Chapter 1. Skin: Your Outer Layer

Every practical person knows the value of a good outer layer. That’s why you wear a raincoat when you visit London, use plastic wrap to save today’s dinner for tomorrow’s lunch, and don a ski mask when you climb snow-capped mountains (or make an unscheduled withdrawal from someone else’s bank account). But by far, the most impressive covering you’ll ever encounter is your skin—the 8 to 11 pounds of watertight wrapping that covers virtually every square inch of your body.

Skin does far more than the obvious task of keeping your insides on the inside. It’s a washable, stretchable, self-repairing fabric that lasts a lifetime with minimal care. It’s also home to a few other important bits of human machinery, including your hair, nails, and sweat glands. Removed and laid flat, your skin occupies about 20 square feet of space—enough to cover the top of a twin-size bed and make it the surprise winner of the “largest organ in your body” award.

Learning about your skin is a great way to spend a Sunday afternoon (and a surefire way to impress your dermatologist). That’s because the study of skin holds secrets that can help you smell nicer (Body Odor), stave off wrinkles (Wrinkles), and commit the perfect crime (see the box about fingerprints on How Fingerprints Work). In this chapter, you’ll learn everything you need to know to care for your very own body wrapper.

The Protective Wrapper

When people think about the purpose of skin, most settle on the obvious—the way a few millimeters of tissue keeps their blood from oozing messily out of their body.

While a bit of skin certainly helps hold you together, it also plays several additional roles. First and foremost, it’s a protective barrier that separates you from the harsh world outside. It helps keep water and nutrients inside your body, where they belong, and it keeps undesirable elements—like toxins and marauding bacteria—outside.

Building a Barrier

To understand how your skin works its defensive mojo, you first need to understand that it’s actually made up of two distinct layers: the epidermis (which is on the very outside) and the dermis (which is just underneath the epidermis).

The epidermis is your body’s first line of defense. It transforms dead skin cells into a tough, protective layer.


Cells are the smallest building block of life. All living creatures—from slimy amoebas to still slimier car salesmen—are made up of cells. Your body contains trillions of cells, many of which don’t belong to you at all. (In fact, the teeny bacteria that digest food in your intestines account for more than half of the cells in your body, as you’ll learn on Bacteria: Your Extended Family.)

Healthy skin cells start at the bottom of your epidermis, about ⅓ of an inch down, living an easy life and cheerily reproducing. As these cells mature, they get ready to face the outside world by producing a fibrous, waterproof compound called keratin. Keratin is a biological wonder substance. Your body uses it to build your nails and hair, and it’s the basis of some of the sexier trimmings of other animals, including claws, horns, hooves, scales, shells, and beaks.

When your body produces fresh skin cells, these newcomers push the older cells out of the crowded neighborhood at the base of the epidermis and toward the surface of the skin. The trip takes anywhere from a couple of weeks to a month. By the time a skin cell reaches the surface, it’s little more than a dead, scale-like structure that’s filled with keratin but none of the ordinary cellular machinery. Each surface skin cell lasts about 30 days on the outside, which means you get an entirely new skin every month.

image with no caption


Although the process isn’t as dramatic, humans shed their skin (and replace it) more often than snakes do. So the next time you act all repulsed by a reptile, perhaps it should really be the other way around.

On most of your body, the epidermis is barely thicker than this page. However, the skin on the palms of your hands and the soles of your feet is much thicker, so it can spend all day slapping up against the outside world without wearing off.

Shedding Your Skin

Every day, you lose millions of dead skin cells. They don’t fall off all at once—instead, you leave a trail of shed skin everywhere you go. We could tell you how many you lose each minute, but it’s really not that important and likely to make you a little nauseous. (All right, if you insist—30,000 or so scales of skin flake off your body every minute. Right now, they’re collecting on the pages of this book, on your clothes, on whatever piece of furniture you’re sitting on, and so on. Over the course of a year, you lose about a pound of the stuff.)

You might wonder why you never see much of this skin lying around. That’s because once your skin leaves your body, it’s known by another name: dust. Good estimates suggest that the majority of the material you vacuum off your carpet every week (or every month, or every year) are errant skin flakes. That means that when you clean your house, you’re vacuuming up bits and pieces of yourself and the people who live around you. Yes, there’s some genuine sock lint in there, some cookie crumbs, and a bit of tracked-in-from-outside dirt, but it’s mostly skin. Because skin flakes are thin and nearly transparent, your household dust almost always has a light, silvery-grey color.


If you want to take a look at your dead skin before it ends up somewhere else, you can try this somewhat unsettling experiment: Stick a piece of clear tape on the back of your hand, strip it off, and then hold it up to a light. You’ll find hundreds of freshly shed skin cells preserved for your inspection.

The Creature That Eats Your Skin

It turns out that your skin flakes have yet another name: lunch. That’s what they are to an unusual family of creatures that exists on a diet made up entirely of dead skin. (And no, they’re not zombies.)

The culprits are dust mites—very tiny, distant relatives of the common household spider. Dust mites live in our houses by the millions, with most of them taking up residence in upholstered furniture, drapery, carpets, and—above all—mattresses. Dust mites need just three things for a life of contentment: warmth, moisture, and a steady diet of skin flakes. In your bed, they get all three.

image with no caption

You won’t actually see the dust mites that share your home, because they’re vanishingly small (a family of mites could pack themselves into the period at the end of this sentence). But if you looked at one under a microscope, you’d see an otherworldly, eight-legged creature.


Before you let the idea of dust mites ruin your day, remind yourself that, unlike some other mites and other nasties, dust mites don’t actually live on your skin—they live in the fabric of the objects around you. In fact, dust mites have absolutely no interest in crawling on your body.

If you’re like most people, dust mites are no big deal and you can safely forget about them. But for some people (estimates suggest one to three people out of 10), dust mites can trigger allergies and even asthma attacks. Common symptoms of dust-mite allergies include sore eyes, an itchy throat, and sneezing fits. If you think you might be allergic to dust mites, it’s worth going to an allergy specialist, who can give you a quick and painless skin-prick test. If you are allergic, you may want to use some of the tips in the box on the next page to help reduce your symptoms.

The problem isn’t the mites themselves—it’s their excrement and (ironically enough) the skin they shed. And here’s more information you probably don’t want to know: Dust mites actually eat and excrete the same skin flake several times, until they’ve finally digested all the goodness out of it.


As far as critters you don’t want to think about go, there’s good news, too: Two stubborn skin dwellers that have plagued humankind for generations—the human flea and the body louse—are no longer much to worry about. In Elizabethan times, these creatures crawled into bed with virtually everyone, rich and poor. Today, thanks to relatively simple conveniences like scalding-hot water and laundry machines, these pests (and the unrelenting itchiness they cause) are virtually unknown in the Western world.

Skin and the Sun

The thought of voracious mites crawling through your bed linens and devouring your dead skin is an unpleasant one. However, the real danger to skin lies in something that seems a lot more innocent—leisurely summer days and the warming rays of the sun.

Your skin has a bit of a love-hate relationship with the sun. On one hand, the sun fuels miniature chemical factories in your skin that create vitamin D, a key nutrient for your body. On the other hand, the powerful rays of the sun damage skin cells, occasionally scrambling their genetic material enough to trigger deadly cancers. In the following sections, you’ll find out what you can do about it.

Vitamin D

Not long ago, vitamin D was considered dull and definitely unsexy. Sure, it was known to help your body absorb calcium and prevent rickets (a childhood disease that softens the bones and causes debilitating deformities). But adding a dash of vitamin D to milk and a few other vitamin-fortified foods solved the problem, and no one thought much about vitamin D—until recently.

Today, vitamin D has leapt to the forefront of the supplement world, thanks to several new studies that suggest it plays a role in the prevention of cancer and other diseases. It’s no longer treated as a simple calcium-booster—vitamin D now has its own starring role as a hormone that triggers a range of cellular processes. Time will tell if science validates this promising new research, or if it becomes another dead end in the vast maze of nutrition science. In the meantime, there’s good reason to make sure your body has a solid dose of the stuff.

Vitamin D is naturally present in very few foods, but your skin has the ability to create this wonder drug when you expose it to the ultraviolet rays of the sun. The cells that carry out this operation lie at the bottom of your epidermis. You need surprisingly little exposure to the sun to maintain a healthy supply of vitamin D. The rule of thumb is 10 or 15 minutes of direct sun exposure, two or three times a week, on just part of your body (say, your face, hands, and arms). After that, it’s time to reach for the sunscreen.

Unfortunately, the vitamin D manufacturing process doesn’t work well in diffuse sunlight—say, in the winter months of a Northern state. Cloud cover and pollution also dramatically reduce the amount of ultraviolet light that reaches your skin. For example, in Boston, sunlight is too weak to trigger vitamin D synthesis from November through February. To make up the difference, you can take a vitamin D supplement—typically, 1,000 IU each day (look for this measure on the bottle), until summer rolls around again. This is roughly the amount of vitamin D that you’d get from 10 glasses of milk.

Supplementing your diet with vitamin D is particularly important if you have brown or black skin, because this natural sunscreen makes it more difficult to synthesize vitamin D.


The key point to remember is that the amount of sun exposure you need to synthesize vitamin D is very little in the summer months (or in a tropical climate). But in late fall and winter, you can run around in boxer shorts without producing a microgram of vitamin D.

Sun Damage

So far, you’ve heard about the good side of the sun—its ability to fuel your skin’s vitamin D factory (at least in warmer seasons). But here’s the scary part: The dangers of the sun far outweigh its benefits.

The problem, of course, is skin cancer. Skin cancer is by far the most common form of cancer, trouncing lung, breast, prostate, and colorectal cancers. However, many skin cancers disfigure the skin without threatening your life. Only the type of skin cancer called melanoma is likely to spread from your skin to the rest of your body, which it can do quite quickly.

The culprit is the ultraviolet radiation in sunlight. Ultraviolet rays can be divided into three types: UVA, UVB, and UVC, from least worrisome to most dangerous. UVA can age the skin and may play a role in skin cancer, but science still considers it to be the least harmful. UVB is the type of radiation that fires up vitamin D production and triggers sunburns (and eventually skin cancer). UVC is more dangerous still, but in most parts of the world it’s blocked by the ozone layer, which means it never reaches your skin.

UV exposure increases your risk for all types of skin cancer. But like many things that have adverse health consequences (smoking, obesity, and so on), there’s a lag between the behavior and its effect. In the case of skin cancer, the blistering sunburn you get in your early twenties might lead to skin cancer 30 or 40 years later. Furthermore, the effects of sun exposure are cumulative, so it may take many years of sun-inflicted damage before you harm some of your skin’s genetic material beyond repair.

Most dermatologists believe that skin cancer is highly preventable if you practice good sun habits:

  • Reduce sun exposure. Don’t linger in the sun between 10 a.m. and 3 p.m. (11 a.m. and 4 p.m. during daylight-saving time). If you find yourself in strong sun, seek shade.

  • Cover up. Always wear a wide-brimmed hat, long-sleeved shirt, and long pants on sunny days.

  • Use sunscreen. Look for a product that protects against both UVA and UVB rays and offers a sun protection factor (SPF) of at least 15. Applied properly, a sunscreen of SPF 15 protects the skin from 93% of UVB radiation. Higher SPF numbers are better and may block more UVA, but the difference is not nearly as significant as the numbers imply.

  • Use sunscreen properly. Apply sunscreen 15 to 30 minutes before going out. Repeat every two or three hours, more often if you’re swimming.


    Despite decades of use and study, the science of sunscreens isn’t settled. Although sunscreens clearly reduce the occurrence of less harmful types of skin cancer, several studies have found that they offer no protection from deadly melanoma. The reason for this discrepancy is unknown—some experts believe sunscreen gives people a false sense of security, allowing them to stay out longer in potentially harmful sun. Others believe the culprit is not using enough sunscreen or not applying it properly, while still others blame old sunscreen formulations that failed to block UVA rays and contained now-banned ingredients. The best advice is to use sunscreen in conjunction with all the good advice in this list.

  • Avoid tanning beds. Although tanning beds use UVA rays rather than the more damaging UVBs, they’re far from harmless. Even occasional tanning sessions accelerate skin aging and are likely to increase your risk of skin cancer.

  • Learn from your mistakes. If you end up with a sunburn—even a mild one—figure out which rule you broke and resolve to avoid the risk next time. Remember: Sun damage accumulates over your lifetime.

These guidelines are particularly important if you have light skin, a large number of moles, or a family history of melanoma, all of which single you out for greater risk.


Scientists believe that skin exposure is particularly risky for children. Each severe sunburn before the age of 18 ratchets up the risk that skin cancer will develop later in life. So make an extra effort to follow these rules with children and teenagers, keep babies under one year old out of direct sun in the summer, and never leave infants playing or napping in the sun.

Tans, Moles, and Melanoma

Excessive sun exposure and blistering sunburns are clearly a bad idea. But is there anything to fear from the healthy glow of a modest tan?

According to current-day science, the answer is “probably.” Biologically speaking, tanning is what happens when ultraviolet-light exposure causes your skin to start producing more melanin. Melanin is a pigment found deep in the skin, at the base of the epidermis. It’s responsible for freckles, moles, and beauty marks—which are all more or less the same thing—and for the differences in human skin color.

But here’s the bad news. Your tan is a defense mechanism that responds to damage caused by the sun. As ultraviolet light strikes your skin, it shatters the DNA in some unlucky cells. Your body has ways of catching and repairing this sort of damage, but when the rate of damage escalates, it presses the panic button and triggers a tan. In other words, the process that prompts melanin production and tanning is the same process that causes sunburns and may, eventually, cause skin cancer.


In the not-so-distant past, tans were a sign of low-class laborers who had to toil in the fields to make a living. Nobility avoided the sun and strove to keep their skin a perfect shade of porcelain white. Today, those views are reversed, and the tan is a symbol of health and wealth. Rightly or wrongly, we assume that someone who has a tan has probably spent the day outside, engaged in vigorous physical activity (say, mountain climbing or windsurfing), rather than working all day in an office cubicle. Similarly, someone who sports a tan in the dead of winter clearly has the financial resources to jet away to tropical climes.

Finally, it’s worth being on the lookout for melanoma, particularly if you’re at high risk. A melanoma begins life looking like a harmless mole. However, a few red flags suggest the possibility of a problem. If you notice any of the signs below (known as the "ABCDE rule”), see your doctor and get checked. If you catch a melanoma developing in your skin before it goes too deep, the chance of successful treatment is high.

image with no caption

If you have a large number of moles on your skin, it’s important to keep track of your collection. That way you’ll notice when a new mole appears or an existing one changes. Your doctor can help by taking yearly pictures.

Going Deeper

So far, you’ve focused on the epidermis, which is the mostly dead, top layer of your skin. Under the epidermis is a thicker second layer of skin with a whole lot more going on. This layer is called the dermis.

The dermis shapes and supports your skin with tough connective tissue that uses strong, flexible fibers made of collagen and elastin. These fibers make your skin stretchable and resilient so you don’t tear a hole in your armpit when you reach for a box of macaroni on the top shelf. These fibers also keep your skin toned and wrinkle-free through the first half of your life, after which they start to loosen and unravel.

image with no caption

Underneath the dermis is a layer of tissue that isn’t part of your skin. Known as the hypodermis, it binds your skin to your body and stores the sorry globs of fat that you’ll consider in the next chapter.

Unlike the epidermis, which is constantly dying off and renewing itself, your dermis is yours for life. A dense network of blood vessels supplies your dermis with essential nutrients. If you have an open cut, immune cells in the dermis fight infectious evildoers that try to creep in (a process you’ll learn about in Chapter 9).


Tattoo machines work by puncturing the skin thousands of times and inserting small globules of ink. The reason that tattoo ink doesn’t wash off or flake away is because it’s not in the epidermis—instead, it sits near the top of your permanent dermis layer. However, the fine detail in a tattoo will fade with time, because the ink droplets drift slightly.

The dermis is also home to many more important bits of body hardware. For example, every square inch of skin has thousands of buried nerve endings that react to different types of sensations, including cold, heat, pain, and pressure. The dermis also holds oil glands, sweat glands, and hair follicles—three key players you’ll learn about in the rest of this chapter. But first, it’s time to consider what happens as your skin ages, loosens, and gives way to wrinkles.


As you age, the style of your skin changes. You start with a tight-fitting sports jacket, and you wind up with something closer to a pair of baggy pajamas. This transition is quite traumatic for many people, as our culture considers it deeply embarrassing for one’s body to betray any sign that it’s a day over 18. If given the choice to look wise and experienced or young and nubile, most of us would choose the baby face every time.

Many factors work together to cause midlife wrinkles and the pruniness of old age. As the years tick by, the collagen and elastin fibers in your dermis—those components that make your skin flexible and resilient—begin to break down, loosening their hold on your skin. Unfortunately, there’s not much you can do to intervene. But if you must try, here are a few wrinkle-avoiding strategies:

  • Choose the right parents. Your genes have the greatest say in deciding how elastic your skin is, and how long it stays relatively smooth and unwrinkled. That’s why some people in their sixties look like they’re in their thirties, much to the chagrin of everyone around them. If you want a quick prediction of how your skin will fare over the next few decades, look at your parents. And if this leaves you too depressed to continue through the rest of this chapter, consider the possibility that you were adopted from a passing circus.

  • Don’t use your face. Many of the deeper grooves in your face are usage lines that mark where your skin folds when you scowl, smile, frown, or look utterly confused. To reduce the rate at which these wrinkles form, stop expressing any of these emotions. Or just accept the fact that wrinkles add character to your face.


    A variation of this wrinkle-avoiding technique is Botox injections, which paralyze the face muscles using a highly toxic nerve agent. (It’s the same substance that causes death by paralysis in improperly canned foods.) A Botoxed face temporarily loses some of its ability to move, and a face that can’t move has a hard time furrowing up a decent wrinkle. What you get is a sort of blander, wax museum version of your face. If you prefer being wrinkle-free to being able to move your forehead, Botox just might be your ticket.

  • Don’t smoke. Cigarette smoke damages skin, causing it to wrinkle prematurely. This probably happens because cigarette smoke reduces blood flow to your skin, starving it of important nutrients. And while quitting the habit may improve your lungs, it won’t repair skin that’s already sagging.

  • Limit sun exposure. Ultraviolet light (both UVA and UVB) breaks down the collagen in your skin. This weathering process speeds up aging and increases wrinkles. To prevent sun damage, slap on some sunscreen and follow the good sun habits described on Tans, Moles, and Melanoma.

These techniques may slow the rate at which your skin becomes progressively more wrinkled, but what can you do to remove the wrinkles you already have?

There’s certainly no shortage of cosmetic products that promise age-conquering miracles. However, most skin creams do relatively little. On the practical side, they may moisturize your skin (as dry skin looks older) and shield it from sun damage (with sunscreen). The effect of other ingredients is less clear-cut. Although many anti-aging skin creams are packed with anti-inflammatory ingredients, their concentrations are low and there’s little independent research to suggest that they actually do anything. Similarly, vitamins, collagen, antioxidants, and other useful-sounding substances are unlikely ever to reach the lower-level dermis, which is where wrinkling takes place. Some creams contain ingredients that obscure fine wrinkles or scatter light, giving skin a “soft-focus” effect. Whatever the case, these creams can only hide aging rather than make lasting improvements. And lotion lovers beware: Some ingredients can actually aggravate sensitive skin or clog pores, exacerbating acne (Acne).

More drastically, cosmetic procedures like chemical peels, laser resurfacing, and microdermabrasion can improve wrinkles by removing excess dead skin in a strategic way. The effect is temporary, usually limited to fine lines rather than deep wrinkles, and may cause redness and peeling. For all but the most wrinkle-averse, it hardly seems worth the trouble.

The truth is that if you live in your body for half a century, it will gradually develop the creases of use and abuse. The over-80 crowd will tell you that the relentless march of time leaves the human face with more grooves than a 45-rpm record (but first they’ll have to explain what a 45-rpm record is). The real decision you have to make is not how to fight wrinkles, but whether you want to accept them with dignity or become an increasingly desperate chaser of youth.

The Oil Factory

Human skin has two types of glands, and both are happily squirting their stuff onto your skin right now.

Most obvious are the sweat glands, which dampen your skin to help you cool down. (You’ll learn about these on Sweat.) However, your skin also has a set of oil glands, which are properly called sebaceous glands. Most sebaceous glands wrap around one of the millions of fine hairs that line your body. When your body kicks them into action, they release an oily substance called sebum, which oozes down the hair follicle and out onto your skin.

image with no caption

Although it may not be the most suitable dinnertime topic, sebum does a lot for your body. It contains a mixture of substances that softens your hair, moisturizes your skin, and discourages bacteria growth. (In some water-loving animals, sebaceous glands help waterproof fur.)


Many skin lotions soothe and moisturize the skin using lanolin, which is the greasy yellow sebum produced by domestic sheep. In fact, the product name Oil of Olay is derived from rearranging some of the letters in the word “lanolin” (and adding a few more). So the next time you visit a farm, don’t hesitate to rub your face against a sweaty sheep—it could do wonders for your skin.


Less helpfully, sebaceous glands cause acne, the scourge of teenagers everywhere. The problem starts when puberty ramps up the production of certain hormones, most significantly, testosterone (in both boys and girls). At the first sign of these hormones, the highly excitable sebaceous glands begin pumping out huge quantities of sticky sebum. Inevitably, they clog themselves up. But the real nightmare is that they keep producing sebum even when the glands are blocked, causing a swelling that eventually appears on the surface of the skin as a whitehead. With chronic acne, swollen sebaceous glands become inflamed, and trapped sebum can form a cyst. Cysts, in turn, can lead to permanent scarring.

So a blockage deep in a sebaceous gland causes acne, which itself is usually caused by the sudden onrush of hormones at puberty. It’s just as important to note what doesn’t cause acne, including chocolate, fried foods, and poor hygiene. (In fact, aggressive washing can exacerbate the inflammation.) Stress may make acne worse, which is rather unfair, considering that the stress was probably caused by the monster zit that appeared Saturday night before your big date.

If you’re suffering from acne, here’s some practical advice:

  • Think before you squeeze. Virtually all dermatologists will tell you to resist the urge to pop a zit. After all, the risks are legion—you might force the sebum deeper into your skin, worsen the inflammation, and cause scarring. However (and this is not the best topic among polite society), if you have a pimple that isn’t inflamed and is white, ripe, and raised above the surface of your skin, it’s safe to give it a tentative nudge. But if blood or clear liquid emerges, just walk away from the mirror before you do any damage.

  • Try an over-the-counter lotion. Treat mild cases of acne with an over-the-counter cream. The key ingredient to look for is benzoyl peroxide. There’s no magic formula, so don’t plunk down serious cash for the miracle cures shilled on late-night infomercials.

  • Get help for acne that doesn’t improve. Don’t be an acne hero. Living with a bad case of acne can lead to scarring (and not just the psychological kind). Your friendly neighborhood dermatologist can prescribe an antibiotic lotion or an oral antibiotic that will change the balance of bacteria on your skin, ultimately reducing the inflammation.

Your Temperature Control System

In some respects, the life of a reptile has a lot of appeal. When the sun rises on a Monday morning, springing out of bed is the last thing on any lizard’s mind. Much as you may need a second or third cup of coffee before you can string a coherent sentence together, a lizard can’t do much of anything until it’s spent a long, lazy morning basking in the sun, heating its body to operating temperature.

Warm-blooded humans like you don’t work that way. Your internal temperature stays at a balmy 98.6 degrees Fahrenheit (or thereabouts). This is quite a feat, because your body continuously generates heat—primarily by your muscles contracting during routine activity and by major organs like the liver. To cool down, your body needs to release some of that heat into the air around you.

Using a design that’s the human equivalent of a hot-water radiator, your body sends warm blood to the surface of your skin so it can radiate heat away to the cooler world outside. When you need to conserve heat, your body clamps down on this process, tightening the blood vessels in your skin. That reduces the flow of blood near the skin and slows your rate of heat loss.

image with no caption

This system explains why people become flushed when they’re hot (it’s from the increased blood flow). It also explains how frostbite inflicts its damage. The cold itself doesn’t harm your body—instead, the extremely reduced blood flow starves your cells of the oxygen they need to survive.


One thing this system doesn’t explain is the uniquely human habit of blushing, in which sudden embarrassment causes increased blood flow and pronounced reddening, particularly in the face. Scientists guess that blushing may be an involuntary skin signal designed to solve social problems. It works like this: If you get into a sticky situation with a more dominant member of your social tribe, blushing expresses your remorse and gets you off the hook without the need for physical violence. Experts agree that the best way to deal with blushing is to announce it and accept it (for example, by saying something along the lines of, “Oh drat, I’m about to blush again!”). Trying to hide it usually triggers a cycle of increased embarrassment and increased blushing, turning the skin of a sensitive person to a distinct shade of cranberry jelly.


The body’s heat-exchange system makes perfect sense, but on its own it’s just not enough. Sure, your body can radiate heat through your skin, but on a hot day it won’t lose a sufficient amount to keep you cool. To lose heat more efficiently, you need the help of sweat.

Sweat is part of your body’s messy air-conditioning unit. Your body sweats continuously, but you don’t notice the small amounts of moisture that trickle out because it’s truly miniscule, and your body reabsorbs some of it. But when the outside temperature rises or the activity in your body soars (say, when you run to catch the last bus home), your body ramps up its sweat production.


Stress also causes sweating. Other than the obvious purpose (to embarrass you in your third-grade public-speaking competition), sweating in response to stress works as part of your body’s fight-or-flight response. Essentially, your body assumes that you’re either going to run away from or attack the threat in front of you, so it prepares for the imminent increase in body heat by switching on your natural air conditioning.

Sweat is mostly water, with a pinch of salt and tiny amounts of other waste products thrown in. As sweat evaporates, it takes some of the heat from your skin, noticeably cooling it. (And if you don’t think it’s noticeable, try taking a hot shower and then walk around the house without drying yourself.)

But the real point of sweat isn’t to cool your skin, but to cool your blood, thereby maintaining your internal body temperature. To accomplish this, your body uses the blood redirection trick you saw on Blushing. When you sweat, your body sends more blood to the newly cooled surface of your skin. The blood gets a chance to cool down, and then it gets pumped back deeper into your body. This isn’t all that different from the way a refrigerator works—it circulates a special substance (ammonia gas) through coils at the back. Once this substance cools, it’s returned to the inside of the fridge so it can keep your rutabagas fresh.


Say what you like about farm animals and zoo dwellers, but humans are the undisputed sweating champions of the natural world. In fact, many mammals barely sweat at all. Cats and dogs, for example, sweat only on their paws. (This is why dogs pant—they can’t cool themselves sufficiently by sweating alone. The air they inhale cools the surface of their lungs and the blood that runs nearby.) Our habit of sweating probably explains why we don’t have thick fur covering our bodies like some other animals—if we did, it would interfere with our ability to evaporate sweat from our skin.

Your skin is studded with several million sweat glands. They cover every square inch of your skin, with just a few exceptions (namely, your lips, nipples, and sexual equipment). The structure of a sweat gland is simple: It looks like a coiled tube that sits in the dermis (where your body produces sweat) and opens out through a pore. Some, but not all, sweat glands squirt their liquid out onto a hair, like your sebaceous glands do.

image with no caption

If you live in a cold or moderate climate, you can produce about one quart of sweat every hour. Move to the tropics and a few weeks later your body doubles or triples its maximum sweat-producing capacity. At the same time, your sweat becomes less salty.

Body Odor

So far, we’ve skirted over one nagging question—namely, why does your personal air conditioner smell like dirty socks?

Surprisingly, sweat itself has no odor. You can douse yourself in the stuff without picking up the faintest scent. However, the bacteria that live on your skin aren’t so innocent—they feed on your sweat and produce a rich collection of stinky substances. (This is the reason a discarded workout shirt smells worse the next day—the bacteria living on it have had some extra time to digest its tasty payload.)

As you’ve probably noticed, body odor seems to emanate from specific places in your body. To understand why, you need to recognize that your body actually has two types of sweat gland:

  • Eccrine. These are the most numerous sweat glands. They’re found all over your body and are particularly dense on the palms of your hands, the soles of your feet, and on your forehead. The eccrine glands do most of your body’s temperature control.

  • Apocrine. These sweat glands are concentrated in the forested areas—the armpits and genitals. Instead of secreting ordinary salt water, they squirt out a thick, milky fluid that has plenty of fats and proteins.

Apocrine glands almost always dump their contents onto a hair follicle. Unlike eccrine sweat glands, apocrine glands don’t do much for temperature control, and they react more readily to emotions and sexual stimulation. Bacteria devour rich, apocrine sweat, leaving their signature gamey odors behind. Bacteria aren’t nearly as interested in the watery sweat that leaks out of the eccrine glands, but under the right conditions they can still make a meal of it, along with skin oils and dead skin cells. (That’s why a warm, moist, poorly ventilated foot can develop a room-clearing odor that rivals the sweatiest armpit.)


Apocrine glands develop during puberty, which is why babies and toddlers don’t have body odor problems.

This raises an excellent question—if apocrine glands don’t help you cool your body, why are they there stinking up the place? It seems that the chief purpose of apocrine sweat is to create your distinctive body odor. Several studies have shown that women, when asked to smell a lineup of used undershirts, can pick out their man’s shirt by smell. Thus, apocrine glands are the human equivalent of the sexual-scent glands of other animals—and whether they have a real effect or are just an evolutionary leftover depends on whom you ask.

Deodorants and Antiperspirants

When people learn how body odor works, the first question they usually ask is how they can stop it.

The first line of defense is bathing, which reduces your sweaty residue, leaving bacteria with a whole lot less to lunch on. However, soap and water won’t kill the bacteria itself, which is a more-or-less permanent resident on your body. (You’ll learn more about your skin-dwelling colonies of bacteria on Living in a Bacterial World.)

Another popular tactic is to use deodorant or antiperspirant, which you usually apply to bacteria’s favorite dining spot—the underarms. These two products work differently. Deodorants mask body odor (which should rightly be called bacteria odor) with a different smell. They may also contain powders that absorb moisture and chemicals that can kill some of the bacteria. Because you can never completely eradicate the bacteria, deodorant is really a population-control tactic.

Antiperspirants may include musky perfumes and germicides like deodorants, but they also have an aluminum-based chemical that temporarily blocks sweat glands. To be labeled an antiperspirant, clinical tests must show that the product actually works. This involves rather amusing studies that put a number of people in very hot rooms and get lab technicians to collect the resulting sweat. The rule of thumb is that a basic antiperspirant must reduce underarm sweating by 20 percent in most people. A high-powered antiperspirant (one with “maximum” or similar language on the label) must hit the 30-percent mark. Prescription antiperspirants can reduce sweating even more.

Now that you understand the science of your armpit, you’re ready to learn about a significant drawback to antiperspirants: They only work on the comparatively harmless eccrine glands. So while antiperspirants do decrease the amount of wetness (which does slow down your armpit bacteria), they can’t suppress the strong-smelling apocrine glands. So, after an hour at the gym, you’ll still smell like, well, yourself.

Finally, it’s important to address one of the very real risks of antiperspirants. No, it’s not breast cancer or Alzheimer’s disease, despite what you might have read in imaginative email chain letters. For the record, aluminum, the key ingredient in antiperspirants, is the third most common element on our planet, and it’s found in food, air, and over-the-counter medications like antacids, all of which provide more aluminum than you can absorb from an antiperspirant through your skin. Furthermore, the amount of waste your sweat glands excrete is small, so there’s no reason to think that slowing down a few sweat glands can increase the level of toxins in your blood.

The real danger of antiperspirants is staining. That’s because the aluminum can react with your sweat to create an embarrassing yellowish stain on your favorite clothes. If this is a problem, apply your antiperspirant and walk around shirtless until it dries. Or consider switching to deodorant.


Deodorants and antiperspirants are simple ways to deal with ordinary sweat, but if you suffer from excessive sweating, you may need the help of the medical community. Your doctor can determine if your sweating is linked to another problem, such as thyroid disease, or if it’s just genetic bad luck (in which case you have a range of treatment options, from stronger antiperspirants to underarm Botox injections and surgery). Lastly, look out for body-odor changes. For example, suddenly sweet body odor may hint at diabetes, or it could just be the result of a change in diet. If in doubt, have it checked.


Along with the different sweat glands and oil glands, there’s one more type of body equipment rooted in the dermis—your hair.

Hair consists of long, flexible strands of dead cells. These cells are filled with keratin, the same wonder substance that strengthens the outer layers of your skin (Building a Barrier). The figure on the next page shows a hair erupting from the surface of the skin. As you can see, the surface of the hair shaft consists of overlapping scales, like shingles on a roof.

This close-up holds the secret to hair frizz. On a humid day, tiny water droplets work their way in between the scales of the hair shaft, making the hair thicker and rougher. Conditioners try to prevent the problem by leaving an oily, water-repelling coating on the hair. Some anti-frizz products accomplish the same thing using silicone, which simultaneously seals the hair and weighs it down, straightening it.

image with no caption

Your body creates each hair in a hair follicle—a tiny pouch deep in your dermis (Going Deeper), where your hair is woven together out of living cells. As each new layer of cells is tacked onto the bottom of the hair, the cells die, and the hair becomes just a little bit longer. In other words, your body treats your hair the same way it treats your skin—it keeps the living cells on the inside and puts the dead stuff on the outside. Which is good in a way, because a head full of living hair would make for an agonizing day at the barbershop.


Your hair stores a permanent record of the toxins you ingest, including illegal drugs like cocaine, amphetamines, heroin, and marijuana. A standard hair drug test searches for traces of drugs consumed over the last 90 days. But take longer strands of hair or some slow-growing body hair and you can easily put the last year of your life under the microscope.

Vellus Hair

Compared to other animals, humans appear relatively hairless. But the surprising truth is that you have more hair follicles crammed onto each square inch of your skin than the hairiest chimpanzee, monkey, or gorilla. The difference is that most of your hair (whether you’re a man or a woman) is nearly invisible. It consists of a fine, slow-growing, almost colorless covering of downy hair called vellus hair.

Vellus hair blankets your body, insulating your skin and heightening your sensitivity to touch. It’s the reason you can sometimes “feel” a person moving past you in a darkened room—the passing air currents disturb your fine hairs and trigger the sensitive nerves attached to them. However, vellus hair is easily overlooked and nearly invisible without a magnifying glass. It’s sometimes known as “peach fuzz.”

Terminal Hair

Terminal hair is the more obvious hair found on your body, including the hair on your head, your eyebrows, and your eyelashes. After puberty, terminal hair appears in many more places on your body—some where it’s wanted, and some where it’s decidedly inconvenient.

Terminal hair is thicker, longer, and darker than vellus hair, although some individuals can have light-colored and fine terminal hair. Terminal hair also boasts a range of textures and colors. The difference between wavy, curly, and straight hair is all in the shape of the follicle that produces it. For example, a perfectly round follicle constructs straight hair, while an oval follicle produces wavy hair. All the hair-care products in the world can’t alter the shape of your follicles.

Like most of your body’s equipment, terminal hair has good reasons for existing:

  • The terminal hair on your eyelashes keeps dirt and insects out of your eyes. Your ear hairs and nose hairs play a similar role.

  • The terminal hair on your eyebrows prevents sweat and rain from dripping onto your face.

  • On your head, terminal hair helps prevent sunburns on sunny days and heat loss on cold ones.

  • Pubic hair is a type of terminal hair that serves as a secondary sexual characteristic. That means it’s there to advertise that you’re a fully functioning adult with the appropriate baby-making abilities.

And, of course, humans have picked up the habit of using hair for something entirely different—as a powerful expression of self-identity that can announce everything from your gender to your political affiliation.

Shampoo and Conditioner

Few products make the bold, imaginative, and highly delusional claims that shampoos and conditioners do. Almost every brand describes mystical powers that can revitalize, energize, volumize, and therapize hair (and the last two aren’t even real words).

Unfortunately, the science of hair pours some distinctly unsudsy water on the whole idea. Because each one of your hairs is a sorry strand of dead material, there’s really nothing you can do to “nourish” it. That means you don’t need to shampoo with vitamins or amino acids. The best botanicals are the ones you grow in pots and water twice a week, and you’re better off rubbing herbs and fruit extracts on your dinner than on your scalp.

And forget other hair-care health claims—the government doesn’t regulate shampoo, so manufacturers don’t need to substantiate their fanciful promises. (For example, some shampoos boast that they protect hair from ultraviolet rays. This typically means that the manufacturer has added a UV-protective ingredient, which you’ll only end up rinsing down the drain, and which isn’t present in strong enough concentrations to have an effect in the first place.)

The truth of the matter is that shampoo provides a rather straightforward hair-cleaning service. To understand how it works, you need to know how your hair gets dirty in the first place. Ordinarily, the same sebum that lubricates your skin (Acne) moisturizes your hair. This is mostly a good thing, because the thin layer of oil protects your hair from damage. But as the hours pass and you go about your daily business, your hair collects natural oil and skin flakes that shed from your scalp. This is where shampoo comes in—it includes powerful surfactants that dissolve these substances, in much the same way that you rinse dirt out of clothes with detergent or grease out of pots with dish soap. The problem is that, in the process, shampoo strips out most of the sebum, leaving your hair dry and fragile (although the effect is far gentler than if you showered with laundry detergent or dish soap).

To balance this effect, many shampoos have conditioning agents, and many people use a separate conditioning product. There’s a bit more variability to the way that conditioners work, but essentially they all aim to coat the hair shaft with protective sebum-like compounds. Some creamy conditioners feel heavy in the hair and glue together damaged fibers and loose scales. Other conditioners are lighter and oilier. But all these substances cling to the hair shaft and don’t rinse out with plain water.

The best hair-care advice for a biology wonk is this: Don’t break your budget on high-end products. Buy the shampoo that matches your hair type (oily or dry) and use conditioner to manage excessive dryness. Finally, don’t pressure yourself into washing your hair every day. If you’re just as happy waiting a day or two, your dead hair will probably be a bit better off.

Hair Growth and Hair Loss

Your hair follicles have a tiring job, and every once in a while they take a break. At the moment, roughly 90 percent of the hair on your head is growing, while the rest is taking some time off. Some of that hair will resume growing again after a pause of a week or two. A smaller proportion will simply fall out—about 50 hairs a day. But don’t panic, because the same hair follicle will begin creating a new hair in its place. An average hair takes six years of abuse on your head before it drops out and the hair follicle starts over.

Eyebrows and eyelashes have a different growing schedule. Eyebrow hairs grow for about 10 weeks, and then rest for the better part of a year. (This is what makes eyebrow shaving such a dastardly revenge tactic.) Eyelash hairs last about three months apiece before falling out and being replaced.


Hair growth is an issue that comes with a boatload of baggage. Hair embarrasses us when it appears in certain places (inside our ears, for example). In other places, it mortifies us when it vanishes. But other than cutting your hair, you have little control over its comings and goings.

Here are some quick facts that can help separate the bare facts from the follicle folklore:

  • Hair doesn’t grow faster or thicker after you shave it (on any part of your body).

  • Hair doesn’t grow faster at night. Female hair doesn’t grow faster during menstruation. Instead, all hair grows at a constant rate with a brief resting period.

  • Frequent washing, blow drying, and dyeing your hair doesn’t destroy hair follicles or slow hair growth. However, these activities might make your current hair more brittle and fragile. But even if you damage a hair to the point of falling out, the same hair follicle will produce a new one to take its place.

  • You’re born with all the hair follicles you’ll ever have. As you grow and your skin stretches from infant-sized to adult proportions, your hair follicles simply become more spread out.

  • While you’re pregnant, each hair clings on a little bit longer, eventually giving you a fuller head of hair. After you give birth, your body sheds its hair more quickly to make up for lost time.

  • The only ways to remove hair permanently are laser hair removal and electrolysis. Both treatments take numerous sessions over the course of many months, and neither treatment works for all people or all hair.

  • Wearing hats doesn’t cause hair loss.

Male-pattern baldness, which causes the infamous ring-around-the-bald-spot effect, develops gradually and eventually affects about two-thirds of all men. Its causes are genetic, and its treatments are few. A small set of medications give some improvement to some people, but these drugs are often ineffectual. There are only two guaranteed solutions: hair-transplant operations (which are expensive, time-consuming, and may look odd, since hair loss continues around the transplanted patches), and head shaving. If you opt for the latter, you’ll likely tell people that you deliberately chose baldness to emphasize your virile, youthful manliness. Everyone will know the truth, of course, but they’ll also be quietly relieved that you aren’t practicing the dreaded comb-over.

Get Your Body: The Missing Manual now with the O’Reilly learning platform.

O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.