a heat storage equal to zero.

Convection (C)

Radiation (R)

Metabolic Heat (M)

Respiratory Conduction (Cres)

Respiratory Evaporation (Eres)

Mechanical power (W)

Ereq = M – W – Cres – Eres – C - REquation

M = W/m2…………………………………(1) M: Metabolic Heat

W = 0 W: skin wettedness

Cres = 0.0014M (Eres – ta) …………(2) Cres : Respiratory Conduction

= 0.0014M 350C – ta)

Erec = 0.0173M (Pex– Pn) Erec : Respiratory Evaporation

= 0.0173M (5.624KPn – Pn)…..(3)






(Normal convection)…..(5) ta : Ambiant temp.

Var = Va + 0.0052 (M – 58) Var : relative air velocity

Fcl = 1/[(hc + hr)/Icl + 1/fcl] Va : air velocity

fcl = 1+1.97 Icl Icl : thermal insulation of the

R = hr Fcl(tsk – tr) clothing in square meter kelvin per watt

Required skin wettedness (Wreq dimensionless)

Wreq = Ereq / Emax

Required skin wettedness (Wreq)

Required Evaporation (Ereq)

Maximum Evaporation (Emax)

Required skin wettedness (Wreq dimensionless) is defined as the ratio between the required evaporation

rate, Ereq and the maximum evaporation rate, Emax

Reference

BS EN 12515 : 1997

The Required Sweat Rate (SWreq, in Watts per square meter)

SWreq = Ereq / rreq

Evaporated efficiency of sweating (dimensionless), which corresponds to the required skin wattedness (rreq)

Required evaporation rate (Ereq)

The sweat rate in watts per square meter represents the equivalent in heat of the sweat rate expressed in grams of sweat per meter of skin surface and per hour. 1 W/m2 corresponds to a flow of 1.47 g/(m2-h) (1.8 m2 of body surface), a flow of 2.6 g/h).

Evaporative Efficiency of Sweating r (dimensionless)

r = 1 – W2/2

W skin wettedness.

Interpretation of required sweat Interpretation of required sweat

raterate

Basis for method of interpretationBasis for method of interpretation CriteriaCriteria

– Maximum skin wettedness (wMaximum skin wettedness (wmaxmax))

– Maximum sweat rate (SWMaximum sweat rate (SWmaxmax))

LimitsLimits– Max heat storage (QMax heat storage (Qmaxmax in watt hours per squire in watt hours per squire

meter)meter)

– Maximum water loss (DMaximum water loss (Dmaxmax in grams) in grams)

Interpretation of required sweat Interpretation of required sweat raterate

Analysis of the situations If wreq<wmax and SWreq<SWmax then

wp = wreq

Ep=Ereq

SWp=SWreq

However if wreq>wmax thenWp=wmax

Ep=wp.Emax

SWp=SWmax Where w=skin wettedness, E= evaporation rate and SW = sweating rate ; and p=

predicted, req=required and max= maximum

Interpretation of required sweat Interpretation of required sweat raterate

Determination of allowable exposure Time Determination of allowable exposure Time Ep=Ereq

SWp<Dmax/8Then no limit has been suggested

If one or other of these condition is not satisfied and If one or other of these condition is not satisfied and if the required evaporation rate is not achieved , the if the required evaporation rate is not achieved , the difference (Ereq – Ep) represents heat storage rate difference (Ereq – Ep) represents heat storage rate which will be responsible for an increase in core which will be responsible for an increase in core temperature temperature

The exposure tile limit will be The exposure tile limit will be DLE1 = 60.Qmax/ (Ereq – Ep) in minute DLE1 = 60.Qmax/ (Ereq – Ep) in minute If predicted sweate rate involves exaggerated If predicted sweate rate involves exaggerated

sweate loss then sweate loss then DLE2 = 60 Dmax/ SWpDLE2 = 60 Dmax/ SWp

Criteria for Thermal Limits Based on Average Values

Heat ( Nonacclimatized ) Heat

( Acclimatized ) Alert Danger Alert Danger

Heat Storage Q max W. h/m2 50 60 50 60

Sweat rate, m< 65 w/m2 ( SWmax) ( rest)

g/h 260 390 520 780

W/m2 100 150 200 300

Sweat rate, m> 65 w/m2 (SWmax) (work) g/h 520 650 780 1040 W/m2 200 250 300 400

Maximum water loss (D max)

W . h/m2 1000 1250 1500 2000

g 2600 3250 3900 5200

Skin wettedness (wmax) 0.85 0.85 1.0 1.0

Body TemperatureBody Temperature

Shell temperature:Shell temperature:– Temperature closer to skinTemperature closer to skin– Oral temperatureOral temperature

36.636.6oo-37.0-37.0ooC (97.9C (97.9oo-98.6-98.6ooF)F)

Core temperature:Core temperature:– Most important temperatureMost important temperature– Temperature of “core” (organs in cranial, Temperature of “core” (organs in cranial,

thoracic and abdominal cavities)thoracic and abdominal cavities)– Rectal temperatureRectal temperature

37.237.2oo-37.6-37.6ooC (99.0C (99.0oo-99.7-99.7ooF)F)

Measuring body temperatureMeasuring body temperature Body temp. measured via mouth, arm, rectum Body temp. measured via mouth, arm, rectum

and via the ear.and via the ear. Celcius and Fahrenheit scalesCelcius and Fahrenheit scales Clinical thermometer holds value. Clinical thermometer holds value. Uses mercury as:Uses mercury as:

easily seen easily seen good conductor good conductor uniform expansionuniform expansion doesn’t stick to glassdoesn’t stick to glass Liquid, from -39 to 357 Liquid, from -39 to 357 ooCC

Poisonous!Poisonous!

Cree & Rischmiller, Fig 2.1

Thermistor thermometersThermistor thermometers

Uses a covered wire and Uses a covered wire and digital readout.digital readout.

Accurate & rapidAccurate & rapid Flexible-probe versions for Flexible-probe versions for

constant monitoringconstant monitoring Can be inserted internally but Can be inserted internally but

need lubrication & irritation need lubrication & irritation checks.checks.

Body temperature factorsBody temperature factors

Kept optimum in the range 36.1 – 37.1 Kept optimum in the range 36.1 – 37.1 ooC C for cell chemistry & enzymes.for cell chemistry & enzymes.

Varies between places.Varies between places. Varies withVaries with

* Time of day* Time of day* Hormonal changes* Hormonal changes* Age* Age* Exposure, illness, stress & exercise* Exposure, illness, stress & exercise



Thermal Comfort

Thermal Comfort

PMV

ASHRAE

Thermal Comfort

Please answer the following questions concerned with YOUR THERMAL COMFORT.1. Indicate on the scale below how you feel Now. Hot Warm Slightly warm Neutral Slightly cool Cool Cold2. Please indicate how you would like to be NOW Warmer No change Cooler3. Please indicate how you Generally feel at work: Hot Warm Slightly warm Neutral Slightly cool Cool Cold4. Please indicate how you would Generally like to be at work:Warmer No change Cooler5. Are you generally satisfied with your thermal environment at work? Yes No

Are you in Thermal Environment Comfort?

Predicted Mean Vote (PMV)

The PMV is an index that predicts the mean value of the votes of a large group of persons on the following 7-point thermal sensation scale:

0 1 2 3-1-2-3

Neutral

Slightly

warm

HotWarmSlightly cool

CoolCold

Reference

ISO 7730:1994 (E)

Predicted mean vote (PMV)Predicted mean vote (PMV)

Predicted mean vote (PMV)Predicted mean vote (PMV)

PMV Required:

M: is the metabolic rate, in watts per square meter of body surface area2

W: is the external work, in watts per square meter, equal to zero for most activities;

Icl: is the thermal resistance of clothing, in square meters degree Celsius per watt3 ;

Fcl : is the ratio of mans surface area while clothed, to mans surface

area while nude:

ta : is the air temperature, in degree Celsius;

tr: is the mean radiant temperature, in degree Celsius;

Var : is the relative air velocity (relative to the human body), in meters per second;

Pa : is the partial water vapour pressure, in Pascal;

hc : is the convective heat transfer coefficient, in watts per square meter degree Celsius;

tcl : is the surface temperature of clothing, in degree Celsius;

Predicted percentage of dissatisfied (PPDPPD)

PMV Values for Air Temperature, Clothing and Activity

(assume: mean radiant temperature=air temperature, air velocity=0.15ms-l and relative humidity = 50 0/0)

Clothing (CLO)

Activity(Wm-2)

Air temperature (OC)

16 18 20 22 24 26 28

0.65 58 -- -2.7 -2.0 -1.3 -0.6 0.0 0.8

1.0 58 -2.1 -1.6 -1.1 -0.5 0.0 0.6 1.2

1.5 58 - 1.1 -0.7 -0.3 0.2 0.6 1.1 1.5

0.65 70 -2.2 -1.7 - 1.2 -0.6 0.0 0.5 1.0

1.0 70 -1.3 -0.9 -0.5 0.0 0.4 0.9 1.3

1.5 70 -0.5 -0.2 0.2 0.5 0.9 1.2 1.6

0.65 100 -0.9 -0.5 -0.1 0.3 0.6 1.0 1.4

1.0 100 -0.3 0.0 0,3 0.6 1.0 1.3 1.6

1.5 100 0.3 0.5 0.7 1.0 1.3 1.5 1.8

PMV: Predicted Mean Vote

Estimates of Typical Metabolic Heat Production Values

Activity Metabolic heat production (W m-2)

Seated, at rest 58

Standing, relaxed 70

Standing, light arm work 100

VDU operation 70

Driving 70-100

Thermal comfort clothing Thermal comfort clothing

clothing (clo )under various clothing (clo )under various conditions conditions

EnvironmEnvironmental ental temp. temp.

Resting Resting sitting sitting

Slow level Slow level walkingwalking

Normal Normal level level walking walking

Fast level Fast level walkingwalking

70°F–70°F–Normal Normal outdoors outdoors

1.501.50 0.70 0.70 0.4 0.4 0.30.3

50°F–50°F–Normal Normal outdoorsoutdoors

3.1 3.1 1.501.50 0.900.90 0.70.7


4.74.7 2.32.3 1.51.5 1.11.1


7.27.2 3.53.5 2.32.3 1.71.7

Estimates of Typical Clothing Insulation Values (1 CLO=0.155m2°CW-1)

Type of clothing Clothing insulation (CLO)

None 0

Light summer clothing (briefs, shorts, short sleeved shirt, light socks, light shoes

0.3

Light work clothing (light underwear, cotton long sleeved work shirt, light longtrousers, socks, shoes)

0.65

Light business suit (including underclothing etc.)

1.0

Heavy business suit (including underclothing etc.)

1.5

ASHRAEASHRAE STANDARD; is to specify the combinations of

indoor space environment and personal factor that will produce thermal environmental condition acceptable to 80% or more of activity and clothing.

Factors

Environment factor

Temperature

Thermal radiation

Humidity

Air speed

Personal factor

Activity

Clothing

ASHRAE Scale

Sensation Cold Cool Slightly cool

Neutral Slightly warn

Warm Hot

PMV -3 -2 -1 0 1 2 3

Same as PMV Scale

psychrometricchart

the dewpoint is:

Td = 8ºC

Relative humidity

Wet bulb temperature (°F)

Saturation vapor pressure (mb)

vapor pressure (mb)

dewpoint (°F)

ASHRAEASHRAE STANDARD: A Standard specific thermal

zone of comfort expressing

On X axis; Operative temp.(to)

On Y axis; Dew temp.(tdp)

Operative temperatureOperative temperature

tO =ta •√ 10 • Va + tr

1+√ 10 • Va

Operative Temperature

Va air velocity, in meters per sec.

tr mean radiant temp in 0C

tr=[ ( tg + 273)4 + 2.5 X 108 X va 0.6 (tg – ta)] ¼ - 273

FORCED CONVECTION

STANDARD GLOBE

tr= [(tg + 273)4 +

1.1 X 108 X va0.6

gx D 0.4(tg – ta)]1/4 - 273

ASHRAE gives optimum and acceptable range of operative temperature for people during exposed to indoor space environment condition

Light, primarily sedentary activity ( 1.2 met) at 50% relative humidity and mean air speed 0.15m/s (30fpm)a

Season Description of typical clothing

Id

(CLO)

Optimum operative temperature

Operative temperature range (10% dissatisfaction criterion)

Winter Heavy slacks 0.9 220C710F

20 – 23.50C68 – 750F

Long sleeve

Shirt and sweater

Summer Light slacks and short – sleeve shirt

0.5 24.50C760F

23 – 260C73 – 790F

Minimal 0.05 270C810F

26 – 290C79 – 840F

a Other than clothing, there are no adjustment for season or sex to the temperature of this Table. For infants, certain elderly people, and individuals who are physically disabled, the lower limits of this table should be avoided

Characteristics of Measuring Instruments

Characteristics of Measuring Instruments, Cont.

Characteristics of Measuring Instruments, Cont.

Main Independent Quantities Involved in The Analysis of the Thermal Balance Between Man and the Thermal Environment

Check List for Controlling Heat Stress and StrainItem Action for consideration

I. Control

M, body heat production of task Reduce physical demands of the work;

powered assistance for heavy tasks

R, Rediative load interpose line-of-sight barrier;

furnace wall insulation,

metallic reflecting screen,

heat reflective clothing,

cover exposed parts of body

C, convective load if air temp is above 350C (950F)

reduce air temp,

reduce air speed across skin,

wear clothing

if air temp is below 350C (950F)

increase air speed across skin,

reduce clothing

Emax, maximum evaporative increase by :

cooling by sweating decrease humidity,

increase air speed, decrease clothing

Item Action for considerationII. Work practices Shorten duration of each exposure;

more frequent short exposures better than fewer long

exposure

schedule very hot jobs in cooler part of day when possible

Exposure limit Self – limiting, based on formal indoctrination of workers and supervisor on signs and symptom of overstrain

Recovery Air – conditioned space nearby

Acclimatization

III. Personal protection R,C, and Emax Cooled air, cooled fluid, or ice cooled conditioned clothing

reflective clothing or aprons

IV. Other considerations determine by medical evaluation,

primarily of cardiovascular status

Careful break – in of unacclimatized workers

Water intake at frequent intervals to prevent hypohydration

Fatigue or mild illness

not related to the job may temporarily contraindicate exposure ( e,g., low grade infection, diarrhea,

sleepless night, alcohol ingestion )

V. Heat Wave introduce heat alert program

Check List for Controlling Heat Stress and Strain

WHO RecommendationNo worker should be exposed to

any combination of environmental heat and physical work which would cause the workers body core temperature to exceed 380 C (100.40F) . (1969).

Criteria for Thermal Limits Based on Average Values

Heat ( Nonacclimatized ) Heat

( Acclimatized ) Alert Danger Alert Danger

Heat Storage Kcal 58 70 5870

Rectal temp increase 0c (0F) 0.8 (1.4) 1.0 (1.8) 0.8 (1.4) 1.0 (1.4)

Skin temp increase 0c (0F) 2.4 (4.3) 3.0 (5.4) 2.4 (4.3) 3.0 (5.4)

Sweat rate, max rest g/h 260 390 520 780

Sweat rate, max work g/h 520 650 780 1040

Max 8h sweat production to-

prevent excessive dehydration 2.60 3.25 3.905.20

Skin wettedness, rest 0.85 1.0

Skin wettedness, work 0.50 0.85

Interpretation of PMV Values in terms of Thermal Sensation and Predicted Percentage of Dissatisfied (PPD)

Sensation

Cold Cool Slightly cool

Neutral

Slightly warn

Warm

Hot

PMV -3 -2 -1 0 1 2 3

PPD(%) - 75 25 5 25 75 --

Reference

•ISO 7730;1993

Prevention of Heat Prevention of Heat StressStress

AcclimatizationAcclimatization

The longer you do hard work in the heat The longer you do hard work in the heat the better your body becomes at keeping the better your body becomes at keeping cool. cool.

If you are not used to working in the heat If you are not used to working in the heat then you must take a week or two to get then you must take a week or two to get acclimatized or used to the heat. acclimatized or used to the heat.

If you were ill or away from work for a If you were ill or away from work for a week or so you can lose your week or so you can lose your acclimatization. acclimatization.

1.1. If you are experienced on the job, limit your time If you are experienced on the job, limit your time in the hot environment to 50% of the shift on the in the hot environment to 50% of the shift on the first day and 80% on the second day. first day and 80% on the second day.

There are two ways to acclimatize:There are two ways to acclimatize:

2.2. Instead of reducing the exposure times to the hot Instead of reducing the exposure times to the hot job, you can become acclimatized by reducing the job, you can become acclimatized by reducing the physical demands of the job for a week or two.physical demands of the job for a week or two.

• You can work a full shift the third day.You can work a full shift the third day.• If you are not experienced on the job (for example, a new If you are not experienced on the job (for example, a new

worker) you should start off spending 20% of the time in the worker) you should start off spending 20% of the time in the hot environment on the first day and increase your time by 20% hot environment on the first day and increase your time by 20% each following day.each following day.


If you have health problems or are not in If you have health problems or are not in good physical condition, you may need good physical condition, you may need longer periods of acclimatization. longer periods of acclimatization.

Hot spells in tropical countries like Hot spells in tropical countries like Malaysia last long enough to allow Malaysia last long enough to allow acclimatization. acclimatization.

When it is hot, consider some of the When it is hot, consider some of the following engineering and administrative following engineering and administrative controls. controls.


Modifying Work and the Modifying Work and the EnvironmentEnvironment

Management and the Joint Health and Management and the Joint Health and Safety Committee can reduce heat stress Safety Committee can reduce heat stress in the following ways: in the following ways:

• Engineering controlsEngineering controls

• Administrative controlsAdministrative controls

• Personal protective equipmentPersonal protective equipment

Engineering ControlsEngineering Controls

Control the heat at source through the Control the heat at source through the use of insulating and reflective barriers use of insulating and reflective barriers (insulate furnace walls). (insulate furnace walls).

Exhaust hot air and steam produced by Exhaust hot air and steam produced by specific operations. specific operations.

Reduce the temperature and humidity Reduce the temperature and humidity through air cooling. through air cooling.

Provide air-conditioned rest areas. Provide air-conditioned rest areas.

Increase air movement if temperature is Increase air movement if temperature is less than 35°C (fans). less than 35°C (fans).

Reduce physical demands of work task Reduce physical demands of work task through mechanical assistance (hoists, lift-through mechanical assistance (hoists, lift-tables, etc.).tables, etc.).

Engineering ControlsEngineering Controls

Administrative ControlsAdministrative Controls

Health and safety committees should Health and safety committees should assess the demands of all jobs and have assess the demands of all jobs and have monitoring and control strategies in place monitoring and control strategies in place for hot days. for hot days.

Increase the frequency and length of rest Increase the frequency and length of rest breaks. breaks.

Schedule hot jobs to cooler times of the Schedule hot jobs to cooler times of the day. day.

Provide cool drinking water near workers Provide cool drinking water near workers and remind them to drink a cup every 20 and remind them to drink a cup every 20 minutes or so. minutes or so.


Workers should salt their food well, Workers should salt their food well, particularly while they are acclimatizing to particularly while they are acclimatizing to a hot job (workers with a low salt diet a hot job (workers with a low salt diet should discuss this with their doctor). should discuss this with their doctor).

Assign additional workers or slow down Assign additional workers or slow down work pace. work pace.

Make sure everyone is properly Make sure everyone is properly acclimatized. acclimatized.


Train workers to recognize the signs and Train workers to recognize the signs and symptoms of heat stress and start a symptoms of heat stress and start a 'buddy system' since people are not likely 'buddy system' since people are not likely to notice their own symptoms. to notice their own symptoms.

Pregnant workers and workers with a Pregnant workers and workers with a medical condition should discuss working medical condition should discuss working in the heat with their doctor.in the heat with their doctor.

Personal Protective EquipmentPersonal Protective Equipment

Light clothing should be worn to allow free Light clothing should be worn to allow free air movement and sweat evaporation. air movement and sweat evaporation.

Outside, wear light-coloured clothing. Outside, wear light-coloured clothing. In a high radiant heat situation, reflective In a high radiant heat situation, reflective

clothing may help. clothing may help. For very hot environments, air, water or ice-For very hot environments, air, water or ice-

cooled insulated clothing should be cooled insulated clothing should be considered. considered.

Vapour barrier clothing, such as acid suits, Vapour barrier clothing, such as acid suits, greatly increases the amount of heat stress greatly increases the amount of heat stress on the body, and extra caution is necessary. on the body, and extra caution is necessary.

Personal Protective EquipmentPersonal Protective Equipment

Threshold Limit Values for Heat Threshold Limit Values for Heat Stress published by the American Stress published by the American Conference of Governmental Conference of Governmental Industrial Hygienists used as Industrial Hygienists used as reference. reference.

These values are based on These values are based on preventing fit, acclimatized workers' preventing fit, acclimatized workers' core temperatures from rising above core temperatures from rising above 3838ooC. C.

References

Curtis, Rick. Outdoor Action Program. Princeton University. 1995

Occupational Safety and Health Administration 3154

Occupational Safety and Health Administration 3156

Occupational Health Clinics for Ontario Workers. Heat Stress Training. 2001

Plog, Barbara A. et al. Fundamentals of Industrial Hygiene. 1996

USAF. Heat Stress Fact Sheet. 1998www.firstworldwar.com/atoz/trenchfoot.html

Documents

Thermal Lecture