198 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS skin condition unless they are sufficiently occlusive to reduce the water loss into the air and permit the buildup of moisture in the skin (6, 7). The prob- lems of dry skin are those of adequate transport of xvater into the horny layer and prevention of gross water loss. Dry stratum corncure presents a high resistance to penetration of water and xvater-soluble substances. Hydration dramatically increases the permeability of the skin ( 1, 2, 6-42), as well as affecting its mechanical properties ( 1, 2, 13, 43-48). This review is concerned with the nature of the hydration of the skin, water binding, and the "barrier" zone. Hx•mt•r•o• The state of hydration of the stratum comeurn is dependent on the rate at which water reaches the comeurn from the layers beneath it, the rate at which water leaves the skin surface by evaporation, and the ability of the cor- ncure to hold moisture (49). The rate at which water reaches the skin surface is governed by the supply of water from the eccrine sxveat glands and moisture obtained by transepi- dermal transport. Secretion from sweat glands consists mainly of water and electrolytes, and would therefore be expected to influence the water content of the stratum corncure by inducing skin hydration. It is estimated that a total transepidermal water loss of about 85-170 ml/day occurs from an average adult under average conditions. In such conditions, at rest and without visible sxveating, sweat secretion amounts to about 300-500 ml/day, a substantial proportion of which is from the palms and soles. Transepidermal or upward diffusion of water yields smaller amounts of xvater since it is limited by the loxv water vapor permeability of the stratum comeurn itself (50). Hydration of the skin varies with changes in the relative humidity or moisture saturation of the air and not the absolute amount of moisture in the atmosphere (5, 13). The degree of hydration is never constant as the variable environmental rela- tive humidity alters the amount of xvater reaching the surface of the skin. Hence, water is constantly diffusing from the lower cells of the epidermis. As long as the ambient relative hmnidity remains less than 100%, water is con- tinuously transferred outward. Under normal conditions, therefore, a gradient in xvater concentration exists xvithin the stratum corncure. The activation energy for the diffusion of water through fully hydrated stratum corneum is 13-16 kcal/mole (14, 20). Through dry tissue it is most likely larger since hydration sxvells and softens the tissue. Hydration increases the thickness of the corncure several fold, as it changes h'om a tissue which normally contains 5-15% •vater to one which may contain as much as 50% water (10). Upon immersion in water, swelling of the cor- ncure starts in a few minutes and continues up to 3 days. The tissue can ab- sorb up to 5-6 times its weight when fully hydrated and this •vater is strongly bmmd within the intracellular keratin (19, 5l, 52).

WATER AND THE SKIN 199 There is little to no correlation of age and water loss. However, in a studv of human fetal skin it was found that in the early stages of pregnancy the skin is very permeable to tritiated water. During this time the skin may serve as a major pathway for solute and water exchange between amniotic fluid and the fetus. However. as the fetus increases in age a large decrease in skin permea- bility to tritiated water occurs (54). Regardless of the increase in permeability, the highly hydrated horny layer remains quite water-imperlneable and represents a stable and extremely cctive diffusion barrier in comparison with the viable skin layers (28). Hy- drated stratum corncure has a diffusional resistance almost 10,000 times great- er than an equivalent layer of water (55). The activation energy and en- tropy of diffusion show that the mobility of water within the tissue is orders of magnitude less than in liquid water and suggest that the diffusing unit is larger than a single water molecule (28, 56). Measurements of the evaporation rates from stripped and nonstripped skin may be used to calculate the resistance of the stratum corncure to water per- meation (57). The resistance of normal human forearm skin to water trans- port is in the range of 150-600 see/cm. The resistance of the same "stripped" skin was in the range of 0.4-1.25 sec/cm (57). The water transmission rate of fish and frog skins approximates free evaporation of water and is 6-20 times greater than mammalian skins (58). The resistance of hydrated stratum corncure is illustrated by a comparison of the diffusion coefficients of drugs in water and in human stratum corncure. Diffusion coefficients ranging from 10 -'• to 10 -•a cm2/sec could be expected for drugs penetrating hydrated stratum corneum. The diffusion coefficient in water for most drug molecules would be expected to range from 10 -• to 10 -ø cm2/sec (55). Hence, the diffusion of water and low-molecular-weight, water-soluble molecules through hydrated keratin is much more difficult than the corresponding h'ee diffusion in aqueous solution. The filament-matrix ul- trastructure is preserved under hydration and the water appears first to enter between the filaments and only later to diffuse within them (59). The trans- epidermal water loss by passive diffusion varies between 0.2-0.6 mg/cm2/hr dependent on the conditions of temperature, humidity, air flow, and body site measured (7, 11, 12, 14, 53, 57, 60-63). With regard to body sites, Bettley and Grice (61) found a h'ansepidermal water loss of 0.35 mg/cm2/hr on the front of the abdomen as compared with 0.6 on the thigh. Diseased skin in which the process of keratinization is disturbed becomes more permeable to water loss (53, 64, 65). Skin affected by eczema and psoriasis could be 8 to 10 times more permeable than normal (64). Even the apparently normal skin of eczematous subjects shows a higher rate of water loss than normal. When large areas of the body surface are so affected, the whole-body skin water loss may reach as much as 2 1. daily. Comparative studies showed that when compared to the normal, ichthyotic skin showed