Heat kills more people than any other extreme weather event, and deadly heat waves are getting longer and hotter as the climate warms.

Heatstroke is a medical emergency. If you notice signs of heatstroke in a person, call 911 immediately.

This summer, huge swaths of the US have already faced record-breaking heat waves.

Staying cool—and informed about the risks heat poses—is essential.

Thomas Clanton is a professor of applied physiology and kinesiology at the University of Florida and an expert in the effects of heat on the body. Here, he explains how to recognize heat illness and the long-term consequences of this kind of stress: