Dry skin, also known as xerosis, can be a congenital or acquired condition. It can be so mild that it is hardly noticed or so severe that it leads to skin breakdown, severe itching, and infection. Mild dry skin is a condition that affects many patients and is often a complaint of cosmetic patients in particular. Billions of dollars a year are spent worldwide on moisturizing skin care products. It is important, therefore, for the cosmetic dermatologist and cosmetic scientist to understand the underlying causes of dry skin and how current therapies treat this condition.
There are so many products on the market to treat skin dryness that one can become easily overwhelmed. This chapter will discuss what is known about the causes of dry skin with an eye toward elucidating issues that must be understood in order to identify the most effective products or the ones best suited to particular skin types.
Dry skin is characterized by the lack of moisture in the stratum corneum (SC). Water is the major plasticizer of the skin, and when levels are low, cracks and fissures occur.1 For the skin to appear and feel normal, the water content of the SC must be greater than 10%.2 The increase in transepidermal water loss (TEWL) that leads to dry skin results when a defect in the permeability barrier allows excessive water to be lost to the atmosphere. This barrier perturbation is caused by several different factors such as harsh detergents, acetone and other contactants, and frequent bathing. When skin becomes too dry, the outer skin layers stiffen and may develop cracks. The cracks become fissures into the skin that become irritated, inflamed, and itchy. The condition is worse in areas of the body with relatively few oil glands such as the arms, legs, and trunk (Box 11-1).
BOX 11-1 Transepidermal Water Loss
Kligman discussed his observations on the efficiency of the epidermal water barrier as a structure to prevent TEWL in a text in 1964.3 He described covering the orifice of an inverted vial of water with a sheet of SC. This sheet of SC tissue prevented water evaporation. TEWL is now used as a measure of the integrity of the SC. TEWL is defined as insensible water loss through the skin. It is not the same as active perspiration. TEWL is measured in two ways. The first is using a device called an evaporimeter, which calculates the gradient of the humidity at the skin’s surface. The second way to gauge TEWL is to use devices that measure capacitance or conductance. Specifically, these devices measure the electrical capacitance in the skin that is altered by skin hydration. This is actually a measure of hydration of the SC rather than a measure of TEWL; however, the rate of water loss can be extrapolated using capacitance measurements. ...