Summer has just started, and already, cities across the country have seen heat waves and record-breaking temperatures: 96 degrees in Bangor, Maine; 98 degrees in Boston; 101 degrees in Albuquerque; a record-tying 117 degrees in Phoenix.

And that’s just the U.S. There have also been record-high temperatures around the world. Human-caused climate change means heat waves are getting both more frequent and more intense, exposing millions of people to extreme temperatures.

These high temperatures can be deadly. In Greece, multiple tourists recently died during a wave of 100-plus degree days. In Saudi Arabia, more than 1,300 people taking part in the Hajj pilgrimage have died as temperatures reached as high 114 degrees. Among weather-related deaths in the U.S., heat is the leading cause, killing more people annually than hurricanes, floods, and tornados.

But even when heat isn’t deadly, it takes a toll on our bodies. And as climate change worsens, even more people will be exposed to extreme temperatures. Scientists are now working to understand all the ways it impacts both our bodies and minds.

“The field started with recognizing the importance of heat stroke, heat exhaustion, dehydration—some of these things that are sort of easy to picture as very related to heat,” says Gregory Wellenius, an environmental epidemiologist and director of Boston University’s Center for Climate and Health. “As the number of studies has grown, we’ve come to realize that those are still true, and there’s a bigger, more complex picture of more subtle health effects as well.”

HOW HEAT IMPACTS OUR BODIES

Exposure to extreme heat can cause dehydration, heat exhaustion, and heat stroke. There’s also evidence that on hotter days, more people are hospitalized with conditions related to kidney diseases. High temperatures can worsen chronic health conditions, from respiratory diseases and migraines, to arthritis and asthma. That could because of the illnesses themselves (for someone with diabetes, for example, heat causes fluctuations in blood sugar, making the disease more difficult to control) or because some medications can make heat’s effects even worse. Different medications can affect sodium levels so you dehydrate more easily, or even impair your body’s ability to sweat or circulate blood, and thus your its ability to regulate its temperature.

Hot temperatures can also affect pregnant women. Studies have linked heat waves to increased risks of stillbirth and preterm births, as well as conditions like preeclampsia. That heat can affect the developing fetus, as well, leading to low birth weight or birth defects. Higher temperatures are also associated with more skin diseases, such as eczema, dermatitis, and rosacea flare-ups.

Heat also affects sleep—making it harder to fall asleep and reducing the time spent in the REM and deep sleep states—which in turn can have more negative effects on our health. “It’s harder to sleep well when it’s very hot,” Wellenius says, adding that a small but emerging number of studies suggest some of the general health impacts of heat may be brought about in part because of disrupted sleeps.

HOW HEAT AFFECTS OUR MENTAL HEALTH

Heat takes a toll on our minds, too. High temperatures are associated with increased irritability, depression, and even suicide rates. Wellenius says his research has shown a relationship between more days of heat and increased emergency department visits for a variety of mental health conditions and mood disorders. Heat is also associated with higher rates of violence and aggression, including intimate partner violence and gun violence. Researchers have even linked high temperatures to an increase in hate speech online.

Extreme heat also affects how well our brains work. High temperatures are associated with poorer test scores, affecting students’ academic performance—an issue that becomes even more apparent among low-income and minority students, whose schools often lack air conditioning. One study found that for every degree the temperature increased above 72, high school standardized test scores fell 0.2%, which can quickly add up during a heat wave. The impact isn’t limited to students: Research has found that politicians tend to use shorter, simpler words in their speeches on hot days.

There’s “a wide and growing literature,” Wellenius says, that suggests heat impairs judgment and decision making, too. That means people are more likely to make risky decisions or behave impulsively. Heat’s impact on sleep means our brains may be fatigued, and heat also affects our general cognition—including coordination and concentration—meaning productivity may go down, and also accidents may be more common.

Studies have found that occupational injuries happen more on hotter days, whether workers are outside or inside, including incidents like falling off of a ladder or getting your hand caught in machinery. When temperatures top 90 degrees, workers have a 6% to 9% higher risk of injuries. (There’s still no federal heat exposure standard when it comes to protecting workers, and only five states guarantee workers things like shade, breaks, and water. Some states including Florida and Texas even prevent cities from requiring such protections.)

Those incidents can occur outside of workplaces, too. Studies have linked heat with a rise in car accidents. While there are physical elements at play—the sun might be in drivers eyes, for example, or tires may expand and blow out—research has linked those accidents to impaired judgment, concentration issues, slow reactions, and other driving errors.

WHO IS VULNERABLE TO HEAT

In the U.S., nearly two-thirds of the population lives somewhere vulnerable to extreme summer temperatures. As human-caused climate change worsens, even more people will be exposed to extreme heat and all the health effects that come with it. “Heat can impact anybody anywhere anytime,” Wellenius says, “but it’s inescapable that some people and some communities are at much greater risk than others.” Those who work indoors or who have access to air conditioning (and the means to pay for it) are at a lower risk than those who work outside, those who can’t afford air conditioning, or those who are homeless.

When heat occurs can also have an impact. “There are a handful of studies showing that heat waves early in the season have bigger adverse health impacts than heat waves late in the season,” he adds. That could be for a few reasons: Later heat waves might give your body more time to acclimate to the weather, just like the first freeze in winter feels colder. Early heat waves may also catch people off guard, before their AC is installed or public pools are open.

As researchers continue to understand how heat affects us, Wellenius expects we’ll uncover even more health impacts of high temperatures. He also hopes people realize the risks of heat, and that communities prepare for heat waves just as they do a blizzard or hurricane. (There has been a recent push to name heat waves as we do other storms; labor and environmental groups are also asking FEMA to recognize extreme heat as a natural disaster.) Wellenius says there’s even emerging research showing that people do recognize that heat poses a health harm—but for other people, not themselves. “One of the key messages,” he says, “is that heat can impact anybody.”