by Mary Roach
If sweating is so effective, why were there 14,577 cases of heat illness among active US Armed Forces personnel between 2007 and 2011? Because they work too damn hard. When sweaters exert themselves, the muscles they’re using begin to demand the blood that the body needs to use for sweating. The mildest consequences of this competition for blood are heat exhaustion and heat syncope—fainting. With blood flowing out to the skin for cooling purposes and, at the same time, into the muscles to deliver oxygen to fuel the body’s toil, it becomes harder to maintain the blood pressure needed to pump blood up to the brain. Without enough oxygen-carrying blood reaching your brain, you pass out. (Counterintuitively, overheated people sometimes pass out not in the midst of their exertions but when they stop and stand still; this is because contracting the leg muscles helps keep blood from pooling down there.)
Heat exhaustion is embarrassing but not particularly dangerous. Fainting is both symptom and cure. Once you’re horizontal on the ground, the blood flows back into your head and you come to. Someone brings you water and escorts you to the shade and you’re fine.
Heatstroke, however, can kill. Here too, it begins with a competition for blood. On a hot day, when your body is trying to sweat your core temperature down to the safe range and you haven’t been drinking enough water to replenish your blood volume, and on top of that you’re exercising hard and your muscles are clamoring for oxygen—and the exercise itself is generating heat—something has to give. “The body sacrifices flow to the gut in order to put it where it’s needed,” explains Sam Cheuvront, a research physiologist at the US Army Research Institute of Environmental Medicine (USARIEM), part of the Natick Labs complex. The splanchnic organs—a stupendously ugly way to say viscera—are cut off from the things they need: oxygen, glucose, toxic waste pickup. The technical term is ischemia. It is a killer. The digestive organs start to fail. The gasping gut may begin to leak bacteria into the blood. A systemic inflammatory response sets in, and multi-organ damage ensues. Delirium, sometimes coma, even death, may follow.
Other scientists emphasize heat damage to the central nervous system: Brain proteins unfolding—“denaturing” is the technical term—and malfunctioning. (When you cook an egg or a piece of meat, the change in texture is caused by denaturing.) Cheuvront doesn’t buy the “hot brain” theory. Protein denaturing, he said, occurs at temperatures much higher than the 104 degrees Fahrenheit of a heatstroked brain. There are hot tubs in Japan hotter than that. Cheuvront indicated that there’s no real consensus on how heatstroke kills. Except this: “Lots of bad things happen.”
Gut ischemia may help explain why the US military life raft survival food packet appears, at first glance, to be a cruel joke: nothing to eat but packages of colorful old-timey sour balls, brand-named Charms.‡ If you’re baking on tropical seas and your digestive organs are shutting down, you are not impelled to eat. One thing to be said for sour balls: The acidity stimulates saliva flow, a welcome feature for dehydrated, cotton-mouthed lifeboaters.
HUMIDITY IN the cook box is set at a highly bearable 40 percent, which goes a long way toward explaining why I’m still vertical. When the air around you is saturated with moisture, your sweat—most of it, anyway—has nowhere to evaporate to. It beads on your skin and rolls down your face and back. More to the point, it doesn’t cool you. In the 1950s, the US military invented an index for the treacherousness and downright god-awfulness of heat, called wet-bulb globe temperature: wind chill factor’s partner in meteorological misery. WBGT reflects the varying contributions of air temperature, wind, sun strength, and humidity. Humidity is a full 70 percent of it.
It’s the humidity, but it’s also the heat. When the air is cooler than 92 degrees Fahrenheit, the body can cool itself by radiating heat into the cooler air. Over 92—no go. Radiation’s partner is convection: That cloud of damp, heated air your body has generated rises away from your skin, allowing cooler air to take its place. And, provided it’s drier, allowing more sweat to evaporate. Likewise, a breeze cools you by blowing away the penumbra of swampy air created by your body. If the air that moves in to take its place is cooler and drier, so, then, are you.
After fourteen minutes in the cook box, I’m sweating lightly. Josh Purvis, on a treadmill behind me, began sweating much sooner than me. The hair on his forearms is matted to his skin. The dragon on his chest appears to be weeping. I took all this to be an indicator of his inferior heat tolerance, but in fact the opposite is true. People who are heat-acclimated typically, as Dianna Purvis puts it, “sweat early and copiously.” Their thermoregulatory system takes action swiftly. Mine took ten minutes just to figure out what was happening. Hey, is it hot in here? Should I be doing something? I would enjoy a Popsicle right now.
Not only is Josh better acclimated to the heat and humidity, he’s vastly fitter than I am. Aerobic fitness and percentage of body fat are thus far the only factors shown to reliably set people apart in terms of their tolerance for heat. A strong heart pumps more blood per beat, making it more efficient at delivering oxygen to the muscles. That leaves more blood for the rest of the body and for making sweat. This doesn’t mean that fit individuals don’t get heatstroke. In the military, it’s often the fittest who fall prey to exertional heatstroke, because they’re the only ones capable of pushing themselves hard enough to reach that point.
“Are you ready for the pack?” Dianna has put thirty pounds of sandbags in a backpack to give me a sense of the weight that a soldier in Afghanistan would carry on a two-day ruck march. The typical combat load has been more than twice that—95 pounds, including 33 pounds of body armor, 16 pounds of batteries, and 15 pounds of weapons and ammunition. World War II–era desert survival experiments by Edward Adolph determined that carrying a pack half that weight caused a man to sweat an additional half pound of fluids per hour.
My pack holds only sand. I wear no body armor and carry no weapon other than a Thermes rectal probe. I don’t know what mission this qualifies me for, but whatever it is, I’m in no shape or mood to undertake it. Within seconds of donning the pack, my heart rate shot up by 25 beats per minute. “You’ve increased your workload, so now you need a lot more blood to your working muscles.” Dianna is yelling over the sound of the fans. “And your core temp is climbing. You’re at thirty-seven point nine.” 100.2 degrees Fahrenheit. Marching toward collapse.
Exacerbating the scenario is my tendency to underhydrate. I am what’s known in the parlance of the Heat Research Group of USARIEM as a “reluctant drinker.” Allowed unlimited access to water, a reluctant drinker in a perspiration chamber will quickly lose more than 2 percent of her body weight. And you can’t trust thirst to tell you how much water you need to be drinking. Yas Kuno cites studies in which men hiked for three to eight hours without water, after which they were allowed as much water as they wanted. They tended to stop as soon as their thirst felt quenched. On average, that happened after drinking about one-fifth the amount of fluid they had sweated away.
Outside the cook box a big blue plastic tub is filled with cold water for anyone whose temperature passes 103. Immersion in cold water is the quickest fix for heat illness. When a hot solid or liquid comes in contact with a cooler one, the hot one will become cooler and the cool one hotter. That’s conduction. Conduction explains why tropical shipwreck survivors can die of “warm water hypothermia.” As long as the sea is cooler than they are, they lose body heat to the water.
Conduction can also, of course, make a body hotter. Should you find yourself stranded in the desert, don’t rest directly on the ground—or lean against the Land Rover. Sand gets as hot as 130 degrees Fahrenheit; metal well hotter. Conduction helps explain why loose clothing keeps you cooler in the sun. A baggy shirt heats up, but because the cloth is not in contact with your skin, it does not—unlike a form-fitting t-shirt—conduct that heat to your body. (Loose clothes also let sweat evaporate more readily.)
Even better if the baggy shirt is white. Light-colored clothing reflects some of the sun’s radiatio
n, so you get hit with less of it. Going shirtless in the sun makes a person hotter, not cooler. In Edward Adolph’s “‘nude’ men in the sun” study, subjects sitting on boxes wearing nothing but shoes, socks, and underwear suffered the equivalent of a ten-degree rise in air temperature. Adding to their discomfort, the control—a fully clothed man—was seated beside them. It’s not the heat, it’s the humiliation.
You can imagine how heat illness experts feel about sunbathers: people who willingly lie in direct sun, on hot sand, nearly nude. Small wonder it’s done in close proximity to the big blue tub known as the ocean. Just don’t get up off your towel and start lifting weights. Overworking a set of muscles puts you at risk for a potentially fatal condition called rhabdomyolysis. If the body can’t keep pace with a muscle’s extreme demand for fuel, eventually the muscle becomes ischemic. Heat exacerbates the scenario, because of the competing demand for plasma for making sweat. The cells of the oxygen-starved muscle tissue begin to break down, and their contents spill into the bloodstream. One of these breakdown components is potassium; high levels of it can cause cardiac arrest. Another, myoglobin, damages the kidneys—sometimes to the point of failure. Now you are a very buff and picturesque corpse.
Bodybuilding has been the number one pastime on bases in Afghanistan, where it is even hotter than in Venice Beach. The bodybuilding supplements soldiers take to bulk up more quickly exacerbate the risks. They often contain potentially dangerous compounds: stimulants that spur muscle contractility, thermogenic agents that rev the metabolism, and creatine, which accelerates dehydration. All of these increase the competition for the body’s limited blood supply. CHAMP runs an online resource, Operation Supplement Safety, that reviews the dangers of different products; however, with more than ninety thousand different supplements for sale on the Internet—and Amazon.com delivering to the major air bases—it’s a Sisyphean challenge. For those unfamiliar with the myth, Sisyphus was that Greek guy the gods punished by condemning him to roll an enormous boulder uphill forever, or until rhabdomyolysis set in. During 2011, there were 435 cases of exertional rhabdomyolysis among US service members.
Even simple protein supplements amplify the risks. Protein is deliquescent: It draws water from the body’s tissues into the bloodstream to help flush the protein breakdown products, which are tough on the excretory system. If you’re dying of thirst in the desert, drinking your urine won’t help you. The proteins and salts are by that point so concentrated that the body needs to pull fluid from the tissues to dilute them, which puts you back where you began, only worse, because now you are saddled with the memory of drinking your own murky, stinking pee.
Rhabdomyolysis also turns up at the other extreme of the bodybuild spectrum. Morbidly obese patients immobilized on their backs—say, for lengthy gastric bypass surgery—run the risk that their bodies will press down on the muscles of their backsides so hard that circulation is cut off. After four to six hours, the dying cells of the muscle tissue break open and leak, and when the patient finally moves, or is moved, the blood rushes back in and sweeps the breakdown products into the bloodstream in a sudden, overwhelming gush. Being pinned under rubble in an earthquake or in the wreckage of a car poses a similar risk. As does passing out drunk and lying without moving for six hours. This was explained to me by rhabdomyolysis researcher Darren Malinoski, an assistant chief of surgery at the Portland VA Medical Center. He added that rhabdomyolysis is one reason people roll over in their sleep. “The muscles are getting ischemic, and they make you move.”
“Look: Even your thighs are starting to flush,” says Dianna. All that overheated blood being shunted to my skin. “Do you want to try to keep going a full half hour with the pack on?”
Not even slightly. “I think I get it.”
Dianna asks the lads how they’re feeling. Josh’s fellow instructor, whose name is Dan Lessard, replies that he’s bored. Josh doesn’t hear the question because he’s got earbuds in. He removes one, and a tinny musical aggression leaks out. It’s Five Finger Death Punch, a metal band that from what I can tell uses synthesized machine-gun fire in place of a drummer.
Josh says he and Dan plan to do “a real workout” later in the day.
“Mary stopped after seven minutes with the pack on,” Dianna volunteers. Hey!
Josh defends me. “You don’t come out of the womb with a rucksack on. The first time I put it on, I hated my life.” He seems like a good person who has been handed a lot. His frivolity, his pep, whatever innocence we’re all born with, became something tougher in Iraq. War denatures people.
At 11:30, we’re released from the cook box. “And now you can go take out your friend,” says a lab tech named Kaitlin, referring to the probe. Earlier, in the midst of a conversation about idiosyncratic sweating patterns, Kaitlin raised both arms as though she’d just won Wimbledon and announced, “My right armpit sweats way more.” This we confirmed. Which bring us to the point of Dianna’s work: Genetic differences in thermoregulation—efficient/inefficient, left side/right side, you name it—are surprisingly large and well worth paying attention to, given our seemingly permanent posture of fighting extremism in the Middle East.
Dianna suggests heading to a nearby Walter Reed cafeteria to continue the conversation. Josh seconds. “Sustenance. Let’s get it.”
THE PIZZA at Warrior Café does not look healthy. By that I don’t mean that it’s unhealthy to eat it—though it possibly is—but rather that the item itself looks in poor health. The edemic crust. The sweating cheese. The scabs of pepperoni. I follow Josh and Dan to the salad bar. Like many in the US military, they are disciples of CrossFit, a workout that emphasizes real-world, or “functional,” strength over isolated muscle development. And lots of garden greens.
“Everybody wants to get big and look strong,” Josh says between mouthfuls, after we’re seated. He eats with purposeful intensity, the way he speaks or strides on a treadmill. By “everyone” he means today’s infantry. “There are different ways to do that. You can work hard, or you can do the bodybuilding thing, because you don’t care about anything other than looking good. Nobody wants to work. They experiment with steroids. They want to be bigger, faster.” The eyes fixed on the salad. “But that’s not functional strength. And they have to lug it around, that muscle, and they have to cool it . . .”
“And the supplements themselves increase the risk of heat illness,” I hear myself saying.
That’s not Josh’s concern. His concern is this: Unfit soldiers put the rest of the unit at risk. He places it in context for me: a hypothetical mission to clear and secure an insurgents’ compound. “How about this. In the middle of a firefight, where you’re already physically sucking, one of your buddies gets shot. You’ve got a casualty collection point in the first room that you cleared, but to get there, you have to drag him in his body armor. You’re already smoked, and now you’re dragging dead body weight, so now you’re really smoked.” He jabs at salad. Lunch is a syncopation of hunger and spite. Stab, shovel, chew, speak, stab. “Are you ready to deliver some first aid to this guy who’s depending on you to save his life after you just got your ass handed to you, because you wanted to go do some curls at the gym?”
There is quiet at the table. I’m thinking this story maybe isn’t hypothetical. I’m adjusting to the concept of a “casualty collection point,” to the horrible fact that there can be enough casualties for a “collection.”
“So,” Dianna says after a moment. “Back to heat.”
“I’m sorry.” Stab, stab, shovel, chew. “I have very little to say about heat. People used to ask me, ‘What it’s like in Iraq?’” A garbanzo bean dies on a tine. “Open your oven and crawl in.”
Dianna persists. “So Josh, I hear stories of guys superhydrating ahead of time so they don’t have to carry water. So they can carry extra ammo.”
Dr. Adolph looked into this. “By predrinking,” he wrote, “man converts his interior into an accessory storage tank. A man on foot can thus carry as much as
a quart or more of additional water.” Adolph had a group of men fill their tanks by drinking two pints of water, and then sent them out into the heat on a “dehydration hike.” By checking the dilution of their urine, he was able to conclude that only 15 to 25 percent of the “predrunk” water had been peed out. The rest was available to become cooling sweat.
However, desert survival scenarios aside, the US Army does not advocate predrinking. Exerting oneself on a sloshing stomach is uncomfortable and compromises performance. And soldiers who get carried away in their effort to fill the “storage tank” risk water poisoning: overdiluting the body’s salt levels and throwing the system fatally out of whack.
Also, it may not be manly. “If you take extra ammo,” says Josh in response to his mother’s query, “you don’t take it at the expense of water. You just take it. You man up, and you take it.” Somewhere Josh found blueberries for his salad. He goes at them with brisk, well-centered stabs. He’s going to ace the Bayonet Assault Course.
Speaking of hot-weather military dilemmas: Let’s talk about body armor. The current ensemble weighs 33 pounds. You are weightlifting just going up a flight of stairs. Josh had a buddy who was killed on a rooftop without his body armor on. “His command was ridiculed for it. But in all reality, I wouldn’t have had my body armor on, either.”