What a Heat Wave Does to Your Body

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.

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.