A few weeks ago, the world recorded the hottest day ever, breaking a record set just the day before. The milestone for hottest single temperature on the planet may fall any day now as Death Valley approaches 130 degrees Fahrenheit. And last month was the hottest June ever measured across the world.

It’s the hottest summer of our lives, and summertime isn’t even halfway over.

These are the effects at about 1 degree Celsius of warming — just an early warning sign of a world that, on its current trajectory, is poised to warm three or more times that amount.

Heat is a particular kind of killer. In the United States, it leads to more deaths than hurricanes, tornadoes, flooding, and earthquakes — combined. But heat is also called the silent killer because we simply don’t know all of its effects on our bodies. In fact, we only have data on a slice of the deadly consequences. Heat also puts an unequal burden on more vulnerable populations, harming older and lower-income people, outdoor workers, and prisoners while other parts of society try to look away from inside air-conditioned homes.

There are just as many non-fatal health consequences in a heat wave. The toll grows the longer a heat wave lingers, which leads to the exact conditions a large part of the world finds itself in this summer.

Even our best metric for understanding all of the consequences of heat is flawed. Most health research on heat focuses on deaths and to a lesser extent hospital admissions. Vivek Shandas, Portland State University professor of climate adaptation, calls the focus on mortality overly “blunt measures” that fail to capture how climate change is harming the world’s mental and physical health. While most of the studies discussed below track heat-related deaths, they are more likely missing a bigger picture: There’s an even broader population suffering from heat, but in ways that don’t lead to hospitalization or death.

What’s the physical toll of extreme heat?

Cardiovascular

The primary performer in helping cool the body down is the heart. It is under special strain when temperatures rise, and the vast number of deaths associated with a heat wave isn’t directly from heatstroke, when the organs can shut down from overheating, but because the heart can’t keep up.

Several systems in the body work overtime to keep internal organs cool as temperatures rise. In hotter temperatures, the heart moves more blood to the skin, so that the heat can dissipate into the environment. The evaporation of sweat is essential to the process. But as the body ages, it becomes less efficient at achieving all this, less able to pump enough blood with every beat, worse at increasing skin blood flow, and less able to sweat to cool down. Not only does the body become less efficient at cooling down, but older adults are also more likely to be taking medications that affect their ability to regulate internal temperatures.

Healthy individuals, young and old, aren’t immune either. The cause can be environmental: The Texas Tribune reported that last month at least five prisoners died of heart attacks during heat waves, in a state where two-thirds of prisons lack air conditioning. Two of the men were only 34 and 35.

A mock prison cell made of cardboard and paper stands in front of the Texas state Capitol building in Austin, with onlookers and cameras nearby.

A mock cell serves as a protest against the heat inside Texas prisons on July 18, 2023.
Sergio Flores/AFP via Getty Images

Cardiovascular failure is one reason death counts in a heat wave are highly disputed and chronically undercounted. Health researchers try to make up for the gaps in reporting by looking at excess mortality to nail down more accurate data.

A recent study published in Nature Medicine found that the 2022 summer heat waves led to over 60,000 deaths in Europe, with younger men and older women at higher risk. It almost broke the previous record of as many as 70,000 excess deaths across Europe in the summer of 2003. During a heat dome in the Pacific Northwest two years ago, official estimates registered a few hundred deaths, but ultimately the death toll was likely well over 1,000.

Respiratory

Some of the excess mortality in a heat wave can also be blamed on more air pollution.

Hot, sunny weather enables the chemical reactions that form ground-level ozone. Ozone reacts to lung tissue, causing what the American Lung Association calls a “sunburn on the lungs.”

The summertime rise of ozone is a well-documented and understood phenomenon, but the data on widespread health impacts in a heat wave is usually mixed. In the UK, hospitals have reported a spike in respiratory admissions in summer heat waves, and a report looking at hospitals in Finland found that heat waves between 2001 and 2017 were associated with a 20 percent rise in hospital admissions for respiratory illness like pneumonia.

Air temperature doesn’t just affect what’s in the air, but also how it moves. “Heat domes” have become more common weather patterns, trapping air pollution in stagnant air in the lower atmosphere. An analysis by Climate Central of federal air data back to 1973 found that high summer temperatures closely correlate with more stagnant air. It’s only getting worse. “As the climate warms, stagnant days are projected to increase further, with up to 40 more days per year by late-century,” according to the outlet.

Heat waves make for poor air quality in even more indirect ways. Heat dries up the soil, contributing to drought, and in turn creating the perfect conditions for a wildfire to spark and spread. Wildfires are the main contributor to another dangerous air pollutant, particulate matter, which is tracked by the EPA’s Air Quality Index.

Pregnancy

Extreme heat affects the pregnant person as well as the fetus, and exposure is linked to premature birth and low birthweight. It also strains their cardiovascular system, which is already in overdrive during pregnancy.

Since a pregnant person has a harder time cooling down, it’s more likely for them to experience symptoms like fatigue, muscle cramping, dizziness, and dehydration, which can lead to more life-threatening conditions like heatstroke.

The effects of heat are also unequal across pregnant populations. Black women faced the most risk associated with higher temperatures, being more likely to have a preterm birth when researchers studied 60,000 births in California between 1996 and 2006. The study, and a 2018 follow-up, found about a 9 percent increase in deliveries before 37 weeks for every 10-degree rise in weekly outdoor temperatures, according to BuzzFeed News.

A view from inside the back of the ambulance shows several officials and paramedics lifting a pregnant woman in an emergency chair into the back on a ramp.

A pregnant woman suffering from dehydration is loaded onto an ambulance in Texas on July 19, 2023.
Brandon Bell/Getty Images

A wider literature review, published in the journal BMJ in 2020, found that across 24 countries, every 1 degree Celsius (1.8 degrees Fahrenheit) hotter led to a 5 percent increase in preterm birth and a 5 percent increase in stillbirth. Longer periods of abnormally high heat raised the risk even more.

How does hot weather affect our sleep?

Heat is, somewhat counterintuitively, deadliest overnight.

Our internal body temperature naturally drops at night to help us fall asleep; the body achieves this by moving some of its core heat to the hands and feet. The air temperature affects how well the body manages this process. So not being able to cool down makes the heart work harder all over again, after an entire day of added strain.

In theory, nighttime should be a respite for the body to recover. But climate change is making summer nights warm up even faster than days. “In general, since records began in 1895, summer overnight low temperatures are warming at a rate nearly twice as fast as afternoon high temperatures for the US and the 10 warmest summer minimum temperatures have all occurred since 2002,” according to the National Oceanic and Atmospheric Administration. The effect is more pronounced in urban heat islands, where concrete, asphalt, and steel radiate heat back, keeping temperatures artificially high overnight.

Our sleep quality is closely associated with temperature, too. You can toss and turn on a hot night as deep sleep suffers. A hotter room is associated with shorter REM sleep, which is tenuously linked to motor skills, memory formation, and regulating emotions.

What’s the emotional and mental toll?

Heat can affect a lot more than physical health. Research shows learning, emotions, stress, and anxiety can all suffer when it’s hot. Exactly what’s driving this, though, is much harder to pin down. With the age-old warning about not confusing correlation with causation in mind, consider some alternative theories: It could be that poor sleep quality is causing a lot of these problems; that there’s some other bodily process scientists don’t fully understand; that the stress of coping with heat and paying high AC bills causes extra anxiety; or that other things about society’s annual rhythms are making these problems more pronounced.

Still, a large body of research shows a correlation between heat and well-being. ER visits and suicide rates go up when temperatures are high, which suggests that there are a lot more people struggling who go unreported. Violent crime also becomes more frequent in hotter months.

Heat also impacts learning. A study that compared test scores of more than 144 million students on a standardized international exam and 270 million exam scores of US students found days above 80 degrees affected academic performance. “Temperature has been shown to affect working memory, stamina and cognitive performance, and to lead individuals to reduce time spent engaging in labour activities,” the researchers wrote. “This suggests that … heat may directly affect students’ capacity to learn or teachers’ ability and willingness to teach.”

A woman sits under a shaded bus stop, with an orange umbrella resting next to her.

A woman sits under a shaded bus stop in Phoenix, Arizona, where temperatures were above 110 degrees for at least 14 straight days in July.
Brandon Bell/Getty Images

There’s even evidence showing that people’s decision-making at all levels of society may change in heat waves. A 2019 study published in the American Economic Journal: Applied Economics found that 266 US immigration judges made harsher judgments when temperatures were higher, even if they were issuing them from the comfort of a climate-controlled courtroom. After controlling for other factors, the researchers found “a 10°F degree increase in case-day temperature reduces decisions favorable to the applicant by 6.55 percent.”

Some work has also found that individuals’ perception of risk changes when they are in a hotter environment, especially if they haven’t had the weeks needed to adjust or acclimatize. In other words, when caught off guard by heat, people seem to be more likely to engage in riskier behavior.

The toll that research fails to capture: the “human face of heat”

Heat harms mental, emotional, and physical health, but we have even fewer answers for how much these impacts could be minimized if we only responded differently.

“You see more social scientists working with climate scientists, architects, and designers [on heat research],” said Daniel Vecellio, a climate and health scholar at George Mason University. “There’s behavioral issues, mental health, and medical issues. You need a coupled approach [to address] the very complex questions of human behavior.”

What’s happening inside people’s homes is the biggest mystery of them all. Shandas worked on a study for the city of Portland, Oregon, placing sensors in 53 homes last year, and found that even with the AC running at full blast through the night, temperatures in public housing were not getting below 85 degrees.

“We don’t have temperature measurements of indoor spaces,” Shandas said. “We don’t have the behavioral response. We open cooling centers, we provide water bottles, and we put misters in places. But we really don’t know what humans, specifically who are a bit more sensitive or have difficulty coping, are doing in the heat.”

Officials are flying blind in responding to a heat wave more often than they’d like to admit, even though there might be more effective immediate interventions. People may benefit far more by getting direct assistance so they can pay their energy bills in the summertime.

Even if we learn all the answers to what’s happening to the body in a heat wave, Shandas said we still don’t understand “a lot of the human face of heat,” as in, how people actually cope. What scientists do understand is that this is just the beginning of these consequences playing out on a global scale. It’s only going to get hotter from here.

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