But what's surprising is not the size but the notion that skin is an organ. It seems to be in a category different from the heart, the stomach, and the lungs -- more like clothing, something meant simply to hold in the important stuff and make us look good. Indeed, skin serves both of those vital purposes.
But skin does crucial physiological work. It protects the body from dirt, chemicals, and microorganisms and functions as a host to beneficial bacteria. It blocks the sun's ultraviolet (UV) rays even as it soaks in enough to produce the vitamin D we need to absorb calcium and build strong bones. And skin's pores open and close to keep our temperature steady.
Humans' ability to shed heat by sweating enabled the evolution of a large brain, which could not operate without a mechanism for staying cool. "We have many more sweat glands per unit area than other mammals," says Nina Jablonski, head of the anthropology department at Penn State University.
Skin accommodates every stretch and twist we make, expanding for pregnancy or obesity. It renews its topmost layer, the epidermis, every month or so, the better to heal quickly from most cuts and scratches and maintain a fresh line of defense against the outside world.
And skin serves as a conduit for the network of nerves that yields our sense of touch. Of the five senses, touch is the least noted because its influences on human well-being are more subtle," Jablonski writes in "Skin: A Natural History" (University of California; 2006). Perhaps that subtlety arises from the number of ways we use touch -- to feel the warmth of the sun or the strength of a breeze, to assess the quality of garden soil or the ripeness of a tomato, to comfort and care for children. We know from studies that when we exchange caring touches with other people, our levels of stress hormones and anxiety are reduced, Jablonski says. And when we receive touch -- a hug or a massage -- we feel a sense of relief."
Our skin does so much, it's easy to forget it does anything at all -- until it begins to itch, hurt, or, heaven help us, look ugly. Avoiding these calamities altogether would be impossible. But by understanding what can go wrong, we can keep the most troublesome problems to a minimum.
Americans make some 25 million visits a year to dermatologists, and 10 million of these are for help with a rash. Most occur when the immune system goes to work against a perceived threat: a poisonous plant, an insect bite, an irritating chemical, fungal spores, or bacteria. In cases of chronic rash, the offender can be hard to pin down.
Eczema, for example, produces dry, itchy, and sometimes oozy patches, typically on the face, neck, elbows, knees, and ankles. Triggers include dust mites and dander. Rough fabric, heat, and soaps can exacerbate symptoms.
The thick, scaly patches that characterize psoriasis, most commonly seen on the elbows, knees, and lower back, seem to be prompted by medicines, stress, or injury to the skin. An autoimmune disease, psoriasis occurs when the system mistakenly speeds up the production of skin cells.
And the itchy, flaky skin of seborrheic dermatitis, which forms in the eyebrows, in the creases of the nose, behind the ears, and on the scalp (where it creates dandruff), happens when the immune system attacks fungal spores that live in hair follicles. The skin usually tolerates these spores peacefully, but cold weather conditions, infrequent shampooing, or stress can cause an outbreak.
All these conditions -- eczema, psoriasis, and seborrhea -- run in families and ebb and flow with stress and, for women, the hormonal fluctuations that occur with pregnancy, menstrual cycles, and menopause, says Diane Berson, a dermatologist in New York City. For these three chronic rashes, flare-ups can be prevented by using a good moisturizer. Doctors can also prescribe anti-inflammatory treatments and, for seborrhea, antifungal creams.
Yet another form of rash, rosacea is more noticeable in people with fair skin but occurs in those with dark skin as well. It usually appears after age 30, and it can worsen over time, progressing from simple redness on the nose, cheeks, chin, and forehead to the appearance of blood vessels to the development of tiny pimples. Scientists do not know what causes rosacea. Some suspect that drinking alcohol can make symptoms flare up. Sun exposure can, too, as can anything else that causes blood vessels to dilate.
To prevent the condition from worsening, Linda Franks, another New York City dermatologist, suggests using sunscreen and a lotion containing an antioxidant, such as vitamin C. Doctors can treat rosacea with topical and oral medications and reduce redness with lasers, which destroy dysfunctional blood vessels at the skin's surface.
This is caused by an immune-system response, and it develops in three stages, Franks says. First, tiny pieces of skin that should be shed build up gradually in a pore, forming a plug. Second, the oil that's produced naturally by glands deep inside the pore gets blocked behind the plug. And third, the bacteria that live on the skin find their way into that oil. Its like a petri dish for the bacteria," Franks says. Overnight, you find, a pimple has formed."
People often try to prevent acne blemishes by using alcohol toners or medicated cleansers, but these won't stop that crucial first step. A better approach, Franks says, is to use retinol products, which help skin shed more cleanly, and a mild cleanser. Salicylic acid, found in many acne treatments, can also help break up the plugs.
Wrinkles and Age Spots
With age, skin grows drier, thinner, and less elastic. Of course, the more the skin is exposed to sun, the faster this happens. At the same time, making repetitive facial expressions builds grooves gradually beneath the skin surface. And then gravity tugs everything downward. (Smoking worsens all of this by impeding blood supply to the skin.
Many creams contain ingredients designed to fight back. Alpha hydroxy acids help shed dead skin. Retinol does the same and also encourages production of collagen, the skin's connective tissue. Oils restore the skin's barrier function, and humectants, such as glycerin, bind water to the skin.
Doctors have bigger weapons, too. Dermabrasion, microdermabrasion, laser treatments, and chemical peels remove the topmost skin cells, diminishing wrinkles and pigment splotches known as age spots. Botox, injected into grooves in the face, blocks chemical signals that cause the muscles to contract and relaxes the furrow. Fat, collagen, and hyaluronic acid can be injected to plump skin. And a face lift can stretch wrinkles into submission. All such treatments are temporary.
Cancerous growth can occur in the basal and squamous skin cells of the epidermis or in the melanocytes, the cells that make pigment. Basal-cell cancer usually does not spread and can be treated with simple surgery. Squamous-cell cancer is a bit more serious, because it has the potential to metastasize. Melanoma, which is even more likely to spread, is the deadliest of skin tumors.
Sun exposure appears to be directly involved in basal- and squamous-cell cancers, which commonly occur on areas of the skin exposed to sun. UV radiation is thought to be the biggest risk factor for melanoma, but melanoma can also occur on parts of the body rarely exposed to the sun -- even in mucous membranes or on the soles of the feet. But people who have had at least one severe, blistering sunburn are at increased risk for this disease.
To prevent melanoma and other skin cancers, doctors advise wearing sunscreen and otherwise avoiding exposure to direct sunlight.
Color Theory: Beyond Black and White
Why does human skin come in so many colors? A leading hypothesis, proposed by Nina Jablonski, head of anthropology at Penn State University, is that skin color correlates with protection. Different shades, which are determined by the amount of pigment, or melanin, in the skin, allow us to absorb the ideal amount of UV radiation from the sun at different latitudes.
People whose ancestors came from equatorial regions have relatively numerous and large melanocytes, the cells that store melanin, to block harmful UV. People whose forebears hailed from northern latitudes have fewer and smaller melanocytes, because there is less UV light, and they need as much sunlight as possible to make adequate vitamin D. Those from middle latitudes have a medium number of midsize melanocytes. Its a classic evolutionary story, where you come up with the best fit for the environment," Jablonski says.
Text by Mary Duenwald