Division of Agriculture and Natural Resources
Division of Agriculture and Natural Resources
Division of Agriculture and Natural Resources
University of California
Division of Agriculture and Natural Resources

10 key points about heat stress

By Howard Rosenberg, UC Cooperative Extension specialist emeritus

  1. Functions of the human body depend on blood circulation and chemical reactions that best occur at about 98.6 degrees F. Your body has natural ways of gaining or losing heat to maintain that “normal” temperature.

  2. The main source of heat that may stress you is your own body. In using its stored energy for physical work, about three-fourths of the energy turns into heat, only one-fourth into motion. An active body usually generates more heat than it needs and therefore has to release some.

  3. The harder you work, the faster you generate heat, and the more your body has to get rid of. Hot weather, high humidity, and insulating clothes increase your risks of stress mainly by slowing the transfer of excess body heat to your surroundings.

  4. When you produce heat that raises internal temperature, your heart rate quickens and vessels expand to bring more blood to the outer layers of skin, from which heat it carries can gradually flow to the environment.

  5. If excess heat is not released fast enough this way, your sweat glands become more active. They draw water from the bloodstream to make sweat that carries heat through pores and onto your skin surface, where it evaporates and releases the heat.

  6. When more blood flows toward your body surface for cooling, less is available to serve your muscles, brain, and other internal organs. And as prolonged sweating draws water out of the bloodstream, it further reduces capacity to deliver nutrients, clear out wastes, lubricate joints, and cool you later. You can expect to sweat out one quart of water or more during an hour of heavy work in hot weather, 3/4 quart during moderately strenuous work.

  7. Continual loss of water makes you increasingly likely to experience symptoms of “heat illness” -- general discomfort, loss of coordination and stamina, weakness, poor concentration, irritability, muscle pain and cramping, fatigue, blurry vision, headache, dizziness, nausea, confusion, and unconsciousness. These and even milder effects of heat stress also increase your chance of accidental injury.

  8. The single most important way to reduce heat stress risks while working is to steadily replenish the water you lose as sweat. Drinking small amounts frequently, such as 6-8 ounces every 15 minutes, is more effective than taking large amounts less often.

  9. Relying on thirst as the signal to drink is dangerous. Most people do not feel thirsty until their fluid loss reaches 2% of body weight and is already affecting them.

  10. If you notice heat illness symptoms, rest to stop generating heat, get fluids, and tell a supervisor as soon as possible. A person whose fluid loss is 8% of body weight is likely to have a core temperature above 104 degrees and serious risk of heat stroke -- a life-threatening emergency in which the brain is deprived of oxygen and the body can no longer cool itself. Please don't let yourself or a co-worker get to this condition. But if you do, call for medical help right away.
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