Heatstroke occurs when your body’s thermostat cannot keep your body cool. Your body relies on water evaporation to stay cool. As your temperature rises, your body reacts by sweating. When this sweat evaporates, it cools your body. The amount of moisture in the air (humidity) determines how readily sweat evaporates. In very dry air, sweat evaporates easily, quickly cooling your body; but in very humid air, sweat does not evaporate. It may collect on the skin or run off your body without affecting your body’s climbing temperature. Extremely warm and humid temperatures can quickly overwhelm your body’s cooling system—particularly when the air is not circulating. When sweating can no longer keep you cool, body temperature quickly rises, causing the symptoms of heat-related illness. Sunstroke is a type of heatstroke. Heatstroke is a condition that occurs after exposure to excessive heat. In sunstroke—also called heat illness, heat injury, hyperthermia, heat prostration, and heat collapse—the source of heat is the sun. Other types of heatstroke occur after exposure to heat from different sources. Heatstroke—including sunstroke—is considered to be the most severe of the heat-related illnesses. Heat can have punishing effects on your body. After excessive exercise or physical labor, your body can overheat, and you may suffer heat exhaustion. Heat cramps occur after excessive loss of water and salt; usually resulting from excessive sweating, or after strenuous exercise or labor. During heat exhaustion and heat cramps, the heat-controlling system is still intact, but can be overwhelmed. If this happens, heat exhaustion can progress to heatstroke, a life-threatening medical condition. In severe cases, heatstroke can even cause organ dysfunction, brain damage, and death.