It is important to remember that the thirst mechanism lags behind the body's need for fluid - by the time one is thirsty one is already substantially dehydrated. Even small amounts of dehydration can affect performance, and severe dehydration can be life-threatening. In the cold, care must be taken as well. Garments made of fabrics like wool, which will insulate even when wet, are superior to garments made of materials like cotton, which will contain sweat and can contribute to heat lost by evaporation and conduction as the activity level decreases. People at risk for heat-related illnesses include those who work or exercise outdoors, elderly people, young children, and people with health problems.
People usually try to get out of extreme heat before they begin to feel ill.
However, some people do not or cannot. Athletes and those who work outdoors often keep working even after they begin to feel ill. Those living in poorly ventilated or poorly insulated or poorly heated buildings are at risk of heat emergencies Many times, they might not even recognize that they are in danger of becoming ill. Heat cramps, heat exhaustion, and heat stroke are conditions caused by overexposure to heat. Heat cramps are the least severe, and often are the first signals that the body is having trouble with the heat.