They typically occur during or after intense exercise in the heat and usually involve the legs. Sodium is lost in the sweat, and is further diluted in the blood if plain water is used to replace fluid losses. Heat cramps usually resolve with rest, cooling down, and massaging the affected muscles. Prevention involves acclimitization to heat and staying on top of fluid and salt status before exercising. It is most important to note that heat cramps may be a warning sign of a more serious heat illness.
Heat exhaustion is more complex and more dangerous.
Victims continue to sweat, become weak and lightheaded, and may become somewhat confused. Treatment involves rest, cooling, and appropriate rehydration. Cases of heat exhaustion should be treated in a medical facility where sodium can be measured and fluid can be replaced at the right rate. Again, prevention involves acclimitization and good hydration practices.