Measurement of transepidermal water loss by electrical hygrometry. Instrumentation and responses to physical and chemical insults

Abstract

A technique for accurately measuring transepidermal water diffusion in vivo depended upon electrically monitoring the changing humidity of a stream of dry air passed through a skin chamber. The test was performed on skin in which the sweat glands were pharmacologically inactivated. The rate of diffusional water loss was 0.2 to 0.3 mg/hr cm2 for most areas of the human trunk. The loss was 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 were corrected for varying horny layer thickness and expressed as diffusion constants. Compared with that of the back, the diffusion constant is 4 times greater on the forehead, 9 times greater on the back of the hand, and 100 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.