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 •,

TRANSEPIDERMAL MOISTURE LOSS Table II Transepidermal Diffusion Loss Rates for Several Subjects" 365 Height Weight Subject Age Sex (inches) (lb) I 19 F 64 115 2 21 F 66 122 3 23 F 62 100 4 23 F 63 118 5 23 F 67 145 6 24 F 65 135 7 24 F 66 142 8 27 M 74 172 9 30 M 66 165 10 31 M 71 170 11 37 F 59 95 12 45 M 75 195 13 46 M 72 178 14 48 F 62 150 Transepidermal Diffusion Loss (mg/cm2/hr) 0.31 + 0.20 q- 0.28 q- 0.34 q- 025q- 021q- 0464- 0 28+ 0 22+ 0 394- 026q- 0 37+ 0 24q- 045+ 0.04 0.•'4 0.04 0.04 0.04 0.04 0.02 0.04 0.04 C.O6 c. 94 0.04 0.04 0.07 Results were obtained on the medial untreated forearm. the weight per tinit area Tnay be calculated. Knowing the specific gravity (sp gr) at skin temperature, one can calculate the film thickness (d) in millimeters according to eq 1. 10W d - (1) sp gr X A Occlusive Thickness In the past, examination of a product for its effect on the transepider- mal diffusion loss rate has been to determine the rate before use (B) and the rate after use (C), and to calculate the per cent reduction (100 n) according to eq 2. B--C 100 n - X 100 (2) B The average per cent reduction is then presented as a characteristic of the product. In our work it is necessary to use volunteer subjects all of whom have different base rates. For the same thickness of film widely different per cent reductions were obtained requiring use of the same subject for comparing products as well as requiring reproducible sample application. For this reason, an experimental approach was desired which would measure a property of the product independent of human subject variables.