National Wildfire Coordinating Group

Heat Stress


During activity, the body releases more than 70 percent of the energy used by muscles as heat.  As a result, heat loss is crucial to prevent an excessive rise in body temperature during firefighting activities. If heat exchange between the body and the environment is impaired, such as in hot or humid environments, firefighter performance can be substantially impaired.

A heat-related illness (HRI) is a potentially fatal disorder caused by elevated body temperatures from internal heat produced by activity or external environmental heat added to the body that cannot be removed to maintain normal body temperature.

Symptoms of an HRI may be difficult to recognize and may occur in no particular order. They may include:

  • Profuse sweating with cool, clammy skin leading to hot, dry skin.
  • Muscle cramps and weakness.
  • Dizziness, headache, and irritability.
  • Rapid, weak pulse.
  • Vomiting.
  • Loss of consciousness.

If you suspect a heat illness, stop work and begin treatments to cool down the body, and consider further procedures.

Considerations for mitigation during firefighting activity:

  • Ability to handle heat is different between individuals and varies on a daily basis.
  • Performing physical tasks, such as hiking or digging fireline, is our largest producer of body heat.
  • Hiking typically raises your body temperature 1 to 2 degrees Fahrenheit.
  • At elevated body temperatures, the risk of HRI has a lesser margin of error.
  • Maintain low skin temperature when possible, as it allows heat transfer from the body.
  • Pack weights exceeding 25 percent of body weight add to the body’s demands during activity.
  • Recovery of high body temperature requires:
    • Reduction of work output.
    • Removal from sources of heat.
    • Proper nutrition and hydration strategies.

Note: Recovery is not just a shift-to-shift concept. It is also task-to-task and even a minute-to-minute process.


Firefighter Health & Safety
Page Last Modified / Reviewed: 
Mar 2022

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