End of the dirt road. You brake to a stop, swing your leg over the scooter, and kick the stand into place.

The effort makes your head throb. The scooter wobbles. Your sunglasses slide down the mixture of sweat and sunscreen on your nose. You adjust them, look up tentatively at the fiery orb in the deep blue sky, and flinch. You chide yourself for staying out so late the night before, for not getting an earlier start this morning. The sun already feels too hot. But this is your only chance to surf Emerald Cove. It’s gonna be OK, you tell yourself. You’re in good shape. You’ve got the stamina to hike the five miles over the ridge and down to the beach before the tide comes in.

That glaring sun, of course, is essential for life on this planet. But its thermal energy, which we feel as heat, is a force both benevolent and cruel. The human body employs a spectrum of physiological tricks to maintain the steady internal temperature—98.6 Fahrenheit—at which it thrives. There is about eight degrees of difference between an optimal level of internal heat and the limit the body can endure. This threshold is referred to as the critical thermal maximum. Exactly when one reaches it depends on individual physiology, exertion, hydration, acclimation, and other factors. Estimates place it at an internal temperature between 105 and 107 degrees. Heat is a giver of life, but when the human body gets this hot—or hotter—­terrible things occur.

How to Prevent and Treat Heat Stroke Exertional heatstroke can cause devastating damage, but it can also be treated quickly Read More →

Emerald Cove is on an island off the coast of South America. You’d flown over a couple of days ago, after a trek in the mainland’s cool interior highlands. You wanted to take in those thousand-year-old stone statues you’d heard so much about, plus you figured you could cap off your vacation with a couple days of surfing. You’re just a beginner, and already you’re hooked, but it’s hard being a newbie. The locals are reluctant to let you into the lineup. What you need is that perfect undiscovered break, no people, no pressure.

Last night you walked into a popular surf bar and pulled up a stool next to two guys you’d seen in the water that day. If you wanted to find a secret spot along this spectacular wave-battered coast, you figured these guys would be the ones to know. They gave you a cursory nod and continued their conversation.

“Huevón,” one was saying to his pal (or at least that’s what you think he said). Your Spanish is OK, but you’re not catching all the slang. He was talking about a point break.

“Qué bacán!” Rad! “And there’s nobody there. Nobody. You have to try it.”

“Nobody where?” you asked quietly, leaning in.

“La Cala Esmeralda.” He barely turned his head to look at you.

“Emerald Cove?” you repeated.

It had taken a long time, a lot of patience, and too many piscolas—pisco and Cokes—to pry out where it was, but the effort was worth it. It’d be the perfect end to a perfect trip, something to talk about to your well-traveled friends back home. “Seriously, you’ve never been there?” you’ll say to them, acting surprised. “You should definitely check it out. But it’s kinda hard to get to, and the trail’s a secret.”

You had to ask the surfers to repeat themselves, just to be sure you understood. They’d finally turned and looked at you full on.

“Dude,” one said, “I’m not sure I’d try it if I were you.”

Heat-related illnesses in the U.S. claim more lives annually than hurricanes, lightning, earthquakes, tornadoes, and floods; there were over 9,000 heat-related deaths between 1979 and 2014. The fatalities tend to peak during heat waves and hotter-than-average years, and they’re expected to rise as climate change affects global temperatures. One of the deadliest heat waves in modern times swept Europe in 2003, killing over 30,000 people as temperatures soared to 100 degrees for days on end.

The human body is much less tolerant of rises in internal temperature than drops. The lowest body temperature a human has been known to survive is 56.7 degrees, nearly 42 degrees below normal. Anna Bagenholm, a 29-year-old Swedish woman, was backcountry skiing when she broke through eight inches of ice into a frozen stream. Her upper body was sucked down, leaving only her feet and skis visible, but she managed to find an air pocket and was able to breathe. After 80 minutes, she was finally rescued. Bagenholm remained in a coma for about ten days and was in intensive care for two months but ultimately suffered only minor nerve damage. On the other end of the spectrum, the highest body temperature measured was only 17 degrees above normal. Willie Jones, a 52-year-old Atlanta man, was rescued from his apartment during a heat wave in 1980. His internal temperature was 115.7. He spent 24 days in the hospital before being released.

While there is some debate, studies on women in the military have shown that they may be more susceptible to heat illness than men due to their higher body-fat content and lower sweat output. Whether the heatstroke victim is male or female, the odds of surviving depend on the duration of overheating and, once their condition is discovered, how quickly they can be cooled down—most effectively by immersion in ice water within 30 minutes. Survival, moreover, doesn’t guarantee full recovery. A powerful heat wave in Chicago in 1995 caused 739 deaths and 3,300 emergency-room visits. A study reviewing 58 of the severe heatstroke victims found that 21 percent died in the hospital soon after admission, 28 percent died within a year, and all the remaining subjects experienced organ dysfunction and neurological impairments.

An average-size male at rest generates about as much heat as a 100-watt light bulb simply through metabolism. During moderate exercise, temperature increases nearly ten degrees every hour unless you cool yourself by sweating or some other means. You risk a variety of illnesses, starting with heat edema, which entails swelling of the hands and feet and can begin at body temperatures close to normal. No precise temperature marks the onset of the various other heat illnesses, and the order of symptoms varies between individuals, but they may include heat syncope (dizziness and fainting from the dilation of blood vessels), heat cramps (muscular clenching due to low salt), and heat exhaustion (identified by muscular weakness, rapid heartbeat, nausea, headache, and possible vomiting and diarrhea). Finally, an internal temperature of 105 marks the lower boundary of heatstroke territory, with outward symptoms of extreme irritability, delirium, and convulsion. Because of individual variation in how these symptoms appear, and because some may not appear at all, athletes in particular can be overcome quickly and with little warning.

There are two kinds of heatstroke: classic and exertional. Classic heatstroke hits the very young, the elderly, the overweight, and people suffering from chronic conditions like uncontrolled diabetes, hypertension, and cardiovascular disease. Alcohol and certain medications (diuretics, tricyclic antidepressants, antipsychotics, and some cold and allergy remedies) can increase susceptibility as well. Classic heatstroke can strike in the quiet of upper-floor apartments with no air-conditioning.

Exertional heatstroke, on the other hand, pounces on the young and fit. Exercise drastically accelerates temperature rise. Marathon runners, cyclists, and other athletes sometimes push into what used to be known as the fever of exercise and is now called exercise-induced hyperthermia, where internal temperatures typically hit 100 to 104 degrees. Usually, there’s no lasting damage. But as body temperature climbs higher, the physiological response becomes more dramatic and the complications more profound. The higher temperature can ultimately trigger a cascading disaster of events as the metabolism, like a runaway nuclear reactor, races so fast and so hot that the body can’t cool itself down. A person careens toward organ failure, brain damage, and death.

It's February, the height of summer in the Southern Hemisphere. You’d planned to get up early but didn’t hear your alarm after the late night at the bar. Now the sun is well into its arc. The temperature is supposed to hit 93 degrees by midday.

Pulling the keys from your scooter, you sling your rented surfboard onto your back, thread your arms through your chest pack, and hear the reassuring slosh of the water bottle inside. You have a seat on the twice-weekly plane that leaves tomorrow, returning you to the mainland. If you’re going to do this, the moment is now. You launch up the trail, a faint unmarked path on the gentle, grassy slope. You’re not surprised you’re the only one around. The surfers said to follow the volcano’s right flank until you gain the ridge, then drop down a cleft in the rocks to the sea. Good luck finding the cleft, they seemed to say. Maybe they were just trying to deter you. You see the slope steepen as it rises toward the sharp crest, where chunks of volcanic rock protrude like broken dinosaur scales through velvety green nap. No trees, not a wisp of wind. Ancient cultures deforested this island centuries ago and mysteriously disappeared, leaving not a sliver of shade under the tropical sun.

You feel the quick flex of your quads, the push of your glutes, the spring of your calves propelling you up the winding path, and hear the steady mantra of your breathing. You have to make time. The guys at the bar said the shore bristles with stone dientes, teeth—get there at low tide. That gives you just under two hours.

Within only a few steps, your body begins to respond to the sun’s radiation, the moist air pressing against your skin, and the heat generated by your own rising metabolism. Blood coursing through your arteries begins to grow warmer. At less than one degree Fahrenheit above your normal internal temperature, receptors in your brain’s hypothalamus start to fire, signaling the circulatory system to shunt more blood toward your skin’s surface for cooling. Other messages tell peripheral blood vessels to dilate, opening up to allow greater blood flow. Still other signals activate millions of tiny coils and tubes embedded in your skin—your sweat glands. Concentrated within your head, palms, soles, and trunk, the glands pump water from a tiny reservoir at the base, pushing the salty liquid up a long tube through layers of skin to erupt in a miniature gusher at the surface.

Several hundred yards up the grassy slope, sweat is popping onto your face. You feel the slick, dark blue fabric of your shirt sticking to your back, despite its breathability. You wish it was looser, and a lighter color that didn’t so readily absorb the sun’s rays. A trickle of sweat runs down your forehead and into one eye, stinging with dissolved salts, blurring your vision.

The air is smothering, thick with moisture, like a greenhouse. The dripping sweat should bring some relief. Usually, the body’s cooling system operates remarkably efficiently; blood rushes to carry the excess heat from your core out to your sweat glands, which squeeze warm fluids to the surface, where air moving past your skin evaporates the moisture. Your excess heat literally blows away in the wind. But for this to work properly, the sweat must evaporate. When the air lies close and unmoving, heavy with humidity, sweat evaporates more slowly. If the air is saturated enough, or if impermeable fabric—or, in your case, a surfboard and a chest pack—trap the sweat against your skin, the moisture won’t evaporate at all.

Because of individual variation in how symptoms appear, and because some may not appear at all, athletes in particular can be overcome by heatstroke quickly and with little warning.

High school athletes are often afflicted by heatstroke, which ranks as one of the top three leading causes of death among that demographic. And according to an investigation done by the HBO show Real Sports with Bryant Gumbel, since the year 2000, at least 30 college football players have died of heatstroke during practice, when remedies as simple as immersing the overheated player in ice water were available. Minnesota Vikings offensive lineman Korey Stringer died of heatstroke during a preseason practice in 2001, and now the University of Connecticut’s Korey Stringer Institute, established in 2010, specializes in sudden-death prevention in athletes, soldiers, and laborers.

Runners, cyclists, and hikers routinely succumb to heatstroke. If properly acclimated, trained, and managed carefully, the human body can endure grueling events in high temperatures, like the Badwater—a 135-mile running race in California that begins in Death Valley, traverses three mountain ranges, and ends at Mount Whitney—and the six-day Marathon des Sables in the Sahara. However, experts say that due to the high intensity of the pace on shorter courses, heatstroke is more common in races of 30 to 90 minutes than in longer events. Three years ago at the annual Falmouth Road Race, a 12K running event in Massachusetts in August, 48 out of more than 10,000 finishers suffered from heatstroke and another 55 from heat exhaustion. (All of them survived without incident due to the extensive cooling procedures available at the race’s finish.)

The National Weather Service now issues warnings when excessive temperatures are expected and gives predictions of the heat index, which takes into account both temperature and humidity as experienced by a five-foot-seven, 147-pound person walking at a speed of about three miles per hour in a six-mile-per-hour breeze. Like the windchill index, the heat index conveys what it feels like outside. For instance, at the Hot Trot Half Marathon, which is held in Dallas in August, the day is often 97 degrees but can have a heat index of 116 degrees because of the 60 percent humidity.

You pull your water bottle from your pack—a full liter shimmering inside a translucent blue Nalgene—take a warm swig, and strike upward again toward the broken scales of the ridge. For the next hour you push at a fast walk, pausing only occasionally to drink. You know the importance of hydration. What you don’t know is how remarkably fast the human body can expel water to cool itself—one and a half liters or more per hour. (Highly efficient, heat-acclimated marathoners can lose close to four liters per hour while they run.) The human gut, however, can absorb only a little over one liter of water per hour. That means that during maximum rates of water loss, it’s possible to drink steadily and still become dehydrated.

Your core temperature has now climbed to 101.5—three degrees above normal—but you’re still in the exercise-induced hyperthermia zone. Your head throbs. You wish you hadn’t drunk quite so many piscolas last night. In doing so, you unwittingly tricked your body’s water controls. Alcohol is a small molecule that slides easily through the walls of the gut, into the bloodstream, and up into the brain, where it suppresses the release of antidiuretic hormone, or ADH. This is the hormone that inhibits urination, in effect closing your dam’s spillway in order to keep your reservoir full. Typically, when you become dehydrated, the percentage of salt in your blood rises, triggering your pituitary gland to release ADH. But under the sabotaging influence of alcohol, your body may sense that your water stores are being depleted but blithely ignore the warning. Thanks to those piscolas, rather than prehydrating for today’s climb, you started the day in the red.

The incline grows steeper. The grass gives way to a light, loose volcanic rock called tuff. The scrappy path has now completely disappeared, but still you labor toward the ridgetop—two steps up, slide, one step down. You’re panting now. The rocks crunch under your feet. Each footstep produces a gritty dust that crusts your bare legs, which are coated in a paste of sweat and sunscreen. The arteries protruding on your forearms look like grapevines wrapped around a post. Your blood vessels are dilating, trying to move as much overheated blood to the surface as possible. Your heart pumps madly, trying to keep the vessels full, but it can’t keep up. Not enough blood—and the oxygen it carries—reaches your brain. You pause to rest. You feel lightheaded and faint. Your vision dims and narrows. You feel wobbly and strange—the onset of heat syncope (or orthostatic hypotension), a temporary loss of consciousness from falling blood pressure.

Fainting from orthostatic hypotension poses a distinct problem for those whose sworn duty requires standing still for hours in the sun, as it does for Britain’s royal guards. In their bearskin hats and thickly layered uniforms, which are designed to hide sweat, they topple with surprising regularity flat onto their faces, breaking teeth and smashing noses, fainting at full attention with their arms and rifles still rigidly glued to their sides.

But you decide to sit on the rocks, and so you do not topple. You finish your water. You feel limp, like a wrung-out rag. You have a single thought: make it to the ridge and descend to the cool of Emerald Cove. Thirty minutes to go.

At one hundred three degrees internally, you’re pushing into the upper limits of exercise-induced hyperthermia and into heat exhaustion. Your brain is no longer able to deal with large numbers.

One hundred four. Get over the ridge, you tell yourself. Get over the ridge.

Above you the jagged lava rocks begin to distort, reshaping into those ancient giant stone statues erected along the island’s shore. They face you, their enormous heads silhouetted against the blue sky, as if to say, Go back!

But you don’t.

Over millennia, people exerting themselves in hot environments, like the nomadic Maasai of Kenya, have genetically adapted, selecting for tall, slender, long-limbed body types that offer the maximum ratio of cooling surface area to heat-generating body mass. You are not Maasai.

When you finally crest the ridge, your core temperature is pushing 105. You are weak, hot, and thirsty, and you are confused but don’t know it. Gazing back down the way you came, you see the dropping sweep of green. It seems surreal, removed and stylized, like an old hand-painted postcard. Just ahead, the cliff’s edge drops away to crashing ocean far below.

The guy at the bar had said that the top of the trail was marked by a divot where the rock is worn like a V. You walk carefully along the broken ridgetop, afraid to peek over the airy drop. Where’s the guardrail? Your body feels unwieldy.

Maybe it was a mistake to come here straight from the interior highlands, with their evening breezes and cool air. You’d heard that the human body needs time to fully adjust to heat. What you didn’t know is that it generally needs about 7 to 14 days. By gradually building your exercise time outdoors in heat and humidity, your body learns to activate its cooling response at lower temperatures. It learns to increase the rate of sweat production and to trigger a mechanism to conserve sodium, which, along with potassium, is essential for fluid regulation and transmission of nerve signals. (The evolution of this mechanism was honed by our hunter-gatherer ancestors, who struggled to consume enough sodium in their diets.) Acclimation would have slowed your heartbeat but boosted the volume of blood circulated with each contraction to help maintain your blood pressure as your vessels dilated.

But you didn’t acclimate. You relied on the fact that you exercise five days a week at home—also a hot, humid place in the summer. Your heat-addled mind drifts back to those summer days. Instead of this blazing light, you see the tinted windows of your SUV. Instead of this heat smothering your skin, you remember the hair-tingling chill of your car’s air-conditioning, the dim, dank spaces of a parking garage, the cold blasts washing over the treadmill in the climate-controlled gym. It begins to dawn on you that all your life you have relied on artificial sources to keep you cool. You’ve never had to change your behavior or alter your ambitious schedule to accommodate the natural diurnal cycle. You’ve always carried your bubble with you. You’ve never had to truly confront the punishing heat of the midday sun.

And then: you’ve found it! You see a scuffed notch on the ridgetop and, far below, the glint of water. This is why you came! Delirious, you begin to scramble down. You slip, skid on your side, dragging and scraping your hands. You regain your feet and steady yourself against smooth boulders, leaving a bloody handprint. The blood stain looks like a bird, you think, in acrylic paint, textured and thick—another effect of dehydration. Suddenly you notice that a bird (does it have four wings or six?) is swooping toward you, its talons reaching for your face. You try to swat the heat-induced hallucination away, first with your hands, then with your board, but it keeps coming back. You toss aside your board and stumble downward to get out of range.

You come to a ledge. Beyond it is pure drop and yes, there’s the beach, several hundred feet below. You just need to fly, you think foggily, but sense that you have no choice but to climb back up. Your chest pack feels impossibly heavy, as if you’re hauling the 13-ton head of one of those ancient statues. Irritated, you shimmy clumsily out of the straps and watch, mesmerized, as your pack tumbles over the edge and drops into the ocean.

Free at last, you begin to crawl back up. But you feel yourself sliding down the loose tuff. It’s so much easier than climbing. You give into the sensation of increasing speed, like a plane accelerating down a runway. You always loved that. You spread your wings and topple backward down the slope. As your head hits the tuff, you feel the coarse lava grit stick to the drying saliva of your lips and mouth. The ledge stops your descent. And then you feel no more.

As your insides melt and disintegrate, purple hemorrhagic spots appear on your skin. Those, the bloody vomit, and your convulsions are the only external hints of total internal annihilation.

It could be a small measure of good fortune that confusion, semiconsciousness, or coma overcome victims as they succumb to severe heatstroke. The damage about to ensue wreaks so much havoc that almost no major organ escapes untouched. At 105, your metabolism accelerates, so your cells generate heat at a rate that is 50 percent faster than normal. In other words, as your internal temperature rises, rather than cranking your air conditioner, you fire up your furnace. The only effective remedy is to douse the fires with immediate and extensive cooling.

Each heatstroke victim responds differently to these extreme internal temperatures, but a sequence of events might go like this: at 105 to 106 degrees, your limbs and core are convulsed by seizures. From 107 to 109, you begin vomiting and your sphincter releases. At 110 to 111, your cells begin to break down. Proteins distort. Liver cells die; the tiny tubes in your kidneys are grilled. The large Purkinje neurons in your cerebellum vanish. Your muscle tissues disintegrate. The sheaths of your blood vessels begin to leak, causing hemorrhaging throughout your body, including your lungs and heart. There is now blood in your vomit. You develop holes in your intestines, and toxins from your digestive tract enter your bloodstream. In a last-ditch effort, your circulatory system responds to all the damage by clotting your blood, thinking your vessels have been severed. This triggers what physicians call a clotting cascade.

As your insides melt and disintegrate, purple hemorrhagic spots appear on your skin. Those, the bloody vomit, and your convulsions are the only external hints of total internal annihilation.

“Is that a person down there?” the surfer from the bar asks his friend, skidding to a halt in their quick descent through the rocks.

Following the line of a pointing finger, the friend peers at a dark splotch on a ledge far below and a bit to the left, off the winding path and down through the steep rocks.

“Looks like that dude from the bar last night,” he continues.

They continue scrambling down toward the cove, their wide-brimmed hats flapping, surfboards strapped to their backs. As they get closer, they see it is you. They drop their boards and clamber across the rocky slope. When they reach you, you look dead—limbs askew, eyes staring. One of them touches your bare arm. The skin is clammy. He feels for a pulse. It’s faint and quick, like the heartbeat of a bird.

“He’s still alive,” he says. “But he’s way too hot,” he adds, shaking his head. “Let’s get him to the agua dulce.”

Lifting you carefully, they drape you over the stouter surfer’s back and shoulders. You’re several pounds lighter than your normal weight due to dehydration. They scramble down the precipitous path, kicking free tuff that bounces ahead. Ignoring the shimmering water and the sculpted waves curling off the point, they haul you across the beach to a grove of palms against the foot of a cliff. A spring spills from a crevice in the rocks into a clear, quiet pool. Agua dulce. Sweet water.

It’s much cooler than the tepid ocean—­almost cold. They slide your body in and hold you there, immersed, cradling your head above the surface. Two minutes pass, five minutes, ten.

“Está muerto,” says the stouter one.

“No,” says the other, carefully scooping handfuls of cooling water over your head.

Your eyes show a flicker of movement.

You hear splashing, faint at first, from somewhere far away. It comes closer, growing louder, until you realize that it’s right around your ears. You feel the sensation of cold all over your body. When you open your eyes, you can’t make sense of what you see—two faces framed by drooping palm fronds and deep blue sky.

“Descansa,” one says. Rest.

You close your eyes again. A hand brings water to your lips. You drink. You are lucky. With an internal temperature of 106, you peaked within your critical thermal maximum. It’s not yet clear what lasting damage you may have sustained, but you are alive.

Right now, however, all you know is that you’re so very tired. You’ll have to be carried out of here, by stretcher or helicopter or boat. Your thirst feels like a cavernous hollow at your core. You don’t know where you are or where you have been. You remember leaving the scooter and starting up a long grassy slope toward a volcanic ridge. After that there was only the relentless weight of the sun overhead, the heat-blasted lava rock underfoot, and the sense that you were being crushed between them with nowhere to run or hide, a fragile creature of flesh and bone, blood and water, trying to escape the enormity of this force that gives life but, you now understand, can so easily destroy it.

Amy Ragsdale and longtime Outside correspondent Peter Stark (@peterstark_adventure_historian) live in Missoula, Montana. Ragsdale is the author of Crossing The River: A Life In Brazil. In 1997, Stark wrote an article about what it feels like to freeze to death, which led to his book, Last Breath: Cautionary Tales From The Limits Of Human Endurance (Ballantine), in which he first wrote about heatstroke.