Why Extreme Heat Is So Bad for the Human Body

Blistering temperatures aren’t just uncomfortable. They can quickly escalate to become life-threatening: According to data from U.S. Centers for Disease Control and Prevention researchers, more than 700 people nationwide died from heat-related causes annually from 2004 to 2018. Some research suggests that the death toll will rise in the coming years as climate change makes extreme heat more common.

Here’s what you need to know about the health risks of extreme heat and how to understand your own risk.

What happens when the body gets too hot

Human beings evolved in tropical climates and can tolerate heat well by releasing it into the environment through the skin. But when the air gets hotter than skin temperature (which is typically 97-99° F) or if sweat doesn’t evaporate, “we start to gain heat, and our body core temperature—the temperature of our deep body tissues—starts to rise,” says W. Larry Kenney, a professor of physiology and kinesiology at Penn State University who studies human temperature regulation. “If that rise is unabated, and it keeps going up, in some people it can lead to heat-related illnesses.”

Some people are at higher risk than others

While everyone is susceptible to the health effects of heat, some people are at much greater risk than others. Infants and children are particularly vulnerable, in part because they lose fluid more quickly than adults and have to rely on caretakers to help cool them down, according to the Environmental Protection Agency (EPA). Certain medications can also make people more vulnerable to heat. Some antidepressants and antipsychotics can affect sweat production, while drugs for treating heart disease, such as ACE inhibitors, can cause dehydration and affect the kidneys. Older people tend to be more likely to have health conditions like diabetes that put them at higher risk, and their bodies respond differently to heat than those of younger people. They produce less sweat per gland, Kenney says, and blood vessels change as people age in ways that makes it harder for blood to get pumped to the skin and cool people down.

It can cause heatstroke

Heatstroke happens when the body reaches a core temperature of at least 104° F, which can lead to organ failure, brain damage, and even death. One of the reasons heatstroke is so dangerous is that it can cause cognitive impairment. Patients with heatstroke often “don’t know where they are, how they got there, they don’t know what day it is. Eventually, they may lose consciousness, and if their body temperature continues to rise, they would eventually die,” says Kenney.

Those most vulnerable to heatstroke include older people and children, but even younger adults can get heatstroke if they don’t take steps to cool off. It most often affects people who work outdoors, military personnel, and athletes who “may ignore warning signs and keep pushing on with intense physical activity,” says Kenney. People are often more vulnerable to heatstroke if there’s a sudden increase in temperature, like a heat wave, and their body isn’t acclimated to the heat. That’s why football players sometimes develop heatstroke when they start training in the summer, says Kenney; they aren’t accustomed to working out in high temperatures.

To address heatstroke, people must be cooled down as quickly as possible—preferably by dunking them up to their neck in ice-cold water, says Kenney. It’s also essential that they’re checked out by emergency health care workers or a doctor, who can ensure that their body temperature has cooled down and that their organs aren’t failing.

Extreme heat can fuel mental-health crises

High temperatures can have profound effects on mental health. Research has found that temperature increases are linked to higher suicide rates; one 2018 study published in Nature Climate Change found that for every 1° C increase in the monthly average temperature in the U.S., suicide death rates increased by 0.7%. The authors hypothesize that high temperatures may induce negative changes in mental state.

Heat may also contribute to more emergency department visits for mental-health crises. In a study published in JAMA Psychiatry in 2022, researchers who evaluated 3.5 million emergency department visits found that higher warm-season temperatures increased the risk of ER visits for any mental-health condition, including substance use disorder, anxiety disorders, and schizophrenia. Amruta Nori-Sarma, an assistant professor at Boston University School of Public Health and the first author of the study, says that the findings likely underestimate how severely temperature affects mental health because the data only included people with commercial health insurance or Medicare Advantage. The study also didn’t capture mental-health crises that didn’t involve hospitalization.

More research is necessary to understand why extreme heat exacerbates mental health issues, but Nori-Sarma suspects that poor sleep is one of several factors. “We know that people struggle to sleep well when it’s very hot outside,” she says. “And we also similarly know that people who have disrupted sleep patterns may experience exacerbations in their existing mental health.”

High heat is bad for the heart

High temperatures can be more dangerous for people with a range of preexisting health conditions—from Type 2 diabetes to COPD—compared to the general population. Cardiovascular problems are particularly risky: high temperatures can strain the heart and lead to heart attacks, endangering people who have heart problems. “When our body temperature starts to rise, one of the things that happens is that our heart rate goes up, and our heart pumps harder to try to pump blood flow to the skin to get rid of that heat,” Kenney explains. About a quarter of heat-related deaths are caused by a combination of heat and cardiovascular disease, according to the EPA.

Rising body temperatures also make people breathe harder, which can be dangerous for people whose respiratory systems are compromised by conditions like asthma and COPD. Air pollution and allergens like pollen—two outdoor conditions that typically accompany high heat—often make this worse.

Dehydration can also lead to kidney problems. As the body heats up, kidneys decrease their outflow of urine as blood flow decreases away from internal organs to the skin and body fluid is secreted as sweat. Over time, chronic dehydration can lead to kidney failure.

As extreme heat becomes more common with climate change, new threats are emerging. Different extreme events are increasingly happening at the same time, says Nori-Sarma, like a heat wave coinciding with a drought, hurricane, or power outage. Extreme heat is also threatening places where high temperatures haven’t been much of a concern before—like the Pacific Northwest—and those regions may not be equipped with tools like air conditioning.

To reduce risk, says Nori-Sarma, regions should develop “heat adaptation plans” that include resources, like cooling centers, targeted to vulnerable communities.

However, people in communities will also need one another, she says. “It’s really important to make sure that neighbors are checking in on neighbors, and friends are checking in on friends, because that can be one of the best ways to make sure that people are okay during these extreme heat periods,” she says.