Dry skin is recognizable by its tight, rough feel and its dull appearance. Dry skin is apparent in its upper-most layer, the epidermis. Roughly 80% of the body's epidermal cells are made of keratinocytes, composed of soft protein keratin. The epidermal cells are born in the lowest layer of the epidermis, the basal layer. As these cells rise toward the outer (horny) layer, they undergo many changes, including the increase in the amount of keratin they produce. By the time the cells reach the top, they are no longer alive, and are formed entirely of keratin. If the cells contain too little keratin, the appearance begins to look cracked and flaky as cells slough off. Keratin needs water to keep it pliable and healthy; when there is not enough water, the keratin crumbles and the cells can't stay together. This is what happens when the skin becomes dry. In order to keep this from happening, a way must be found to keep water trapped in the skin, keeping the keratin healthy.
Moisturizers are a very important part of dermatology since keeping the skin moist is one of the key factors in healthy skin. Most moisturizers available are composed of some formulation of oil and water, with added ingredients that may or may not help to combat dry skin. However, moisturizers are only capable of keeping moisture from escaping out of the skin, not putting moisture back into the skin.
For moisturizers that work by trapping moisture in the skin, the most effective available is petrolatum or Vaseline, which provides a coating over the skin through which water cannot escape. Unfortunately, because it is greasy, not many people like to use it. Ointments are usually like petrolatum, since most have an oily base. They are also very greasy and are usually used at bedtime rather than during the day. Moisturizing creams contain more oil than water and, as a result, are also very effective. These also may feel a little greasy, but less so than ointments or petrolatum. Lotions are the most popular moisturizer because the absorb quickly, but because they contain more water than oil, they are not as effective.
Very dry skin often lacks the lipids found in healthy, young skin. These lipids help the skin keep moisture in the skin and keep it well hydrated. Without these lipids, water escapes easily and skin is left even more dry and cracked. If your skin is extremely dry, a lipid-replacement moisturizer will be the most helpful. Many moisturizers are now adding the lipids into their formulas in order to restore lipids where they might be lacking in the skin. Lipids are located in the outer layer of the skin and work by trapping water within the skin. When the lipid content is reduced, water in the skin evaporates quickly, resulting in dry skin. Use of lipid products can help restore the lipid content in the skin, allowing the skin to retain water naturally.
Other ingredients that are often added to moisturizers are lanolin (which may cause an allergic reaction), vitamins, essential fatty acids, collagen, elastin, and keratin. Things such as collagen, elastin, and keratin may add a little to the moisturizer, but they also drive the price of the moisturizer up more than it is worth. Vitamins may be of benefit (please refer to the vitamin page for more info), and many moisturizing products with a fair price contain vitamins. One important tip in the use of moisturizers is to apply to damp skin. This helps trap more moisture in the skin, keeping it better lubricated.
Those with dry skin should stay away from deodorant or antibacterial soaps, since they can be drying to the skin. If the odor-fighting capabilities of these soaps are desirable, then it is recommended that they be used only on areas such as the armpits, etc...Cleansing creams and lotions are the most useful for particularly dry or sensitive skin. They leave behind a layer of oil that helps lubricate the skin much as moisturizers do, but they are only moderately effective at cleansing the skin. Mildly moisturizing soaps are probably the most useful for general use since they clean adequately without stripping the skin of it's natural oils.