The diffusion of water across the stratum corneum as a function of its water content.

The flux of tritiated water (HTO) through pieces of stratum corneum at four different levels of hydration has been measured. The concentration of water in the stratum corneum, the concentration of HTO in the presenting solution, and the thickness, density, and weight per unit area of the dry stratum corneum are known. The thickness of the hydrated stratum corneum and the permeability and diffusion constants of HTO were calculated. From these in vitro data it is possible to calculate the in vivo thickness of the stratum corneum, its water concentration profile, and the flux of water (transepidermal water loss) at environments of different relative humidities. Both the transepidermal water loss and the water concentration profile change very little as the environmental relative humidity increases from 0 to 80%. The small decrease in the water concentration of the surface layers of cells as the relative humidity becomes very low, however, may cause an observable alteration in the physical characteristics of the surface layers.