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Section III: Chapter 4

Section III: Chapter 4

Heat Stress

Table of Contents:

  1. Introduction
  2. Heat Disorders and Health Effects
  3. Investigation Guidelines
  4. Sampling Methods
  5. Control
  6. Personal Protective Equipment
  7. Bibliography

List of Appendices:

The mention of trade names, commercial products, or organizations does not imply endorsement by OSHA or the U.S. Government.
I. 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.

A. Causal Factors
  1. Age, weight, degree of physical fitness, degree of acclimatization, metabolism, use of alcohol or drugs, and a variety of medical conditions such as hypertension all affect a person's sensitivity to heat. However, even the type of clothing worn must be considered. Prior heat injury predisposes an individual to additional injury.
  2. It is difficult to predict just who will be affected and when, because individual susceptibility varies. In addition, environmental factors include more than the ambient air temperature. Radiant heat, air movement, conduction, and relative humidity all affect an individual's response to heat.
B. Definitions
  1. 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).
  2. Heat is a measure of energy in terms of quantity.
  3. A calorie is the amount of heat required to raise 1 gram of water 1°C (based on a standard temperature of 16.5 to 17.5°C).
  4. Conduction 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.
  5. Convection 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.
  6. Evaporative 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.
  7. Radiation 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.
  8. Globe temperature is the temperature inside a blackened, hollow, thin copper globe.
  9. Metabolic heat is a by-product of the body's activity.
  10. Natural wet bulb (NWB) temperature is measured by exposing a wet sensor, such as a wet cotton wick fitted over the bulb of a thermometer, to the effects of evaporation and convection. The term natural refers to the movement of air around the sensor.
  11. Dry bulb (DB) temperature is measured by a thermal sensor, such as an ordinary mercury-in-glass thermometer, that is shielded from direct radiant energy sources.
II. Heat Disorders and Health Effects
A. Heat Stroke

Occurs when the body's system of temperature regulation fails and body temperature rises to critical levels. This condition is caused by a combination of highly variable factors, and its occurrence is difficult to predict. Heat stroke is a medical emergency. The primary signs and symptoms of heat stroke are confusion; irrational behavior; loss of consciousness; convulsions; a lack of sweating (usually); hot, dry skin; and an abnormally high body temperature, e.g., a rectal temperature of 41°C (105.8°F). If body temperature is too high, it causes death. The elevated metabolic temperatures caused by a combination of work load and environmental heat load, both of which contribute to heat stroke, are also highly variable and difficult to predict.

If a worker shows signs of possible heat stroke, professional medical treatment should be obtained immediately. The worker should be placed in a shady area and the outer clothing should be removed. The worker's skin should be wetted and air movement around the worker should be increased to improve evaporative cooling until professional methods of cooling are initiated and the seriousness of the condition can be assessed. Fluids should be replaced as soon as possible. The medical outcome of an episode of heat stroke depends on the victim's physical fitness and the timing and effectiveness of first aid treatment.

Regardless of the worker's protests, no employee suspected of being ill from heat stroke should be sent home or left unattended unless a physician has specifically approved such an order.

B. Heat Exhaustion

The signs and symptoms of heat exhaustion are headache, nausea, vertigo, weakness, thirst, and giddiness. Fortunately, this condition responds readily to prompt treatment. Heat exhaustion should not be dismissed lightly, however, for several reasons. One is that the fainting associated with heat exhaustion can be dangerous because the victim may be operating machinery or controlling an operation that should not be left unattended; moreover, the victim may be injured when he or she faints. Also, the signs and symptoms seen in heat exhaustion are similar to those of heat stroke, a medical emergency.

Workers suffering from heat exhaustion should be removed from the hot environment and given fluid replacement. They should also be encouraged to get adequate rest.

C. Heat Cramps

Are usually caused by performing hard physical labor in a hot environment. These cramps have been attributed to an electrolyte imbalance caused by sweating. It is important to understand that cramps can be caused by both too much and too little salt. Cramps appear to be caused by the lack of water replenishment. Because sweat is a hypotonic solution (±0.3% NaCl), excess salt can build up in the body if the water lost through sweating is not replaced. Thirst cannot be relied on as a guide to the need for water; instead, water must be taken every 15 to 20 minutes in hot environments.

Under extreme conditions, such as working for 6 to 8 hours in heavy protective gear, a loss of sodium may occur. Recent studies have shown that drinking commercially available carbohydrate-electrolyte replacement liquids is effective in minimizing physiological disturbances during recovery.

D. Heat Collapse ("Fainting")

In heat collapse, the brain does not receive enough oxygen because blood pools in the extremities. As a result, the exposed individual may lose consciousness. This reaction is similar to that of heat exhaustion and does not affect the body's heat balance. However, the onset of heat collapse is rapid and unpredictable. To prevent heat collapse, the worker should gradually become acclimatized to the hot environment.

E. Heat Rashes

Are the most common problem in hot work environments. Prickly heat is manifested as red papules and usually appears in areas where the clothing is restrictive. As sweating increases, these papules give rise to a prickling sensation. Prickly heat occurs in skin that is persistently wetted by unevaporated sweat, and heat rash papules may become infected if they are not treated. In most cases, heat rashes will disappear when the affected individual returns to a cool environment.

F. Heat Fatigue

A factor that predisposes an individual to heat fatigue is lack of acclimatization. The use of a program of acclimatization and training for work in hot environments is advisable. The signs and symptoms of heat fatigue include impaired performance of skilled sensorimotor, mental, or vigilance jobs. There is no treatment for heat fatigue except to remove the heat stress before a more serious heat-related condition develops.

III. Investigation Guidelines

These guidelines for evaluating employee heat stress approximate those found in the 1992-1993 ACGIH publication, Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices.

A. Employer and Employee Interviews
  1. The inspector will review the OSHA 200 Log and, if possible, the OSHA 101 forms for indications of prior heat stress problems.
  2. Following are some questions for employer interviews: What type of action, if any, has the employer taken to prevent heat stress problems? What are the potential sources of heat? What employee complaints have been made?
  3. Following are some questions for employee interviews: What heat stress problems have been experienced? What type of action has the employee taken to minimize heat stress? What is the employer's involvement, i.e., does employee training include information on heat stress? (Appendix III:4-1 lists factors to be evaluated when reviewing a heat stress situation, and Appendix III:4-2 contains a follow-up checklist.)
B. Walkaround Inspection

During the walkaround inspection, the investigator will: determine building and operation characteristics; determine whether engineering controls are functioning properly; verify information obtained from the employer and employee interviews; and perform temperature measurements and make other determinations to identify potential sources of heat stress. Investigators may wish to discuss any operations that have the potential to cause heat stress with engineers and other knowledgeable personnel. The walkaround inspection should cover all affected areas. Heat sources, such as furnaces, ovens, and boilers, and relative heat load per employee should be noted.

C. Work-Load Assessment
  1. Under conditions of high temperature and heavy workload, the CSHO should determine the work-load category of each job (Table III:4-1 and Figure III:4-1). Work-load category is determined by averaging metabolic rates for the tasks and then ranking them:

    1. Light work: up to 200 kcal/hour
    2. Medium work: 200-350 kcal/hour
    3. Heavy work: 350-500 kcal/hour
  2. Cool Rest Area: Where heat conditions in the rest area are different from those in the work area, the metabolic rate (M) should be calculated using a time-weighted average, as follows:

    Equation III: 4-1. Average Metabolic Rate

    AverageM = (M1)(t1)+ (M2)(t2)+...+(Mn)(tn) / (t1)+(t2)+...(tn)


    M  =   metabolic rate

    t  =   time in minutes

    In some cases, a videotape is helpful in evaluating work practices and metabolic load.

    • Light hand work: writing, hand knitting
    • Heavy hand work: typewriting
    • Heavy work with one arm: hammering in nails (shoemaker, upholsterer)
    • Light work with two arms: filing metal, planing wood, raking the garden
    • Moderate work with the body: cleaning a floor, beating a carpet
    • Heavy work with the body: railroad track laying, digging, barking trees

    Sample Calculation: Assembly line work using a heavy hand tool

    Walking along - 2.0 kcal/min

    Intermediate value between heavy work with two arms and light work with the body - 3.0 kcal/min

    Add for basal metabolism - 1.0 kcal/min

    Total: - 6.0 kcal/min

    Source: ACGIH 1992

    Body position and movement kcal/min*
    Sitting 0.3
    Standing 0.6
    Walking 2.0-3.0
    Walking uphill add 0.8 for every meter (yard) rise

    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 1.7  
    Work: Both arms
    Light 1.5 1.0-3.5
    Heavy 2.5  
    Work: Whole body
    Light 3.5 2.5-15.0
    Moderate 5.0  
    Heavy 7.0  
    Very heavy 9.0  
    * For a "standard" worker of 70 kg body weight (154 lbs) and 1.8m2 body surface (19.4 ft2).

Source: ACGIH 1992

IV. Sampling Methods
A. Body Temperature Measurements

Although instruments are available to estimate deep body temperature by measuring the temperature in the ear canal or on the skin, these instruments are not sufficiently reliable to use in compliance evaluations.

B. Environmental Measurements

Environmental heat measurements should be made at, or as close as possible to, the specific work area where the worker is exposed. When a worker is not continuously exposed in a single hot area but moves between two or more areas having different levels of environmental heat, or when the environmental heat varies substantially at a single hot area, environmental heat exposures should be measured for each area and for each level of environmental heat to which employees are exposed.

C. Wet Bulb Globe Temperature Index
  1. Wet Bulb Globe Temperature (WBGT) should be calculated using the appropriate formula in Appendix III:4-2. The WBGT for continuous all-day or several hour exposures should be averaged over a 60-minute period. Intermittent exposures should be averaged over a 120-minute period. These averages should be calculated using the following formula:

    Equation III:4-2. Average Web Bulb Globe Temperature (WBGT)

    AverageWBGT = (WBGT1)(t1)+ (WBGT2)(t2)+...+(WBGTn)(tn) / (t1)+(t2)+...(tn)

    For indoor and outdoor conditions with no solar load, WBGT is calculated as:

    WBGT = 0.7NWB + 0.3GT

    For outdoors with a solar load, WBGT is calculated as

    WBGT = 0.7NWB + 0.2GT + 0.1DB


    WBGT = Wet Bulb Globe Temperature Index

    NWB = Natural Wet-Bulb Temperature

    DB = Dry-Bulb Temperature

    GT = Globe Temperature

  2. The exposure limits in Table III:4-2 are valid for employees wearing light clothing. They must be adjusted for the insulation from clothing that impedes sweat evaporation and other body cooling mechanisms. Use Table III:4-3 to correct Table III:4-2 for various kinds of clothing.
  3. Use of Table III:4-2 requires knowledge of the WBGT and approximate workload. Workload can be estimated using the data in Table III:4-1, and sample calculations are presented in Figure III:4-1.
D. Measurement

Portable heat stress meters or monitors are used to measure heat conditions. These instruments can calculate both the indoor and outdoor WBGT index according to established ACGIH Threshold Limit Value equations. With this information and information on the type of work being performed, heat stress meters can determine how long a person can safely work or remain in a particular hot environment. See Appendix III:4-2 for an alternate method of calculation.

------------- Work Load* ------------
Work/rest regimen Light Moderate Heavy
Continuous work 30.0°C (86°F) 26.7°C (80°F) 25.0°C (77°F)
75% Work, 25% rest, each hour 30.6°C (87°F) 28.0°C (82°F) 25.9°C (78°F)
50% Work, 50% rest, each hour 31.4°C (89°F) 29.4°C (85°F) 27.9°C (82°F)
25% Work, 75% rest, each hour 32.2°C (90°F) 31.1°C (88°F) 30.0°C (86°F)

*Values are in °C and °F, WBGT.

These TLV's are based on the assumption that nearly all acclimatized, fully clothed workers with adequate water and salt intake should be able to function effectively under the given working conditions without exceeding a deep body temperature of 38°C (100.4° F). They are also based on the assumption that the WBGT of the resting place is the same or very close to that of the workplace. Where the WBGT of the work area is different from that of the rest area, a time-weighted average should be used (consult the ACGIH 1992-1993 Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices (1992).

These TLV's apply to physically fit and acclimatized individuals wearing light summer clothing. If heavier clothing that impedes sweat or has a higher insulation value is required, the permissible heat exposure TLV's in Table III:4-2 must be reduced by the corrections shown in Table III:4-3.

Source: ACGIH 1992.

E. Other Thermal Stress Indices
  1. The Effective Temperature index (ET) combines the temperature, the humidity of the air, and air velocity. This index has been used extensively in the field of comfort ventilation and air-conditioning. ET remains a useful measurement technique in mines and other places where humidity is high and radiant heat is low.
  2. The Heat-Stress Index (HSI) was developed by Belding and Hatch in 1965. Although the HSI considers all environmental factors and work rate, it is not completely satisfactory for determining an individual worker's heat stress and is also difficult to use.

    Clothing type Clo* value WBGT correction
    Summer lightweight working clothing 0.6 0
    Cotton coveralls 1.0 -2
    Winter work clothing 1.4 -4
    Water barrier, permeable 1.2 -6
    *Clo:   Insulation value of clothing. One clo = 5.55 kcal/m2/hr of heat exchange by radiation and convection for each degree °C difference in temperature between the skin and the adjusted dry bulb temperature.

    Note: Deleted from the previous version are trade names and "fully encapsulating suit, gloves, boots and hood" including its clo value of 1.2 and WBGT correction of -10.

Source: ACGIH 1992

V. Control

Ventilation, air cooling, fans, shielding, and insulation are the five major types of engineering controls used to reduce heat stress in hot work environments. Heat reduction can also be achieved by using power assists and tools that reduce the physical demands placed on a worker.

However, for this approach to be successful, the metabolic effort required for the worker to use or operate these devices must be less than the effort required without them. Another method is to reduce the effort necessary to operate power assists. The worker should be allowed to take frequent rest breaks in a cooler environment.

A. Acclimatization
  1. The human body can adapt to heat exposure to some extent. This physiological adaptation is called acclimatization. After a period of acclimatization, the same activity will produce fewer cardiovascular demands. The worker will sweat more efficiently (causing better evaporative cooling), and thus will more easily be able to maintain normal body temperatures.
  2. A properly designed and applied acclimatization program decreases the risk of heat-related illnesses. Such a program basically involves exposing employees to work in a hot environment for progressively longer periods. NIOSH (1986) says that, for workers who have had previous experience in jobs where heat levels are high enough to produce heat stress, the regimen should be 50% exposure on day one, 60% on day two, 80% on day three, and 100% on day four. For new workers who will be similarly exposed, the regimen should be 20% on day one, with a 20% increase in exposure each additional day.
B. Fluid Replacement

Cool (50°-60°F) water or any cool liquid (except alcoholic beverages) should be made available to workers to encourage them to drink small amounts frequently, e.g., one cup every 20 minutes. Ample supplies of liquids should be placed close to the work area. Although some commercial replacement drinks contain salt, this is not necessary for acclimatized individuals because most people add enough salt to their summer diets.

C. Engineering Controls
  1. General ventilation is used to dilute hot air with cooler air (generally cooler air that is brought in from the outside). This technique clearly works better in cooler climates than in hot ones. A permanently installed ventilation system usually handles large areas or entire buildings. Portable or local exhaust systems may be more effective or practical in smaller areas.
  2. Air treatment/air cooling differs from ventilation because it reduces the temperature of the air by removing heat (and sometimes humidity) from the air.
  3. Air conditioning is a method of air cooling, but it is expensive to install and operate. An alternative to air conditioning is the use of chillers to circulate cool water through heat exchangers over which air from the ventilation system is then passed; chillers are more efficient in cooler climates or in dry climates where evaporative cooling can be used.
  4. Local air cooling can be effective in reducing air temperature in specific areas. Two methods have been used successfully in industrial settings. One type, cool rooms, can be used to enclose a specific workplace or to offer a recovery area near hot jobs. The second type is a portable blower with built-in air chiller. The main advantage of a blower, aside from portability, is minimal set-up time.
  5. Another way to reduce heat stress is to increase the air flow or convection using fans, etc. in the work area (as long as the air temperature is less than the worker's skin temperature). Changes in air speed can help workers stay cooler by increasing both the convective heat exchange (the exchange between the skin surface and the surrounding air) and the rate of evaporation. Because this method does not actually cool the air, any increases in air speed must impact the worker directly to be effective.

    If the dry bulb temperature is higher than 35°C (95°F), the hot air passing over the skin can actually make the worker hotter. When the temperature is more than 35°C and the air is dry, evaporative cooling may be improved by air movement, although this improvement will be offset by the convective heat. When the temperature exceeds 35°C and the relative humidity is 100%, air movement will make the worker hotter. Increases in air speed have no effect on the body temperature of workers wearing vapor-barrier clothing.
  6. Heat conduction methods include insulating the hot surface that generates the heat and changing the surface itself.
  7. Simple engineering controls, such as shields, can be used to reduce radiant heat, i.e. heat coming from hot surfaces within the worker's line of sight. Surfaces that exceed 35°C (95°F) are sources of infrared radiation that can add to the worker's heat load. Flat black surfaces absorb heat more than smooth, polished ones. Having cooler surfaces surrounding the worker assists in cooling because the worker's body radiates heat toward them.

    With some sources of radiation, such as heating pipes, it is possible to use both insulation and surface modifications to achieve a substantial reduction in radiant heat. Instead of reducing radiation from the source, shielding can be used to interrupt the path between the source and the worker. Polished surfaces make the best barriers, although special glass or metal mesh surfaces can be used if visibility is a problem.

    Shields should be located so that they do not interfere with air flow, unless they are also being used to reduce convective heating. The reflective surface of the shield should be kept clean to maintain its effectiveness.
D. Administrative Controls and Work Practices
  1. Training is the key to good work practices. Unless all employees understand the reasons for using new, or changing old, work practices, the chances of such a program succeeding are greatly reduced.
  2. NIOSH (1986) states that a good heat stress training program should include at least the following components:

    • Knowledge of the hazards of heat stress;
    • Recognition of predisposing factors, danger signs, and symptoms;
    • Awareness of first-aid procedures for, and the potential health effects of, heat stroke;
    • Employee responsibilities in avoiding heat stress;
    • Dangers of using drugs, including therapeutic ones, and alcohol in hot work environments;
    • Use of protective clothing and equipment; and
    • Purpose and coverage of environmental and medical surveillance programs and the advantages of worker participation in such programs.
  3. Hot jobs should be scheduled for the cooler part of the day, and routine maintenance and repair work in hot areas should be scheduled for the cooler seasons of the year.
E. Worker Monitoring Programs
  1. Every worker who works in extraordinary conditions that increase the risk of heat stress should be personally monitored. These conditions include wearing semipermeable or impermeable clothing when the temperature exceeds 21°C (69.8°F), working at extreme metabolic loads (greater than 500 kcal/hour), etc.
  2. Personal monitoring can be done by checking the heart rate, recovery heart rate, oral temperature, or extent of body water loss.
  3. To check the heart rate, count the radial pulse for 30 seconds at the beginning of the rest period. If the heart rate exceeds 110 beats per minute, shorten the next work period by one third and maintain the same rest period.
  4. The recovery heart rate can be checked by comparing the pulse rate taken at 30 seconds (P1) with the pulse rate taken at 2.5 minutes (P3) after the rest break starts. The two pulse rates can be interpreted using Table III:4-4.
  5. Oral temperature can be checked with a clinical thermometer after work but before the employee drinks water. If the oral temperature taken under the tongue exceeds 37.6°C, shorten the next work cycle by one third.
  6. Body water loss can be measured by weighing the worker on a scale at the beginning and end of each work day. The worker's weight loss should not exceed 1.5% of total body weight in a work day. If a weight loss exceeding this amount is observed, fluid intake should increase.
F. Other Administrative Controls

The following administrative controls can be used to reduce heat stress:

  • Reduce the physical demands of work, e.g., excessive lifting or digging with heavy objects;
  • Provide recovery areas, e.g., air-conditioned enclosures and rooms;
  • Use shifts, e.g., early morning, cool part of the day, or night work;
  • Use intermittent rest periods with water breaks;
  • Use relief workers;
  • Use worker pacing; and
  • Assign extra workers and limit worker occupancy, or the number of workers present, especially in confined or enclosed spaces.
Heart rate recovery pattern P3 Difference between
P1 and P3

Satisfactory recovery

High recovery (Conditions may require further study)

No recovery (May indicate too much stress)







A. Reflective Clothing

Which can vary from aprons and jackets to suits that completely enclose the worker from neck to feet, can stop the skin from absorbing radiant heat. However, since most reflective clothing does not allow air exchange through the garment, the reduction of radiant heat must more than offset the corresponding loss in evaporative cooling. For this reason, reflective clothing should be worn as loosely as possible. In situations where radiant heat is high, auxiliary cooling systems can be used under the reflective clothing.

B. Auxiliary Body Cooling
  1. Commercially available ice vests, though heavy, may accommodate as many as 72 ice packets, which are usually filled with water. Carbon dioxide (dry ice) can also be used as a coolant. The cooling offered by ice packets lasts only 2 to 4 hours at moderate to heavy heat loads, and frequent replacement is necessary. However, ice vests do not encumber the worker and thus permit maximum mobility. Cooling with ice is also relatively inexpensive.
  2. Wetted clothing is another simple and inexpensive personal cooling technique. It is effective when reflective or other impermeable protective clothing is worn. The clothing may be wetted terry cloth coveralls or wetted two-piece, whole-body cotton suits. This approach to auxiliary cooling can be quite effective under conditions of high temperature and low humidity, where evaporation from the wetted garment is not restricted.
  3. Water-cooled garments range from a hood, which cools only the head, to vests and "long johns," which offer partial or complete body cooling. Use of this equipment requires a battery-driven circulating pump, liquid-ice coolant, and a container.

    Although this system has the advantage of allowing wearer mobility, the weight of the components limits the amount of ice that can be carried and thus reduces the effective use time. The heat transfer rate in liquid cooling systems may limit their use to low-activity jobs; even in such jobs, their service time is only about 20 minutes per pound of cooling ice. To keep outside heat from melting the ice, an outer insulating jacket should be an integral part of these systems.

  4. Circulating air is the most highly effective, as well as the most complicated, personal cooling system. By directing compressed air around the body from a supplied air system, both evaporative and convective cooling are improved. The greatest advantage occurs when circulating air is used with impermeable garments or double cotton overalls.

    One type, used when respiratory protection is also necessary, forces exhaust air from a supplied-air hood ("bubble hood") around the neck and down inside an impermeable suit. The air then escapes through openings in the suit. Air can also be supplied directly to the suit without using a hood in three ways:

    • by a single inlet;
    • by a distribution tree; or
    • by a perforated vest.

    In addition, a vortex tube can be used to reduce the temperature of circulating air. The cooled air from this tube can be introduced either under the clothing or into a bubble hood. The use of a vortex tube separates the air stream into a hot and cold stream; these tubes also can be used to supply heat in cold climates. Circulating air, however, is noisy and requires a constant source of compressed air supplied through an attached air hose.

    One problem with this system is the limited mobility of workers whose suits are attached to an air hose. Another is that of getting air to the work area itself. These systems should therefore be used in work areas where workers are not required to move around much or to climb. Another concern with these systems is that they can lead to dehydration. The cool, dry air feels comfortable and the worker may not realize that it is important to drink liquids frequently.

C. Respirator Usage

The weight of a self-contained breathing apparatus (SCBA) increases stress on a worker, and this stress contributes to overall heat stress. Chemical protective clothing such as totally encapsulating chemical protection suits will also add to the heat stress problem.


American Conference of Governmental Industrial Hygienists (ACGIH). 1990. Documentation of the Threshold Limit Values and Biological Exposure Indices. 6th ed. Cincinnati: American Conference of Governmental Industrial Hygienists.

American Conference of Governmental Industrial Hygienists (ACGIH). 1992. 1992-1993 Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices. Cincinnati: American Conference of Governmental Industrial Hygienists.

American Industrial Hygiene Association (AIHA). 1975. Heating and Cooling for Man in Industry. 2nd ed. Akron, OH: American Industrial Hygiene Association.

Electric Power Research Institute (EPRI). 1987. Heat-Stress Management Program for Nuclear Power Plants. Palo Alto, CA: Electric Power Research Institute.

Eastman Kodak Company. 1983. Ergonomic Design for People at Work. Vol. II. Belmont, CA: Lifetime Learning Publications.

National Institute for Occupational Safety and Health. Criteria for a Recommended Standard--Occupational Exposure to Hot Environments. DHHS (NIOSH) Publication No. 86-113, April 1986.

National Institute for Occupational Safety and Health. Occupational Safety and Health Guidance Manual for Hazardous Waste Site Activities. DHHS (NIOSH) Publication No. 85-115, 1985.

National Institute for Occupational Safety and Health. Standards for Occupational Exposures to Hot Environments: Proceedings of a Symposium. DHHS (NIOSH) Publication No. 76-100, January 1976.

National Institute for Occupational Safety and Health. Working in Hot Environments. DHHS (NIOSH) Publication No. 86-112. Revised 1986.

National Safety Council. 1985. Pocket Guide to Heat Stress. Chicago, IL: National Safety Council.

Ramsey, J. D., Buford, C. L., Beshir, M.Y., and Jensen, R .C. Effects of Workplace Thermal Conditions on Safe Work Behavior. Journal of Safety Research 14:105-114, 1983.

Zenz, C. 1988. Occupational Medicine: Principles and Practical Applications. 2nd ed. St. Louis, MO: Mosby Year Book, Inc.


Note: Listed below are sample questions that the Compliance Officer may wish to consider when investigating heat stress in the workplace.

Workplace Description
  1. Type of business
  2. Heat-producing equipment or processes used
  3. Previous history (if any) of heat-related problems
  4. At "hot" spots:

    • Is the heat steady or intermittent?
    • Number of employees exposed?
    • For how many hours per day?
    • Is potable water available?
    • Are supervisors trained to detect/evaluate heat stress symptoms?
Are Exposures Typical For A Workplace In This Industry?
  1. Weather at Time of Review
  2. Temperature
  3. Humidity
  4. Air velocity
  5. Is Day Typical of Recent Weather Conditions? (Get information from the Weather Bureau)
  6. Heat-Reducing Engineering Controls
  7. Ventilation in place?
  8. Ventilation operating?
  9. Air conditioning in place?
  10. Air conditioning operating?
  11. Fans in place?
  12. Fans operating?
  13. Shields or insulation between sources and employees?
  14. Are reflective faces of shields clean?
Work Practices To Detect, Evaluate, And Prevent Or Reduce Heat Stress
  1. Training program?
  2. Content?
  3. Where given?
  4. For whom?
  5. Liquid replacement program?
  6. Acclimatization program?
  7. Work/rest schedule?
  8. Scheduling of work (during cooler parts of shift, cleaning and maintenance during shut-downs, etc.)
  9. Cool rest areas (including shelter at outdoor work sites)?
  10. Heat monitoring program?
  11. Personal Protective Equipment
  12. Reflective clothing in use?
  13. Ice and/or water-cooled garments in use?
  14. Wetted undergarments (used with reflective or impermeable clothing) in use?
  15. Circulating air systems in use?
  16. First Aid Program
  17. Trained personnel?
  18. Provision for rapid cool-down?
  19. Procedures for getting medical attention?
  20. Transportation to medical facilities readily available for heat stroke victims?
  21. Medical Screening and Surveillance Program
  22. Content?
  23. Who manages program?
  24. Additional Comments: (Use additional pages as needed.)
  1. Describe events leading up to the episode.
  2. Evaluation/comments by other workers at the scene.
  3. Work at time of episode (heavy, medium, light)?
  4. How long was affected employee working at site prior to episode?
  5. Medical history of affected worker, if known.
  6. Appropriate engineering controls in place?
  7. Appropriate engineering controls in operation?
  8. Appropriate work practices used by affected employee(s)?
  9. Appropriate personal protective equipment available?
  10. Appropriate personal protective equipment in use?
  11. Medical screening for heat stress and continued surveillance for signs of heat stress given other employees?
  12. Additional comments regarding specific episode(s): (Use additional pages as needed.)

Measurement is often required of those environmental factors that most nearly correlate with deep body temperature and other physiological responses to heat. At the present time, the Wet Bulb Globe Temperature Index (WBGT) is the most used technique to measure these environmental factors. WBGT values are calculated by the following equations:

Equation III:4-4. Indoor or Outdoor Wet Bulb Globe Temperature Indexes (WBGI)

Indoor or outdoors with no solar load

WBGT = 0.7NWB + 0.3GT

Outdoors with solar load

WBGT = 0.7NWB + 0.2GT + 0.1DB


WBGT = Wet Bulb Globe Temperature Index

NWB = Natural Wet-Bulb Temperature

DB = Dry-Bulb (air) Temperature

GT = Globe Thermometer Temperature

The determination of WBGT requires the use of a black globe thermometer, a natural (static) wet-bulb thermometer, and a dry-bulb thermometer. The measurement of environmental factors shall be performed as follows:

  1. The range of the dry and the natural wet-bulb thermometers should be -5°C to +50°C, with an accuracy of ±0.5°C. The dry bulb thermometer must be shielded from the sun and the other radiant surfaces of the environment without restricting the airflow around the bulb. The wick of the natural wet bulb thermometer should be kept wet with distilled water for at least one-half hour before the temperature reading is made. It is not enough to immerse the other end of the wick into a reservoir of distilled water and wait until the whole wick becomes wet by capillarity. The wick must be wetted by direct application of water from a syringe one-half hour before each reading. The wick must cover the bulb of the thermometer and an equal length of additional wick must cover the stem above the bulb. The wick should always be clean, and new wicks should be washed before using.
  2. A globe thermometer, consisting of a 15 cm (6-inch) in diameter hollow copper sphere painted on the outside with a matte black finish, or equivalent, must be used. The bulb or sensor of a thermometer (range -5°C to +100°C with an accuracy of ±0.5°C) must be fixed in the center of the sphere. The globe thermometer should be exposed at least 25 minutes before it is read.
  3. A stand should be used to suspend the three thermometers so that they do not restrict free air flow around the bulbs and the wet-bulb and globe thermometer are not shaded.
  4. It is permissible to use any other type of temperature sensor that gives a reading similar to that of a mercury thermometer under the same conditions.
  5. The thermometers must be placed so that the readings are representative of the employee's work or rest areas, as appropriate.

    Once the WBGT has been estimated, employers can estimate workers' metabolic heat load (see Tables III:4-1 and III:4-2) and use the ACGIH method to determine the appropriate work/rest regimen, clothing, and equipment to use to control the heat exposures of workers in their facilities.

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