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NIOSH Safety and Health Topic:

Heat Stress

Workers drinking waterFire fighter working in intense heatWorkers casting hot metal


Workers who are exposed to extreme heat or work in hot environments may be at risk of heat stress. Exposure to extreme heat can result in occupational illnesses and injuries. Heat stress can result in heat stroke, heat exhaustion, heat cramps, or heat rashes. Heat can also increase the risk of injuries in workers as it may result in sweaty palms, fogged-up safety glasses, and dizziness. Burns may also occur as a result of accidental contact with hot surfaces or steam.

Summer Hazards for Workers – now on the NIOSH Science Blog. Read it and share your comments...

Workers at risk of heat stress include outdoor workers and workers in hot environments such as firefighters, bakery workers, farmers, construction workers, miners, boiler room workers, factory workers, and others. Workers at greater risk of heat stress include those who are 65 years of age or older, are overweight, have heart disease or high blood pressure, or take medications that may be affected by extreme heat.

Prevention of heat stress in workers is important. Employers should provide training to workers so they understand what heat stress is, how it affects their health and safety, and how it can be prevented.

Types of Heat Stress

Heat Stroke | Heat Exhaustion | Heat Syncope | Heat Cramps | Heat Rash

Heat Stroke

Heat stroke is the most serious heat-related disorder. It occurs when the body becomes unable to control its temperature: the body's temperature rises rapidly, the sweating mechanism fails, and the body is unable to cool down. When heat stroke occurs, the body temperature can rise to 106 degrees Fahrenheit or higher within 10 to 15 minutes. Heat stroke can cause death or permanent disability if emergency treatment is not given.


Symptoms of heat stroke include:

First Aid

Take the following steps to treat a worker with heat stroke:

Heat Exhaustion

Heat exhaustion is the body's response to an excessive loss of the water and salt, usually through excessive sweating. Workers most prone to heat exhaustion are those that are elderly, have high blood pressure, and those working in a hot environment.


Symptoms of heat exhaustion include:

First Aid

Treat a worker suffering from heat exhaustion with the following:

Heat Syncope

Heat syncope is a fainting (syncope) episode or dizziness that usually occurs with prolonged standing or sudden rising from a sitting or lying position. Factors that may contribute to heat syncope include dehydration and lack of acclimatization.


Symptoms of heat syncope include:

First Aid

Workers with heat syncope should:

Heat Cramps

Heat cramps usually affect workers who sweat a lot during strenuous activity. This sweating depletes the body's salt and moisture levels. Low salt levels in muscles causes painful cramps. Heat cramps may also be a symptom of heat exhaustion.


Muscle pain or spasms usually in the abdomen, arms, or legs.

First Aid

Workers with heat cramps should:

Heat Rash

Heat rash is a skin irritation caused by excessive sweating during hot, humid weather.


Symptoms of heat rash include:

First Aid

Workers experiencing heat rash should:

Recommendations for Employers

Employers should take the following steps to protect workers from heat stress:

Recommendations for Workers

Workers should avoid exposure to extreme heat, sun exposure, and high humidity when possible. When these exposures cannot be avoided, workers should take the following steps to prevent heat stress:

CDC Resources

MMWR: Heat-Related Deaths among Crop Workers, 1992-2006

CDC: Extreme Heat
Additional information on heat stress illnesses and prevention.
En Español

NIOSH: Criteria for a Recommended Standard: Occupational Exposure to Hot Environments (Revised Criteria 1986)
This document presents the criteria, techniques, and procedures for the assessment, evaluation, and control of occupational heat stress by engineering and preventive work practices. Included are ways of predicting health risks, procedures for control of heat stress, and techniques for prevention and treatment of heat-related illnesses.

NIOSH: Working in Hot Environments
Workers who are suddenly exposed to working in a hot environment face additional and generally avoidable hazards to their safety and health. This publication discusses the safety and health consequences of heat stress.

Health Hazard Evaluations

Other Government Resources

Occupational Safety and Health Administration (OSHA) Safety and Health Topics: Heat Stress
Provides a guide to information regarding the recognition, evaluation, control, and compliance actions involving heat stress.
External link:

OSHA Technical Manual Section III: Chapter 4 - Heat Stress
Provides descriptions of heat disorders, investigative guidelines, sampling methods, control, and PPE.
External link:

OSHA Sawmills eTool: Heat Stresses
Provides information on the hazards of heat stress and possible solutions or controls.
External link:

OSHA Quick Card: Heat Stress
Provides heat stress factors, symptoms, prevention tips, and first aid recommendations.
External link:
En Español
External link:

OSHA Fact Sheet: Protecting Workers from Effects of Heat
Provides information that will help workers understand what heat stress is, how it may affect their health and safety, and how it can be prevented.
this document in PDF  22.18 KB (2 pages)

OSHA Fact Sheet: Working Outdoors in Warm Climates
Hot summer months pose special hazards for outdoor workers who must protect themselves against heat, sun exposure, and other hazards. Employers and employees should know the potential hazards in their workplaces and how to manage them.
this document in PDF 25.38 KB (2 pages)


National Oceanic & Atmospheric Administration's (NOAA) National Weather Service: Heat Index
External link:

NOAA: Heat Wave - A Major Summer Killer
Provides general information regarding the recognition and control of heat stress.
this document in PDF 267.79 KB (2 pages)

Mine Safety & Health Administration (MSHA): Heat Stress - What to Do.
Provides documents related to heat stress in the mining industry.
External link:

United States Department of Agriculture (USDA): Wildland Fire Safety - Heat Stress
This brochure focuses on the risks of heat stress, and what the firefighter should do to minimize those risks.
External link:

U.S. Army: Heat Index Calculator
External link:

Additional Resources

American Conference of Governmental Industrial Hygienists: Product Store - Threshold Limit Values and Biological Exposure Indices
This document is available for purchase at:

American National Standards Institute (ANSI) - Ergonomics of the Thermal Environment: Analytical Determination and Interpretation of Heat Stress Using Calculation of the Predicted Heat Strain
This document specifies a method for the analytical evaluation and interpretation of the thermal stress experienced by a subject in a hot environment. It describes a method for predicting the sweat rate and the internal core temperature that the human body will develop in response to the working conditions.
This document is available for purchase at:

ANSI - Ergonomics of the Thermal Environment: Medical Supervision of Individuals Exposed to Extreme Hot or Cold Environments
This International Standard provides advice to those concerned with the safety of human exposures to extreme hot or cold thermal environments.
This document is available for purchase at:

ANSI - Hot environments: Estimation of the Heat Stress on Working Man, Based on the WBGT-index (Wet Bulb Globe Temperature)
This document gives a method, which can easily be used in an industrial environment for evaluating the stresses on a individual. It applies to the evaluation of the mean effect of heat on man during a period representative of his activity but it does not apply to very short periods, nor to zones of comfort.
This document is available for purchase at:

Texas A&M University - Texas Cooperative Extension: Coping with Hot Work Environments
this document in PDF 124.95 KB (3 pages)
this document in PDF PDF en Español  124.95 KB (3 pages)

State of New Mexico Workers' Compensation Administration - Safety Facts: Working Outdoors
this document in PDF 35 KB (3 pages)

Page last updated: October 22, 2008
Page last reviewed: October 22, 2008
Content Source: National Institute for Occupational Safety and Health (NIOSH)

Heat Stress

Workers in hot environments