Mary Roach - Grunt

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. [28] Charms used to be part of ground rations, too. They were removed partly because of a persistent belief that they brought bad luck. No one at the Natick Labs Combat Feeding Directorate knows the origins of the unlucky-Charms superstition. I like this guess best, from the gun-enthusiast website AR15.com: “Because the plastic wrapper sticks… and results in you getting drilled in the brainpan because you were picking at a piece of candy and not paying attention.” 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.)