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.

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.

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.

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. 

Relying on thirst as the 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, rest to stop generating heat, get fluids, and tell a supervisor as soon as possible.