Your sweat is 99.5 percent water. The rest is about half salt and half other chemicals. Although salt is only a tiny part of your overall sweat, you can lose as much as three teaspoonfuls of it in a day in hot weather, which can be a dangerously high amount, so it is important to replenish salt as well as water. Sweating is activated by the release of adrenaline, which is why when you are stressed, you break into a sweat. Unlike the rest of the body, the palms don’t sweat in response to physical exertion or heat, but only from stress. Emotional sweating is what is measured in lie detector tests.
Sweat glands come in two varieties: eccrine and apocrine. Eccrine glands are much the more numerous and produce the watery sweat that dampens your shirt on a sweltering day. Apocrine glands are confined mostly to the groin and armpits (technically the axilla) and produce a thicker, stickier sweat.
It is eccrine sweat in your feet—or more correctly the chemical breakdown by bacteria of the sweat in your feet—that accounts for their lush odor. Sweat on its own is actually odorless. It needs bacteria to create a smell. The two chemicals that account for the odor—isovaleric acid and methanediol—are also produced by bacterial actions on some cheeses, which is why feet and cheese can often smell so very alike.
Your skin microbes are exceedingly personal. The microbes that live on you depend to a surprising degree on what soaps or laundry detergents you use, whether you favor cotton clothing or wool, whether you shower before work or after. Some of your microbes are permanent residents. Others camp out on you for a week or a month and then, like a wandering tribe, quietly vanish.
You have about 100,000 microbes per square centimeter of your skin, and they are not easily eradicated. According to one study, the number of bacteria on you actually rises after a bath or shower because they are flushed out from nooks and crannies. But even when you try scrupulously to sanitize yourself, it isn’t easy. To make one’s hands safely clean after a medical examination requires thorough washing with soap and water for at least a full minute—a standard that is, in practical terms, all but unattainable for anyone dealing with lots of patients. It is a big part of the reason why every year some two million Americans pick up a serious infection in the hospital (and ninety thousand of them die of it). “The greatest difficulty,” Atul Gawande has written, “is getting clinicians like me to do the one thing that consistently halts the spread of infections: wash our hands.”
A study at New York University in 2007 found that most people had about 200 different species of microbes on their skin, but the species load differed dramatically from person to person. Only four types appeared on everyone tested. In another widely reported study, the Belly Button Biodiversity Project, conducted by researchers at North Carolina State University, sixty random Americans had their belly buttons swabbed to see what was lurking there microbially. The study found 2,368 species of bacteria, 1,458 of which were unknown to science. (That is an average of 24.3 new-to-science microbes in every navel.) The number of species per person varied from 29 to 107. One volunteer harbored a microbe that had never been recorded outside Japan—where he had never been.
The problem with antibacterial soaps is that they kill good bacteria on your skin as well as bad. The same is true of hand sanitizers. In 2016, the Food and Drug Administration banned nineteen ingredients commonly used in antibacterial soaps on the grounds that manufacturers had not proved them to be safe over the long term.
Microbes are not the only inhabitants of your skin. Right now, grazing in the divots on your head (and elsewhere on your oily surface, but above all on your head) are tiny mites called Demodex folliculorum. They are generally harmless, thank goodness, as well as invisible. They have lived with us for so long that according to one study their DNA can be used to track the migrations of our ancestors from hundreds of thousands of years ago. At their scale, your skin to them is like a giant crusty bowl of cornflakes. If you close your eyes and use your imagination, you can almost hear the crunching.
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One other thing the skin does a lot, for reasons not always understood, is itch. Although a great deal of itching is easily explained (mosquito bites, rashes, encounters with poison ivy), an awful lot of it is beyond explanation. As you read this passage, you may feel an urge to scratch yourself in various places that didn’t itch at all a moment ago simply because I have raised the matter. No one can say why we are so suggestible with respect to itches or even why in the absence of obvious irritants we have them at all. No single location in the brain is devoted to itching, so it is all but impossible to study neurologically.
Itching (the medical term for the condition is pruritus) is confined to the outer layer of skin and a few moist outposts—eyes, throat, nose, and anus primarily. No matter how else you suffer, you will never have an itchy spleen. Studies of scratching showed that the most prolonged relief comes from scratching the back but the most pleasurable relief comes from scratching the ankle. Chronic itching occurs in all kinds of conditions—brain tumors, strokes, autoimmune disorders, as a side effect of medications, and many more. One of the most maddening forms is phantom itching, which often accompanies an amputation and provides the miserable sufferer with a constant itch that simply cannot be satisfied. But perhaps the most extraordinary case of unappeasable suffering concerned a patient known as M., a Massachusetts woman in her late thirties who developed an irresistible itch on her upper forehead following a bout of shingles. The itch became so maddening that she rubbed the skin completely away over a patch of scalp about an inch and a half in diameter. Medications didn’t help. She rubbed the spot especially furiously while asleep—so much so that one morning she awoke to find a trickle of cerebrospinal fluid running down her face. She had scratched through the skull bone and into her own brain. Today, more than a dozen years later, she is reportedly able to manage the scratch without doing severe damage to herself, but the itch has never gone away. What is most puzzling is that she has destroyed virtually all the nerve fibers in that patch of skin, yet the maddening itch remains.
Probably no mystery of the outer surface causes greater consternation, however, than our strange tendency to lose our hair as we age. We have about 100,000 to 150,000 hair follicles on our heads, though clearly not all follicles are equal among all people. You lose, on average, between fifty and a hundred head hairs every day, and sometimes they don’t grow back. About 60 percent of men are “substantially bald” by the age of fifty. One man in five achieves that condition by thirty. Little is understood about the process, but what is known is that a hormone called dihydrotestosterone tends to go slightly haywire as we age, directing hair follicles on the head to shut down and more reserved ones in the nostrils and ears to spring to dismaying life. The one known cure for baldness is castration. Ironically, considering how easily some of us lose it, hair is pretty impervious to decay and has been known to last in graves for thousands of years.
Perhaps the most positive way to look at it is that if some part of us must yield to middle age, the hair follicles are an obvious candidate for sacrifice. No one ever died of baldness, after all.