The sweltering triple-digit heat is not only uncomfortable, it can add significantly to risks of strenuous work and other physical activity. To help people avoid the dangers of becoming overheated, the University of California Cooperative Extension has heat-stress information online at http://ucanr.org/News/Heat  and by phone at 1 (800) 514-4494.
A downloadable card for farmworkers explains in English and Spanish how heat-related illnesses develop and how to avoid them.
Although the advice is directed at farmworkers, it is useful to anyone who works in the heat.
"Excess heat affects your body and can impair your functioning even before you feel ill," warned Howard Rosenberg, UC Cooperative Extension specialist emeritus. He advises people working outdoors in hot weather to drink fluids to replenish the water the body loses as it sweats, whether or not they feel thirsty, and to slow their bodies' internal production of heat by moderating their level of exertion.
Rosenberg's 10 key points about heat stress:
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 to 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 percent 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. Don't let yourself or a co-worker get to this condition. But if you do, call for medical help right away.