How extreme heat affects the body — and who's most at risk

Pittsburghers flock to the Water Steps at the Riverfront Park along the Allegheny River
People enjoying Riverfront Park in Pittsburgh. (Jeff Swensen/Getty Images)

Nearly 77 million Americans across the Midwest and Northeast are under heat alerts this week, with the National Weather Service warning of dangerously hot temperatures as high as the triple digits in many areas.

In Phoenix, temperatures are forecast to reach 113 degrees on Thursday — the first day of summer — followed by 115 degrees on Friday.

Extreme heat like this can be lethal, according to the Centers for Disease Control and Prevention. Heat-related deaths in the U.S. have been increasing over the past few years, with about 1,600 in 2021; 1,700 in 2022, and 2,300 in 2023, according to the Department of Health and Human Services.

As temperatures are expected to scorch parts of the U.S. this week, here’s some important info on what extreme heat can do to the body, who is most at risk and heat-related illnesses to watch out for:

🌡️ What happens to your body in extreme heat

A normal human’s body temperature ranges from 97°F-99°F. The body’s temperature needs to be regulated in order for internal organs to function properly. When your brain senses a change in body temperature, either hot or cold, it tries to help your body readjust.

When the body’s temperature is too hot, one of the most common ways the body cools itself is through sweat, which then evaporates in dry heat, thus cooling the body.

The other way the body cools itself is by moving warmer blood away from the internal organs to capillaries at the surface of the skin. That’s why people look flushed when their body temperature is elevated.

Heat-related illnesses can set in when the air temperature is hotter than the skin’s temperature, around 90°F, because it’s more difficult for your body to cool itself. When there’s extreme heat combined with humidity, sweat doesn’t evaporate as easily. That means your body’s temperature rises even higher, according to the Mayo Clinic.

👶 Who is most vulnerable to extreme heat?

According to the National Institute of Health and the CDC, the following groups are most at risk in extreme heat:

  • Children: The way their bodies regulate internal body temperatures can make them overwhelmed more quickly.

  • Older adults: They’re more likely to have a chronic medical condition or to be taking medications that affects the body’s response to heat.

  • People with chronic medical conditions: They’re less likely to sense and respond to changes in temperature.

  • Pregnant people: Their bodies must work harder to cool down not only their body, but the developing baby’s as well.

  • People experiencing homelessness: Those unsheltered or experiencing housing insecurity are more exposed to extreme heat.

  • Athletes and outdoor workers: Those who exercise or do strenuous work outside in extreme heat are more likely to become dehydrated and develop a heat-related illness

  • Pets: They can develop heat-related illnesses too.

To find out information on cooling centers in your state, the National Center for Health Housing provides a list.

🥵 Heat-related illness symptoms to watch out for:

The Centers for Disease Control provides a guide for what to watch for and what to do to prevent heat-related illnesses like heat stroke, heat exhaustion, heat cramps, sunburn and heat rash.