Heatstroke, also called sunstroke, is the most serious heat-related illness. It occurs when the body's temperature is 104ºF or higher, and it is a life-threatening medical emergency. If not treated immediately, heatstroke can damage multiple organs.

Heat exhaustion is less serious than heatstroke. Anyone who suspects that they have heat exhaustion should immediately rest and rehydrate. If symptoms do not improve, seek medical attention to prevent heatstroke.