August 14, 2007
By ABIGAIL ZUGER
Like the finish line of a long road race, the Times Square subway platform one recent hot afternoon was a study in wet humanity, from drenched (a large woman in shorts and a skimpy, sweat-splotched top, flushed and vigorously fanning herself), to barely bedewed (an elderly man in a suit and tie calmly reading his paper).
Who would believe that every sufferer had the same model of personal air-conditioner operating at full blast?
Sweat is our interior coolant, part of a uniquely human biologic machine. The machine drips and occasionally stalls: long waits on torpid platforms can inspire glum reflections on how it will hold up as the planet heats up. But experts counsel optimism: the system is sturdy, adjustable and even reproducible by engineers working to make our future sweaty selves more comfortable.
Humans operate in a tiny range of preferred internal temperatures. We can tolerate overcooling, routinely recovering from long periods of hypothermia with body temperatures diving 20 or more degrees below normal.
But we have little tolerance for even brief overheating: the brain malfunctions with six or seven degrees of fever, and an internal temperature of 110, barely a dozen degrees above normal, is often cited as the upper limit compatible with life. So a good internal air-conditioner is essential, both to dissipate the heat generated by the body’s metabolism and to relieve the heat absorbed from miserable summer weather.
“It is plain old unglamorous sweat that has made humans what they are today,” writes the evolutionary anthropologist Nina G. Jablonski in her recent book “Skin.” “Without plentiful sweat glands keeping us cool with copious sweat, we would still be clad in the thick hair of our ancestors, living largely apelike lives.”
Fur inhibits sweat-induced cooling, and furry animals generally have other ways to lose heat. In humans, Dr. Jablonski argues, sweat glands evolved as body hair vanished, allowing optimal cooling of the enlarging hominid brain and an active lifestyle even in the blazing sun.
For sedentary pursuits in temperate weather, people have no need to sweat: excess metabolic heat easily moves from blood vessels at the surface of the skin into the surrounding air. Because the skin is not completely waterproof, some evaporation of water from skin cells adds a little extra cooling.
But when the body’s owner decides to exercise, the muscles generate too much heat for the air to absorb. The same thing happens when the temperature climbs into the 90s: the skin stops losing heat to the air and absorbs it instead. Then temperature-sensing nerves in the skin and the body’s interior tell the brain to unleash a flow of sweat for heavy-duty evaporation and cooling.
Humidity reduces evaporation and makes everyone sweatier. A breeze enhances evaporation and makes skin cooler (unless the air is so hot the body absorbs its heat instead). Dehydration markedly reduces sweat production. So does sunburn.
But individual sweat patterns still vary enormously. Age, sex, genes, weight and shape play a role, said Craig Crandall, a thermoregulation expert at University of Texas Southwestern Medical Center and Presbyterian Hospital, both in Dallas. So does nonexercise activity, and so, according to a pivotal set of sweat studies done during World War II, does clothing, although not in the way one might predict.
Some people have fewer than two million sweat glands; some have as many as four million. Heavy sweaters may have glands five times average size; their big glands are more sensitive to nerve stimuli and make more sweat.
Everyone’s inner temperature cycles around a slightly different genetically determined version of 98.6 set by the hypothalamus, the brain region that serves as thermostat. We run a little cooler in the morning, a little warmer in the late afternoon. Women run about half a degree higher after ovulation. With menopause the female thermostat becomes notoriously trigger-happy, imagining excess heat where none exists and generating unnecessary sweat.
Men may be more thermally stable, but not for long: beginning about age 60 both sexes sweat less, even if they are in good physical condition, and even if they become seriously overheated. Thus the statistics that during heat waves the elderly are at highest risk of heat stroke.
As for obesity, it is complicated, Dr. Crandall said. Fat may insulate the interior from very hot external temperatures, but it also may compromise heat transfer from interior to skin. Carrying more weight generates more metabolic heat to get rid of. That means more sweat, but research suggests that large people cannot grow more sweat glands to cope with the extra heat load. Radiation of heat from skin to air may become especially important in their heat control.
Over all, though, these factors make small difference in sweat rate. The bigger differences come from activities that may fall short of exercise. Even brief spurts of walking or leg jiggling generate metabolic heat that turns into sweat, as do anger and frustration. The sweatiest person on the subway platform is probably the one who just ran for a train and missed it, Dr. Crandall said.
And as for clothing: less is not always better. In studies during World War II, researchers sat volunteers on wooden boxes in the California desert, some wearing standard olive drab military fatigues, some in light tan summer uniforms, and some “near naked.” The unclothed “soldiers” sweated about 30 percent more than the others — an indication of how much heat their unprotected skin was absorbing from the environment.
And so the average urban warrior might be forewarned that near-nudity on hot subway platforms may be counterproductive, as may be vigorous fanning, pacing and gesticulating if the train is late.
What will happen as the planet heats and more is asked of our sweat glands? No problem, experts say: the system can easily rev up into a high, efficient gear.
The process is called heat acclimation and is routinely seen in athletes training in hot weather. At first their internal temperatures climb, they sweat profusely, lose large quantities of salt in their sweat and feel miserable. But as the days pass they sweat even more, their salt loss diminishes, both skin and internal temperatures drop, and their endurance improves.
At least in part, heat acclimation reflects bigger, juicier sweat glands: in monkeys exposed to continuous heat and humidity, individual sweat glands more than doubled in volume after only two months.
And it took only a week or so for the research subjects in the California desert to develop high sweat rates, low pulse rates and low rectal temperatures. They could work more comfortably, with greatly improved well-being.
In the words of the scientists, they had become “desertworthy.”
All in all, Dr. Crandall pointed out, global warming is likely to be far less thermally dramatic for the individual person than a relocation from Canada to Florida, with its accompaniment of larger, more efficient sweat glands and slightly moister skin.
If the world does become a sweatier place, some engineers are primed to cope. Two years ago, a team at the United States Department of Energy’s National Renewable Energy Laboratory in Colorado completed work on a mannequin, christened Adam, who sweats like a human being and can complain like one, too.
Adam’s slim carbon frame is covered with 120 separate temperature-sensing and sweating zones; water seeps from an interior two-quart reservoir out through his porous skin. He is wirelessly connected to a computer whose software forms his hypothalamus. Other software based on human reactions to a range of temperatures provides estimates of his comfort in various situations.
Adam was devised to help reduce automobile fuel consumption by evaluating ways to limit air-conditioner use. Fully dressed in a car parked in the hot sun, he gets as wet on his back and rear end as any human driver, and just as irritable. Programmers can also rev up his metabolic rate to provide a good, sweaty simulation of road rage, said one of his creators, John Rugh, a senior mechanical engineer.
Adam has helped evaluate clothes for astronauts to wear underneath their spacesuits, and devices to warm injured soldiers. Currently unemployed, he is looking for other work mimicking the human experience in temperature extremes, Mr. Rugh said.