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.

366 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The fractions reduced (n) for five thicknesses of white petrolatum (normally marketed product) were determined on four different subjects. The results are shown in Fig. 3 for two of these subjects. Figure 3 shows a plot of thickness vs. fraction reduced. These data can be linearized by replotting on log-log paper. When this is done, Fig. 4 results. If the lines are extrapolated to the 1In z 1 intercept, it can be observed that they intersect. The thickness of the fil•n at which 1/n = 1 is the occlu- sive thickness, represented as (S). It has as its dimension millimeters and represents the theoretical thickness of products necessary to reduce the rate of transepidermal moisture loss to zero. C.5 0.35 o .30 o .2• o .Io • o.•0 (mm) O d o.15 o. 0õ 0.05 o 0•_ o 6[0 I 0 I 0 910 50 7 8 100 001 n 10 Figure 3. The fraction reduction (•) transepidermal moisture loss for 2 subjects is sho•n as a fimction of the thickness (d) of a layer o[ •hite petro]atum applied to the surface of the skin o 1.5 i i I i ] •o •5 1/rl Figure 4. The thickness (d) in mm of white petrolatum is shown as a function of the rccil)rocal of the fraction reduced (l/n) on a log-log scale Table III shows the results obtained for the occlusive thickness and the per cent reduction of the transepidermal moisture loss by a O.l-mm film of white petrolatum on each of four subjects. If the results are calculated as the per cent reduction of transepider- mal moisture loss achieved by the application of a uniform film of prod- uct, the results vary widely from subject to subject because the subjects have different base rates. By contrast, when occlusive thickness is calcu- lated, essentially the same result is obtained from each subject, despite the variation in base rate.