TRANSEPIDERMAL MOISTURE LOSS 363 Each dew point instrument is attached to one side of a two-pen re- corder. An electronic calibration unit is also attached to equalize the electrical response of the two instruments. The range of moisture levels which may be measured is determined by the flow rate. A rate of 50 ml/min allows measurements of 0 to 10 mg/cm2hr with a 50% scale reading of 0.3 mg/cm2hr. Standardization of Apparatus The Sage Instrument Probe is replaced with a gas chromatographic type injector port. Standardization is accomplished by leaking water at several different rates into the gas stream utilizing a radiometer auto- matic syringe microburet fitted with a 2-v1 gas chromatog•:aphic syringe. Standard response graphs are plotted and used to interpret the instru- mental response. Environmental Precautions Water measured by the apparatus in in vivo experiments could come from three sources. These sources are listed in Table I along with the orders of magnitude of each effect. Eccrine sweating is the largest effect. It is discontinuous and varies greatly in amount. Baker and Kligman (8) utilized pharmacologically induced anhydrosis. Not being so equipped we have attempted to eliminate eccrine sweating by maintaining the thermal and emotional environment below threshold values. Subjects are conditioned to a 19.4-20.0øC room for 20 min and then tested at this temperature. Testing is conducted in an isolated area. While not ab- solutely guaranteeing anhydrosis, eccrine sweating is controlled out of our measurement within practical limits. Table I Basic Mechanisms by which Water Is Lost through Skin Rate Mechanism (mg/cm2/hr) Duration Eccrine sweating Transepidermal diffusion Stratum corneum desorption of normal hydrated skin by a stream of dry gas 32-48 Discontinuous 0.2-0.3 Continuous 0.6-0.7 10-15 rain Stratum Comeurn Desorption During exposure of the surface of normal skin to the dehydrating in- fluenee of a stream of dry air, some stratum comeurn desorption of the

364 JOURNAL OF THE SOCIETY OF COSME•I'IC CHEMISTS normally hydrated skin takes place. This effect is superin]posed on the measurement of the rate of transepidermal moisture loss. An experiment in which the normal hydration level was raised slightly by soaking the arm in water for 5 rain prior to making the measurement was carried out to find out how long it would take to bring the two different arms to the same level of dehydration. The results, shown in Fig. 2, coincided with our finding that about 10 to 15 rain were required to reach equilibrium when dry gas is used on the normally hydrated arm. To prevent stratum comeurn desorption from causing an error in the rate of transepidermal diffusion loss, we wait 15 min after putting the sampling cells into place before determining the response. 0.3 0.2 ARwvf (mg/cm2/hr) 0.• ' I 0 2 4 6 8 10 12 t4 16 TIME (rain) Figure 2. Desorption of hydrated stratum comeurn. l)iffcrcncc in moisture loss rate (A Rwvt) between an experimentally hydrated and a norn,•al ann is shown as a funcliol• of time Influence of Age, Sex, Height, and Weight A brief survey was made of 14 subjects from 19 to 48 years of age, both sexes, and all types of complexions and builds. The results (Table II) ranged from 0.2 to 0.46 mg/cm•/hr with a mean of 0.31 and a standard deviation of --+0.04 mg/cm•/hr. No correlation with age or physical characteristics was noted in this study. Sample Thickness Determination In order to test products for their effect on the rate of transepidermal moisture loss, one must know the thickness or weight per unit area placed on the subject's skin. A 6 X 6 cm area of the media! forearm is marked off. From 100 to 300 mg of sample are placed onto a fiat stainless steel spatula and weighed. The sample is then evenly dispersed over the 36 cm • with even strokes. Excess sample remains on the spatula which is reweighed. Knowing the weight (W) in •ams and the area (A) in cm •,