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Heat Stress
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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.

On the Job

The heat stress chart illustrates how temperature and humidity combine to create moderate or high heat stress conditions.

Be alert for heat stress when radiant heat from the sun or nearby flames is high, the air is still, or when you're working hard, creating lots of metabolic heat.

Some organizations use the WBGT Heat Stress Index. The index uses dry bulb, wet bulb, and black globe temperatures. The temperatures are weighted to indicate the impact of each measure on the worker:

  • Wet bulb (humidity) accounts for 70%
  • Black globe (radiant heat and air movement) accounts for 20%
  • Dry bulb (air temperature) accounts for 10%.




Heat stress indexes do not take into account the effects of long hours of hard work, dehydration, or the impact of personal protective clothing and equipment.

When heat stress conditions exist, you must modify the way you work or exercise. Pace yourself. There are individual differences in fitness, acclimatization, and heat tolerance. Push too hard and you'll be a candidate for a heat disorder.

When possible:

  • Avoid working close to heat sources
  • Do harder work during cooler morning and evening hours
  • Change tools or tasks to minimize fatigue
  • Take frequent rest breaks as you work.

Most important, maintain hydration by replacing lost fluids.


Studies on wildland firefighters indicate that fire suppression activities generate about 7.5 kilocalories of heat each minute worked, or over 400 kilocalories for each hour. Additional heat (about 180 kilocalories per hour) comes from the environment and the fire. The total heat load amounts to 580 kilocalories per hour (400 + 180 = 580).


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