Heat Waves & Your Body
One study has predicted that, if Phoenix experienced a blackout during a heat wave, half its population could require emergency care.


Annals of a Warming Planet What a Heat Wave Does to Your Body The human body is a remarkably effective cooling machine—but it has a limit.
By Dhruv Khullar August 25, 2023, New Yorker
On a sweltering day in June, 2019, David Kim, a third-year medical resident, was working in a Bay Area emergency department when he received a dispatch. The temperature outside was ninety-nine degrees—nearly unprecedented for Northern California—and a woman in her eighties had just been found lying on the ground in a parking lot. Her body temperature was a hundred and four degrees. Paramedics had lifted her off the pavement and applied cold packs to her skin, and she’d regained consciousness. But she was unable to tell them how she had fallen, or even who she was. She was now in an ambulance headed for Kim’s hospital.
In cases of heatstroke, the fastest way to lower a person’s body temperature is to plunge them into cold water. Other interventions—cold towels, misting fans—are far less powerful. But Kim’s emergency department didn’t have a bathtub, and they needed to improvise. In a supply closet, Kim found some gray plastic buckets. He ran with them to the cafeteria for ice and water. Meanwhile, a technician located a postmortem kit—a pre-packed container full of supplies for when a patient dies. It contained a body bag made from white waterproof vinyl.
The woman arrived on a stretcher pushed by a paramedic, and was barely conscious. She was breathing rapidly; she had a black eye and scattered abrasions over her reddened skin. The team quickly cut her clothes off, counted “One, two, three!,” and lifted her off the stretcher and into the bag, which surrounded her like a cocoon. They started pouring buckets of ice and water over her. The bag swelled like a water balloon, and, to keep the slush from spilling out, they pulled the zipper up to her neck. She hardly stirred. Anyone watching might have assumed that she was dead.
It took ten minutes for the woman’s temperature to fall to a hundred and one, at which point she became alert. The doctors unzipped the bag, plunged their hands into the icy water, and eased her onto a dry stretcher. They gave her fluids and stitched up a cut on her arm. A few hours later, after her body temperature had normalized and she was thinking clearly again, she asked to go home.
To understand what heat does to the human body, the author spent two hours walking uphill on a treadmill in a hundred-and-four-degree chamber at forty-per-cent humidity—a test developed in the seventies.
Not long afterward, Kim and his colleagues wrote about what had happened in a case report titled “A body bag can save your life,” published in an emergency-medicine journal. They thought of the body-bag method as a strategy that might prove useful in the most extreme circumstances. But, the following year, a heat dome smothered the Pacific Northwest for nearly two weeks. Temperatures reached a hundred and twenty degrees in a region with limited air-conditioning. One doctor treated nearly two dozen heatstroke patients in a single day, and hospitals ran low on ice packs and cooling catheters. The emergency department at Seattle’s Harborview Medical Center turned to body bags. News reports called the procedure “grim.” But, in a heat wave that melted power cables and buckled roads, and may have killed hundreds of people, it helped avert even more casualties.
Deadly heat, once rare, is spreading. This summer—which is likely to be the hottest in recorded history—Beijing warmed to a hundred and six degrees and Sardinia baked at a hundred and eighteen. For forty-four consecutive days, El Paso recorded temperatures of a hundred or more. We’re all becoming guinea pigs in a vast experiment: How will people of different ages and levels of fitness respond to unprecedented, ongoing heat? What will happen to our bodies when we have no choice but to stay outside, or when the air-conditioning goes out?
One way to study this question is to put people in heat chambers—special rooms where temperature, humidity, and light can be manipulated—while monitoring their vital signs. The Korey Stringer Institute, a nonprofit at the University of Connecticut, operates such chambers. The institute is named for a Minnesota Vikings football player who died of heatstroke at training camp. When I told the institute’s director that I wanted to understand what heat does to our bodies, he agreed to put me in a hundred-and-four-degree chamber for two hours at forty-per-cent humidity, a combination that would put serious strain on my body. (I’d need to sign a waiver and get my doctor’s permission.) I’d spend the time walking uphill on a treadmill—a test developed by the Israeli Defense Forces in the nineteen-seventies. Scientists would monitor my vitals and analyze my sweat to find out how I’d coped.
In August, after several days of scorching temperatures in New York City, I took the train to Connecticut and made my way to the arena where the UConn Huskies play basketball. I was already starting to sweat when I found Rebecca Stearns, the institute’s friendly and efficient chief operating officer.
“Ready to get hot?” she asked.
“I already am,” I replied.
“Just wait!”
We walked together to the heat lab, which resembled a locker room. On the way inside, I saw a photo of Stringer on the wall, along with a football that he’d signed on the day before he collapsed. On a whiteboard, I saw instructions: if someone’s body temperature reaches a hundred and four degrees, reduce the workout’s intensity; if it continues to climb, start a “rehydration protocol” immediately.
I’m a doctor—someone more accustomed to conducting tests than undergoing them—and, as I peered through an observation window into the heat chamber, I felt a mounting nervousness. Ten large circular heat vents loomed over the treadmills. They looked like the jet engines of an airplane. A nearby wall bore a quote attributed to Serena Williams: “Sometimes the heat is my biggest opponent.”
While we looked inside, Stearns unlocked the chamber’s wall-mounted control box and flipped a switch. Using one keypad, she punched in the temperature, and using another, the humidity. I heard a click and a hiss. Throughout the next few minutes, the numbers started climbing: ninety degrees, then ninety-five, then a hundred. It reminded me of a preheating oven. Stearns opened the door, and I stepped inside.
The human body is astonishingly good at cooling itself off. The hypothalamus, an almond-size structure deep inside the brain, responds to heat by stimulating sweat production. It also speeds up the heart, dilates blood vessels, and shunts blood to the extremities. The basic principle is to get hot blood near the skin, where heat can dissipate in a number of ways. When we touch something cold, like gel in an ice pack (or the slush in a body bag), it can dissipate through conduction. When air currents wash over us, it can leave through convection. Heat can be lost directly, through radiation, in the form of electromagnetic waves. Most importantly, as we sweat, evaporation cools our skin. The problem with extreme heat is that it makes the first three mechanisms less effective, or even turns them into routes for gaining heat. When humidity rises, the fourth mechanism weakens, too.
Heat affects us on a molecular level. Excess heat interferes with the chemical bonds that help proteins to twist and fold into shape; just as a hot frying pan can denature the proteins in an egg, high body temperatures can denature the proteins in our cells, preventing them from functioning properly and even killing them off, especially in the liver, blood vessels, and brain.
On the scale of the whole body, meanwhile, overheating can trigger a downward spiral. Sweating can leave a person dehydrated; this, in turn, means that there’s less liquid available to carry heat away. Desperate to release heat, the body diverts more blood to its periphery, starving internal organs of oxygen and nutrients. In very bad cases of heat illness, the gut can lose its integrity, allowing deadly bacteria to leach into the bloodstream, or the heat can set off a frenzy of inflammation known as a cytokine storm. In the most severe heat, the body’s enzymes—the proteins that carry out life’s essential chemical reactions—cease to function.
Doctors divide heat strokes into two categories. Classic heat stroke generally occurs at rest, and is more common among kids, older people, and those with chronic conditions. Exertional heat stroke strikes athletes, laborers, soldiers, and others engaged in strenuous activity. To avoid both kinds of illness, our bodies adjust. In 1962, Ferruccio Ritossa, an Italian geneticist who studied fruit flies, found that someone had accidentally increased the temperature in one of his incubators; when he inspected the chromosomes of his overheated flies, he noticed that they looked oddly puffy. The heat, it seemed, had provoked the chromosomes to unravel, allowing the production of more cellular material. Later, scientists learned that the flies had made heat-shock proteins—molecular chaperones that help other proteins fold correctly. This fundamental defense against heat exists in virtually every species on earth.
Inside the chamber, my heat-shock proteins didn’t feel very helpful. My pulse quickened, and sweat poured from my forehead, burning my eyes. Stearns turned on two large lights set into the ceiling, which simulated the impact of direct sunlight. I was stunned by their force and reflexively turned my face away. My skin felt like it was frying.
The treadmill whirred to life. While I walked, Stearns and two graduate students in exercise physiology, David Martin and Sean Langan, took turns by my side. They had runners’ physiques, and I sucked in my gut, grabbed onto the treadmill’s handles, and kept walking.
“Try not to hold on,” Martin, an élite triathlete, told me.
Every ten minutes, the trio asked me questions. How hard was I working? Very hard. How hot did I feel? Take a guess. Between checks, they told me about the lab’s research and shared stories about people who’d suffered heat-related illnesses.
A half hour had passed. I now found it difficult to concentrate. Instead of talking, I took to nodding silently. Langan showed me a scale of heat that ran from unbearably cold to unbearably hot.
“How do you feel right now?” he asked.
I pointed to “very hot.”
He glanced at a monitor tracking my core temperature and scribbled something on a clipboard. He explained that certain medical conditions—as well as drugs that treat depression, anxiety, and Parkinson’s disease—can stop a person from noticing the intensity of the heat. He seemed satisfied with my level of discomfort.
Over time—on a scale of days or weeks—our bodies can adjust to extreme heat. They learn to increase the amount of blood that circulates and pump it toward the skin more efficiently. Our bodies start to sweat more and at lower temperatures, and we get better at holding on to our electrolytes instead of sweating them away. Yet acclimation takes time—and this is one reason heat waves tend to be deadliest at their start, especially in places where people aren’t used to hot weather. (Last summer, temperatures as high as a hundred and four degrees were thought to have contributed to as many as eleven thousand deaths in France.) Unfortunately, the adaptation process reverses shortly after we return to normal temperatures. Last summer won’t help you much with this one.
At around minute forty, my limbs started to feel heavy. I dragged them along, determined to keep moving. My calves tightened. I listened to the thwap of my feet and the thrum of the chamber. “Deep breaths,” Martin told me. I plucked a towel from the treadmill’s handlebars and wiped my forearms. I’d been told to catch every drop for a sweat analysis, but a puddle had already formed on the treadmill belt. I felt a pain in one of my fingers and looked down at my hands. They were so swollen that I couldn’t make a fist. My wedding ring was cutting into my flesh. I glanced at the monitor and watched my internal temperature rise past a hundred degrees.
What counts as hot depends on where you are. Anyone who’s spent a muggy summer day in the South can tell you that ninety degrees often feels like a hundred and ten. Arizonans and others in the Southwest have long joked that “at least it’s a dry heat.” They mean that a hundred-and-ten-degree afternoon feels like a hundred and ten.
Scientists can measure the severity of humid heat by wrapping a thermometer in a soaked rag. This allows them to determine what’s known as the wet-bulb temperature—the lowest temperature that can be achieved through evaporation, the power of which is limited when the air is exceedingly humid. If the air is full of water, it won’t accept more; the rag stays wetter, and the thermometer stays hot. As wet-bulb temperatures rise, even people who are used to the heat will struggle to work outdoors. Above a wet-bulb of ninety-five—roughly the equivalent of a hundred degrees at eighty per cent humidity, or a hundred and fifteen at fifty per cent—humans can’t live for more than a few hours. Globally, instances of extreme humid heat have doubled in the past half century. In recent years, wet-bulb temperatures have touched the limits of human survivability in Pakistan, India, and Australia.
In the summer of 2014, Zoë Wallis enrolled at the College of Charleston, in South Carolina, on a basketball scholarship. At six feet three, Wallis had been the star center of her high-school team in St. Louis; playing in college was a lifelong dream. One August morning, Wallis woke before 6 A.M. for a preseason training run—two and a half miles across a bridge, and then back again.
The sun was just starting to rise when she started running. The temperature had already pushed past eighty-eight degrees, and the humidity sat at ninety-four per cent. Wallis focussed on putting one foot in front of the other. All the way across the bridge, she kept up with her teammates. But, as she turned around for the second half of the run, she struggled to catch her breath. She felt like she couldn’t suck enough oxygen into her lungs; she had to force herself to keep running.
At one point, she felt a coach’s hands on the small of her back, pushing her forward.
“Keep moving!” one of them said.
Half a mile later, Wallis’s vision started to blur. She felt an overpowering need to close her eyes. Two of her teammates tried to steady her by hooking their arms through hers. “You got this, Zoë!” one said. The end of the bridge came into focus. But, a few feet from the finish line, Wallis collapsed, tearing the skin of her knees on the pavement.
When she arrived at the emergency room, her temperature was a hundred and five degrees, and her organs had started to fail. She opened her eyes and squinted into fluorescent lights, unable to remember who she was. She spent a night in the I.C.U., and a second night in the hospital. After she was discharged, her doctors told her to avoid strenuous activity until her liver and kidneys had recovered. She returned to class, but found that she couldn’t pay attention. She looked down at her notebook and saw that her neat handwriting had devolved into an illegible scrawl. On another occasion, her breathing quickened and her mind raced with anxiety; she ran to the bathroom and started sobbing.
That semester, Wallis suffered panic attacks almost every day. Still, she played basketball all year. The following season, after hyperventilating during a practice, she fell to the floor of the locker room in a fetal position, crying. “I remember thinking, I’m not healthy enough to do this—not mentally, not physically,” she told me. After that, she lost her scholarship and had to transfer to a college back home. (She later sued the university; the case was settled out of court.) For years, she avoided the outdoors in summer, and couldn’t bring herself to exercise indoors during any season.
There is an increased risk of post-traumatic stress disorder after many serious illnesses, but heat in particular has been linked to an array of mental-health problems. Hotter days are associated with anxiety, anger, irritability, disordered sleep, and violent crime; as temperatures rise, so do suicide attempts and overdose deaths. In its 2022 report, the Intergovernmental Panel on Climate Change said, with high confidence, that the warming planet is adversely affecting people’s mental well-being.
Heat illness is only one of the physical ailments caused by heat waves. Kevin Foster, a surgeon at Valleywise Health, in Phoenix, runs the only burn center in Arizona; he was trained to treat wounds from fire, chemicals, and boiling water. As temperatures have risen, however, he has seen new hazards emerge. This July, Phoenix recorded the hottest month ever for an American city, with thirty-one consecutive days of temperatures above a hundred and ten degrees. This was hot enough to warm the pavement to a hundred and eighty degrees. “It takes just a fraction of a second to get a really, really bad burn,” Foster told me. Recently, an older man slipped on a rock on his way to work, fell onto the pavement, and burned twenty per cent of the skin on his back. In another case, a young woman’s golf cart tipped over, pinning her between the vehicle and the searing concrete.
On the hottest days, Foster told me, everyday objects—door handles, metal gates, seat belts, the water in a hose—can become hazards. “When hot coffee gets poured on a leg, that coffee loses its temperature very quickly, so the amount of damage it does is limited,” he said. “Contact burns continue to burn until you’re removed from the surface.” Last summer, the Arizona Burn Center-Valleywise Health treated eighty-five people for serious heat-related injuries. A third required I.C.U.-level care. This year, the center admitted more than fifty such patients in July alone. An elderly woman was scalded after her wheelchair tipped over onto concrete; a construction worker passed out and burned himself on hot equipment. A child was admitted after running barefoot onto a scorching driveway. Some patients have suffered from liver and kidney failure; others have lost limbs.
Across the hospital from Foster’s burn center, E.R. doctors have converted a waiting area that once housed COVID-19 patients into a cold room where medical workers can hand out electrolyte drinks and inject fluids. Frank LoVecchio, one of the hospital’s E.R. doctors, told me that in recent weeks his team has treated around fifty patients a day with heat-related illness. They arrive not only in the peak heat of the afternoon but also early in the morning, because nighttime temperatures often remain above ninety degrees. High maximum temperatures make news, but high minimums can be even more dangerous: the body never gets a respite from the heat.
LoVecchio said that, every day, twenty-five to thirty patients show up with heat illnesses too severe for the cold room; they must be treated in the main emergency department or admitted to the hospital. Around five come in with heatstroke. “We see a lot of people presenting with temperatures of a hundred and seven, and you have to ask yourself, Why?,” LoVecchio told me. “Well, that’s as high as our conventional thermometers go.” He and his colleagues fetch a ten-gallon bucket of ice from the freezer and follow the body-bag protocol. After zipping a patient up to the armpits and filling the bag with ice slush, LoVecchio often needs to force a tube into their trachea and connect them to a ventilator.
Immersive cooling can lower a person’s body temperature by around half a degree per minute. Sometimes this is enough to save a person’s life. But, “as you can imagine, it’s not good to have your brain cooking at a hundred and seven, a hundred and eight, a hundred and eleven, for very long,” LoVecchio told me. According to LoVecchio, almost all of the patients who experience heatstroke at his hospital are admitted to the I.C.U., and about a quarter die or are permanently disabled. Last year, Maricopa County registered four hundred and twenty-five heat-related deaths (the most since it began tracking them, in 2006); this year, it may break the record again. The county’s medical examiner’s office recently prepared ten refrigerated containers to handle a possible overflow of bodies.
Last summer, Austin Davis, a twenty-three-year-old who runs an outreach program for Arizonans experiencing homelessness, drove around with water, ice, fans, misting spray, and electrolyte tablets. Recently, he found a woman collapsed, face down in gravel, fifteen feet from a cooling center that he helps run. Some families wander through air-conditioned grocery stores, pretending to shop. Others take refuge in malls or libraries. One called Davis from the airport. “We have this suitcase—there’s nothing in it, but we’re trying our best to blend in,” they told him. After that, he heard from a woman who’d been sleeping in a truck with her ten-year-old daughter. When the air-conditioning broke, the girl started vomiting, and they rushed to the E.R. “At least we can be inside for a little bit,” the mother told Davis.
Meanwhile, in the warmest parts of the world, more than ninety per cent of people have no air-conditioning. Even those who do may risk their lives if they need to go outside, or if the electricity goes out. One study has predicted that, if Phoenix experienced a blackout during a heat wave, half its population could require emergency care. In spite of these risks, Sun Belt states, including Arizona, Texas, and Florida, have some of the fastest-growing cities in the country. By the middle of this century, more than a hundred million Americans are expected to experience at least one day annually when the heat index reaches a hundred and twenty-five degrees; some Southern states are projected to experience temperatures above a hundred degrees for months each year. Cities like Phoenix, Los Angeles, and Miami have appointed chief heat officers.
It’s possible to work together to blunt the heat. New York’s city government asks volunteers to check on vulnerable neighbors; around the country, communities are planting trees, which provide shade and evaporative cooling. By painting roads, roofs, and parking lots with light instead of dark colors, we can reflect the sun’s rays; warning systems can tell people that extreme heat is on the way, and cooling centers, hydration stations, and public pools can offer a respite. Those who work outdoors could be assigned early or late shifts, and should always be able to take shade and water breaks. These steps are necessary but imperfect; they can dull the heat, but not stop it. The planet is becoming a kind of heat chamber. As we keep burning fossil fuels, we’re getting locked out of the controls.
An hour into the test, I felt a knot in my left thigh. I kneaded my swollen fist into my cramping leg. Ten minutes later, I developed a throbbing headache and felt light-headed. For a moment, I panicked, unable to catch my breath. My temperature passed a hundred and one degrees; my heart rate was pounding at a hundred and sixty beats per minute. Langan asked if I was O.K. Weakly, I gave a thumbs-up. “Almost there,” he said. As I walked on, my temperature climbed toward a hundred and two. When my heart rate jumped to a hundred and seventy beats per minute, I stepped off the treadmill.
The next step was analysis. I threw my shirt, shorts, and towel into a large black tub, then climbed inside. Martin and Langan poured a huge container of water over my head, shoulders, and chest, and then another. I sat, exhausted, in a weirdly refreshing soup of water and sweat.
Later that day, the researchers drained the tub and analyzed the soup. They told me that my performance in the chamber counted as a passing grade. My temperature hadn’t entered the danger zone, in part because I’d sweated out nearly a litre of water each hour. On the other hand, my sweat had contained a lot of sodium—possibly a sign that my body hadn’t adjusted to high-temperature conditions. I may not have been ready for the heat.
One thing was for sure: I was no longer ready for ordinary life. At the Amtrak station, my headache worsened, and a ponderous fatigue set in. In the bathroom, I discovered that my urine was an alarming dark amber color. I drank bottle after bottle of water, trying to quench a thirst that wouldn’t go away. I bought a sandwich, but felt nauseous and set it aside. The train was powerfully air-conditioned, and I sat down and opened my laptop, hoping to respond to the day’s neglected e-mails, but I felt foggy and faint. We passed apartment windows that were crammed with A.C. units, and others that didn’t have any—heat chambers of their own. Against my will, I fell asleep.
After a while, the train came to a stop. I felt a warmth on my face and opened my eyes; the sun was burning through my window. I saw a young girl in a flower-patterned dress waiting with her mother on the platform; the girl leaned against her mom, and her mom fanned her with a piece of cardboard. The girl reached into her backpack, pulled out a small baby-blue water bottle, and took a long sip. The train doors slid open, and they rushed inside. ♦
Dhruv Khullar, a contributing writer at The New Yorker, is a practicing physician and an assistant professor at Weill Cornell Medical College.