A technique for accurately measuring transepidermal water diffusion in vivo depends upon electrically monitoring the changing humidity of a stream of dry air passed through a skin chamber. The test is performed on skin in which the sweat glands have been pharmacologically inactivated.
The rate of diffusional water loss was found to be 0.2 to 0.3 mg/hr-1 cm-2 for most areas of the human trunk. The loss is constant for any given area under standard ambient conditions of humidity and temperature.
Considerable regional variation was noted in certain areas, even after the readings had been corrected for varying horny layer thickness and expressed as diffusion constants. Compared with that of the back, the diffusion constant is four times greater on the forehead, nine times greater on the back of the hand, and one hundred times greater through the palm.
Vasoconstriction, vasodilation, or whealing in the underlying dermis did not influence diffusive water loss. Removing the horny layer by stripping with cellophane tape abolished the barrier, increasing water loss 50-fold.
Sodium lauryl sulfate and dimethyl sulfoxide directly altered the horny layer, immediately increasing outward water diffusion. Superficial defatting of the horny layer had virtually no effect on water loss.
Baker H, Kligman AM. Measurement of Transepidermal Water Loss by Electrical HygrometryInstrumentation and Responses to Physical and Chemical Insults. Arch Dermatol. 1967;96(4):441-452. doi:10.1001/archderm.1967.01610040091018