Whatever your age, there are two guiding principles underlying everything you do as part of your daily basic skin care: one is to increase its moisture content, which you've just learned about. And two is to protect the skin's barrier function. Water is what keeps the cells plump and functioning. Even after the cells die, their ability to absorb moisture continues for some time. As long as the cells are moist and fit tightly together, the barrier function of skin is assured.

Protecting the topmost layers, the stratum corneum, is essential when caring for the skin. Washing, toning, and moisturizing seem so simple and straightforward that many of us take these routine skin care steps for granted, assuming we know what we're doing and using our favorite products. I've found that as knowledgeable as many men and women are, they are often victims of misinformation. They mistakenly believe, for example, that some products will shrink pores. Or that one cleanser is as good as another. "You're just going to wash it off anyway," they may have been told. Some people also believe that the skin type they are in their thirties or fifties is the same type they were a decade earlier or when they lived in the desert or in the mountains.

People often don't realize that skin type may not only change with age, it can also change in response to a shift in the external or internal environment. Traveling to a different climate or a higher or lower altitude and seasonal shifts in temperature and humidity are the most obvious examples. But certain medications, particularly hormone replacement therapy or oral contraceptives, can affect skin type, too. So do drugs designed to get rid of wrinkles, such as Retina-A, or those used to treat acne, such as Acutance. Illness, pregnancy, and menopause cause significant changes in skin physiology. Normal combination skin can become dry. Dry skin can become oily.

SKIN TYPING

Everyone has a unique genetic profile. No two of us are exactly alike, with the exception of identical twins. The characteristics of your skin-its color and pore size, how much hair you have and where you have it, and how much sebum and sweat coats your skin-are dictated by your genes. Your skin characteristics may be similar to your father's skin or your sister's, but there are going to be important differences. After all, the medicines you take, where you live, the facial stress you experience are unique to you. Even identical twins are exposed to different environmental influences. And your own skin will not be the same tomorrow as it is today.

Still, dermatologists have made some for generalizations and created categories of skin types into which most people can fit themselves. The Fitzpatrick Skin Types, for example, named for Thomas Fitzpatrick, the Harvard dermatologist who established them, are based on how the skin reacts to the sun. Another dermatologist, Richard Glogau, devised a system based on visible signs of aging.

I believe that the standard system based on the oil and moisture content of the skin is best because it encompasses the most important factor in skin health: its ability to hold moisture. Of course, there are conditions beyond the basic type of skin that affect its care, so I modify the basic skin types with "special concerns." These include acne, pigmentation, menopause related changes, and sensitivity.

When people think of skin type, they typically mean how oily or dry the skin is. Oil is actually a white, fatty, sticky substance secreted by the sebaceous glands. Except for the lips and eyelids, which have no hair follicles or sweat glands, sebaceous glands empty sebum into the upper part of the hair follicle. As the oil emerges from the follicle opening, or pore, it lightly coats the skin, mixing with the structural lipids within the stratum corneum, creating a kind of protective barrier that keeps water within the layers, helping the skin stay moist and soft. When the sebaceous glands are overactive (usually in response to hormonal stimulation), the excess sebum can make skin look shiny and feel greasy. When sebaceous glands are under active or harsh chemicals or overzealous scrubbing remove the natural lubricant, moisture is lost and the skin becomes dry.