In very hot weather, high humidity, and other conditions, body heat can build to dangerous levels. This can cause heat illness, such as heat cramps, heat exhaustion, or heatstroke. Heat exhaustion starts slowly, but if it's not quickly treated it can progress to heatstroke. In heatstroke, a person's temperature reaches 105°F (40.5°C) or higher. Heatstroke needs emergency medical care right away and can be life-threatening.