What is Heat Stress

Heat stress is the effect that the thermal environment has on a person’s ability to maintain a normal body temperature.  Physical work generates heat in the body which must be lost to the environment through sweating and evaporation. A hot or humid environment makes this more difficult and this can affect both mental and physical performance. Inability to get rid of body heat adequately may result in heat illness.

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.

Risk factors for heat illness

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.

Types of Heat Stress

Heat cramps are involuntary muscle contractions caused by failure to replace fluids or electrolytes, such as sodium and potassium. Cramps can be relieved with stretching and by replacing fluids and electrolytes.

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.

Heat exhaustion is characterized by weakness, extreme fatigue, nausea, headaches, and a wet, clammy skin. Heat exhaustion is caused by inadequate fluid intake. It should be treated by resting in a cool environment and replacing fluids and electrolytes.

Heat stroke is a medical emergency caused by failure of the body's heat controls. Sweating stops and the body temperature rises precipitously. Heat stroke is characterized by hot dry skin, a body temperature above 105.8 F (41 C) mental confusion, loss of consciousness, convulsions, or even coma. Send for medical help at once and begin rapid cooling with ice or cold water, fanning the victim to promote evaporation. Treat for shock if necessary. For rapid cooling, partially submerge the victim's body in cool water.


You can prevent the serious consequences of heat disorders by improving your level of fitness and becoming acclimated to the heat.

Maintaining a high level of aerobic fitness is one of the best ways to protect yourself against heat stress. The fit worker has a well-developed circulatory system and increased blood volume. Both are important to regulate body temperature. Fit workers start to sweat sooner, so they work with a lower heart rate and body temperature. They adjust to the heat twice as fast as the unfit worker. They lose acclimatization more slowly and regain it quickly.

Acclimatization occurs in 5 to 10 days of heat exposure as the body:

  • Increases sweat production
  • Improves blood distribution
  • Decreases the heart rate, and lowers the skin and body temperatures.

You can acclimatize by gradually increasing work time in the heat, taking care to replace fluids, and resting as needed. You maintain acclimatization with periodic work or exercise in a hot environment.

Recommendations for Employers

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

  • Schedule maintenance and repair jobs in hot areas for cooler months.
  • Schedule hot jobs for the cooler part of the day.
  • Acclimatize workers by exposing them for progressively longer periods to hot work environments.
  • Reduce the physical demands of workers.
  • Use relief workers or assign extra workers for physically demanding jobs.
  • Provide cool water or liquids to workers.Provide rest periods with water breaks.
  • Avoid alcohol, and drinks with large amounts of caffeine or sugar.
  • Provide cool areas for use during break periods.
  • Monitor workers who are at risk of heat stress.
  • Provide heat stress training that includes information about:
    • Worker risk
    • Prevention
    • Symptoms
    • The importance of monitoring yourself and coworkers for symptoms
    • Treatment
    • Personal protective equipment

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:

  • Wear light-colored, loose-fitting, breathable clothing such as cotton.Gradually build up to heavy work.
  • void non-breathing synthetic clothing.
  • Schedule heavy work during the coolest parts of day.
  • Take more breaks in extreme heat and humidity.Drink water frequently. Drink enough water that you never become thirsty. Approximately 1 cup every 15-20 minutes.
  • Take breaks in the shade or a cool area when possible.
  • Avoid alcohol, and drinks with large amounts of caffeine or sugar.
  • Be aware that protective clothing or personal protective equipment may increase the risk of heat stress.
  • Monitor your physical condition and that of your coworkers.