According to the passage, what is often the first warning of heat illness?

The body cools itself mainly by sweating. But in very hot and humid weather, that cooling system can be overwhelmed. If the body heats up faster than it can cool down, or loses too much water and salt through sweating, its temperature rises, and a person can fall ill.

Heat illness often comes in stages. The first warning may be heat cramps: painful muscle cramps, usually in the legs or stomach, with heavy sweating. Resting, gently massaging the muscle and taking small sips of water can help.

If the body keeps heating, heat exhaustion can follow. The person may sweat heavily and feel weak, dizzy or sick, with cool, pale, clammy skin and a fast, weak pulse. They should be moved somewhere cooler, their clothing loosened, and cool, wet cloths put on their skin.

The most dangerous stage is heat stroke. The body's temperature rises above about 39.5 degrees Celsius. The person may have a throbbing headache, become confused, faint or lose consciousness, and their skin may be hot and red. Heat stroke is a medical emergency: the person must get to a hospital immediately, because delay can be fatal.