Squeaky-clean is good for kitchen sinks, but not for human skin! While sebum locks moisture in skin, the natural moisturizing factor (NMF) keeps skin hydrated. NMF is a mixture of water, free amino acids, lactic acid, and urea, as well as sodium, potassium, chloride, phosphate, calcium, and magnesium salts that keep the skin moist and supple by attracting and holding water. The water content of the skin’s outer layer is normally about 30 percent; it rises after the skin has been treated with certain humectants, such as hyaluronic acid, that boost the skin’s ability to retain moisture.
To help preserve water, skin cells contain fats and fatty acids, which trap water molecules and provide a waterproof barrier that prevents transepidermal water loss (TEWL). It is important to feed aging skin with substances that resemble the skin’s own oils. TEWL is the constant movement of water through the epidermis. Water evaporates through the epidermis to the surrounding atmosphere. Environmental factors such as humidity, temperature, season, and the moisture content of the skin can all affect TEWL. Our skin gets drier as we get older because it loses some of its intercellular lipids after age forty.
It is important to feed aging skin with substances that resemble the skin’s own oils. These moisturizers should become oilier, but not necessarily heavier, as our skin ages. Essential fatty acids can greatly help skin retain moisture, and since they are natural, our skin accepts them more happily, which means less irritation. Skin Eats, Too! Advocates of synthetic skin care insist that our skin is virtually watertight. Many say skin can be scrubbed, steamed, and washed, and nothing penetrates it deep enough to cause any damage.