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

How heat affects the body

By Howard Rosenberg, UC Cooperative Extension specialist emeritus

Your bodily functions depend on blood circulation and many chemical reactions that occur best at about 98.6 degrees. The body has natural ways of gaining or losing heat to maintain that "normal" temperature.

Your own body is the main source of heat that may stress you. Three-fourths of the energy you convert for physical work turns into heat, only one-fourth into motion. When your body is active, it usually generates more heat than it needs and therefore has to release some.

The harder you work, the faster you generate heat, and the more your body has to get rid of heat. Hot weather and high humidity increase your risks by slowing the transfer of heat to the air around you.

When you produce heat that raises internal temperature, your heart rate increases and vessels expand to bring more blood to the outer layers of skin, where the heat is released.

If excess heat is not released fast enough this way or the surrounding air is warmer than your body, your sweat glands go to work. They draw water from the bloodstream to make sweat that carries heat through pores and onto the skin surface, where it evaporates and releases the heat.

When more blood goes toward your body surface for cooling, less is available to serve your muscles, brain and other internal organs. And prolonged sweating draws a lot of water from the bloodstream, further reducing its capacity to deliver nutrients, clear out wastes, lubricate joints and cool you later. You can easily sweat out one quart of water during an hour of heavy work in hot weather, 3/4 quart in less strenuous work.

If your body continues to lose fluid, you are likely to experience increasingly severe 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 symptoms and even mild effects of heat stress also increase your chance of suffering an accidental injury.

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

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

If you notice heat illness symptoms in yourself (or a co-worker), rest to stop generating heat, get fluids, and tell your supervisor as soon as possible. If fluid loss approaches 8 percent of body weight, there is 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. Don't let it get this far! But if it does, seek medical help right away.

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