Normal skin has a soft, supple texture because of its water content. For skin to feel soft, pliable and "normal," its top layer must contain a minimum of 10% water — and ideally between 20% and 35%. To help protect the outer layer of skin from losing water, the skin's sebaceous glands produce an oily substance called sebum. Sebum is a complex mixture of fatty acids, sugars, waxes and other natural chemicals that form a protective barrier against water evaporation. If the skin doesn't have enough sebum, it loses water and feels dry. If environmental factors cause more water evaporation and overwhelm the ability of sebum to prevent water loss, the skin will shrivel and crack.
Dry skin, also called xerosis, is a very common problem in modern societies, affecting people of all ages, even infants. In the United States, most cases of dry skin are related to one or more of the following factors:
Decreased production of sebum — This is often a factor in the elderly, since the number and activity of sebaceous glands in the skin tends to decrease with age.
Loss of existing sebum — This usually is caused by lifestyle factors, such as excessive bathing or showering, excessive scrubbing of the skin while washing, or harsh soaps that dissolve the protective layer of sebum. In some cases, the result is dry skin over the entire body, especially among school athletes who shower several times a day. In other cases, dry skin affects only the hands — for example, in health care workers, food handlers, house cleaners, homemakers, mothers with children in diapers and others who frequently wash their hands.
Environmental conditions that increase water loss — Extreme environmental conditions can overwhelm the skin's natural protective barrier, causing water to evaporate. This is an important reason for dry skin among people who live in sun-baked desert climates, especially in parts of the southwestern United States. Excessively dry indoor air also can cause dry skin and "winter itch" in the northern United States, particularly in people who use forced-air heating systems. Among outdoor athletes, frequent exposure to wind and sun can evaporate water from the skin, making the surface feel itchy and dry. Even swimmers can get dry skin, since the chemical content of pool water actually draws moisture from the skin.
Dry skin is a common problem in people with diabetes or skin allergies (atopic dermatitis). Less often, it can also be a symptom of hypothyroidism, kidney failure or Sjögren's syndrome. In addition, dry skin sometimes develops as a side effect of medication, especially acne products that are applied to the skin.