Heat stroke or heat illness is a form of hyperthermia that comes about as a result of exposure to external environmental heat rather than an internal lack of thermoregulation such as a fever. It is defined as a body temperature of 42 C (about 105 F) combined with an inability to regulate one's own body temperature.
In warm conditions, the body regulates its body temperature by sweating, the evaporation of which cools the body. However, if sweating is inhibited (typically by heavy clothing) or the body is dehydrated due to lack of water intake, body temperature starts to rise rapidly, eventually resulting in unconsciousness as an attempt to stop the generation of heat through the normal metabolism of burning food as much as is possible. This process can start at temperatures as low as 40 C (about 101 F).
Heat stroke requires immediate hospitalization, but in the meantime the patient should be cooled as rapidly as possible with any cool liquid. Immersion in water is acceptable, but very cold water should be avoided as this can lead to constriction of the blood vessels in the skin, actually leading to heat being preserved in the body's organs. Clothing should be removed as far as possible to encourage heat loss from the skin. Wet towels should not be used to wrap the patient as they tend to keep body heat in. If conscious, water should be consumed slowly to prevent secondary complications, and should be consumed with a small amound of salt or food to maintain the balance of electrolytes in the blood. Moving the patient into shade or an air conditioned environment is also recommended. Alcohol should not be used for cooling as this will cause further dehydration. In all cases the patient should be closely monitored until emergency medical personnel arrive.
To avoid heat stroke, avoid alcohol and stimulants of any type in hot weather, wear very light loose fitting clothing, drink plenty of water, and cease exercising immediately if you feel light-headed or nauseous.