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Heat stress is probably not a subject we give much attention to regarding laboratories

Vince McLeod, CIH

Vince McLeod is an American Board of Industrial Hygiene-certified industrial hygienist and the senior industrial hygienist with Ascend Environmental + Health Hygiene LLC in Winter Garden, Florida. He has more...

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Dealing With Heat Stress

Heat stress is probably not a subject we give much attention to regarding laboratories—most of us likely work in comfortable air-conditioned spaces. But there are situations and facilities where heat stress could be a concern for workers. These might include cage wash areas of labs involved in animal research or production, or perhaps there are a large number of furnaces or reactors in a small area of a research lab. And then there are the field situations that occur outdoors, especially in the South and during the summer months all over the country. All of these and many more could pose a potential threat of heat stress. So let us enlighten those of you who may need to address one of these scenarios with the basics of heat stress—the signs, symptoms, and proper management.

Heat stress defined

Workers who are exposed to extreme heat for extended periods or work in hot environments may risk developing heat stress.1 Workers at greater risk of heat stress include older workers (65+ years of age), overweight workers, those with underlying health conditions such as heart disease or high blood pressure, or those taking medications that make them more sensitive to extreme heat.

Dehydration is the primary cause of heat stress, which can range in severity from reduced strength to nausea to unconsciousness—and even to death if not dealt with immediately. After two hours of moderate work activity, workers can lose up to 1.5 liters of fluid, and may begin to experience the initial stages of heat stress. Add heat, humidity, work intensity, and/or protective clothing, and the risk of developing heat stress is magnified. Replacing fluids lost in sweating is a must for proper heat stress control.

Stages of heat stress

Occupational illnesses and injuries resulting from heat stress are generally placed in three categories: heat cramps (with or without rash), heat exhaustion, and heat stroke. We describe the signs and symptoms of each category in detail below and list appropriate counteractions.

Heat cramps

This is the first stage of heat stress and the least troublesome. Profuse sweating while performing strenuous work depletes the body’s salt and water levels, which causes cramps, usually in the muscles of the arms, legs, or abdomen. If ignored, the heat stress will progress to the next levels rather quickly. If cramps develop, workers should stop work and rest in a cool place, drink water or a sports beverage (at ambient temperature for fastest absorption), and avoid strenuous tasks until the cramps have completely subsided.

Heat exhaustion

The second stage of heat stress is heat exhaustion and results from excessive loss of water and salt. Symptoms include heavy sweating, weakness or fatigue, nausea, clammy or moist skin, flushed complexion, and elevated body temperature. Fast, shallow breathing, muscle cramps, and confusion may also develop. To counteract these symptoms, move the worker to a cool and shady or air-conditioned space. Make sure they drink plenty of water or a healthful sports beverage. In addition, a cool shower or bath can be used to lower the worker’s body temperature, if indicated.

Heat stroke

Heat stroke is the most serious stage of heat stress, and it is rapidly reached if the first two stages are ignored. After extreme water and salt loss, the body’s sweating mechanism shuts down, rendering the body incapable of regulating its temperature and cooling down. The internal core temperature rises quickly to 106°F or higher, resulting in hot, dry skin; chills; confusion; slurred speech; hallucinations; and intense headache. If a worker reaches this stage, you must call emergency services immediately. If the progression of symptoms is not stopped and reversed, death is a possibility. Move the worker to a cool area and soak their clothes with water and continue spraying or showering, and fanning them until help arrives.

Heat stress regulations

OSHA does not have a standard governing heat stress prevention in industrial areas. The General Duty clause (§ 5(a)(1) of the Occupational Safety and Health Act) requires that an employer furnish each employee with employment and places of employment free from recognized hazards causing or likely to cause death or serious physical harm.

Heat stress guidelines (e.g., Threshold Limit Values, or TLVs) have been published by the American Conference of Governmental Industrial Hygienists (ACGIH), which establish work/rest regimens based on temperature and work stresses. However, careful consideration regarding the rigid application of work/rest regimens is recommended. For example, moderate work in an 88°F wet-bulb globe temperature (WBGT) area triggers a “25 percent work, 75 percent rest” regimen under current ACGIH TLVs. Each possible heat stress situation needs a detailed evaluation prior to the development of realistic controls.2

Employers should start with measuring the full range of temperature and humidity conditions of the work areas. Measurement is often performed under environmental factors that most nearly correlate with deep body temperature and other physiological responses to heat. At the present time, the WBGT index is the technique typically used to assess these environmental factors. Conduct a heat stress survey using a calibrated heat stress data logger. The WBGT readings are useful in establishing the relative degree in which work areas are hotter than ambient temperature.

OSHA has published Using the Heat Index to Protect Workers,3 which indicates an increased risk when the ambient heat index is greater than 91°F. Strenuous work combined with the use of protective clothing creates an additive effect and can increase risk. Extra precautions are recommended in these circumstances. Finally, understand that there are limitations to human endurance and unique individual responses to situations. Use of the heat index and WBGT are recommended in developing your site-specific heat stress policy. These measurements, combined with the level of work effort, are recommended for decision-making, especially during the summer months, and particularly in light of using the ACGIH guidance.


1. Heat Stress, National Institute of Occupational Safety and Health, US Department of Health and Human Services. Atlanta, GA. 2016 

2. Combating Occupational Heat Stress: Getting Past the Basics, Bancroft, Kris. Occupational Health & Safety Journal. 2003. 

3. Using the Heat Index: A Guide for Employers, Occupational Safety & Health Administration, US Department of Labor. Washington, D.C. 2016.