Here's what happens to the body in extreme temperatures — and how heat becomes deadly

It was 118 degrees after 5 p.m. in Phoenix, Arizona on July 18, 2023. More than 80 million people have been under heat warnings, as relentless temperatures continue to bake the western and southern U.S. (Patrick T. Fallon/AFP via Getty Images)

Of all extreme weather conditions, heat is the most deadly. It kills more people in the U.S. in an average year than hurricanes, tornadoes and floods combined. The human body has a built-in cooling mechanism – sweat. But that system can only do so much, especially in soaring temperatures with high humidity.

Here's a look at what happens to the human body in extreme temperatures – and the three main pathways to fatal consequences.