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 radiation, 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.