OCCLUSIVITY OF OIL FILMS 347 observation of the skin was made by preparing a replica of the skin and examining it with a scanning electron microscope (Nihon Denshi Co., JSM-II). CALCULATION OF OCCLUSIVITY: In order to express the occlusivity of oil films as a measure of a resistance to water loss, the occlusivity was defined by the following equation. Occlusivity (%) = [1 -- (TEWL (1)/ (TEWL (C)/] •,TEWL (2)/x [TEWL (3)/1 x 100 (1) where TEWL (1) = TEWL of the treated skin at t minutes after sample application TEWL (2) = TEWL of the treated skin before sample application TEWL (3) = TEWL of the untreated skin at t minutes after sample application TEWL (C) = TEWL of the untreated skin before sample application Since the occlusivity is generally expressed as a suppression of TEWL, it can be given by an equation of [1 - TEWL(1)/TEWL(2)] x 100. When the oil layer is a strong barrier to water loss, TEWL(1) is zero and the occlusivity becomes 100%. On the other hand, the occlusivity becomes 0% when the oil layer shows no resistance to water loss because TEWL(1) and TEWL(2) take the same value. However, since TEWL of the skin changes with time even though the skin is untreated, it is necessary to correct this change of TEWL with time in determining the occlusivity of oil films. A correction term, TEWL(C)/TEWL(3), was therefore added to the equation, as seen in eq. 1. RESULTS AND DISCUSSION The average value of the casual TEWL level of untreated skin at given test period in 32 subjects ranged between 0.51 and 0.59 mg/cm2/hr. This value was rather higher than that reported by Baker (10). The average skin temperature during the test period was between 31.1 and 32.8øC. Although the TEWL(C) value differed largely between subjects, there was no large difference that was observed in the TEWL(C) value for individual subjects with time. A correlation between TEWL(C) and skin surface temperature was also observed and it generally agreed with the correlation reported by Grice (11). Since a constant value of TEWL(C) was observed for individual subjects, it is believed that the evaluation of occlusivity can be done by eq 1 as long as the experimental conditions such as laboratory temperature and site of oil application are kept the same. Figure 1 shows the effect of coating amount of oils on their occlusivity observed after 15 min and 60 min of the application of their chloroform solutions. As seen from the figure, petrolatum was most effective in inhibiting water loss. The occlusivity of petrolatum and solid paraffin reached to a constant value beyond the coating amount greater than 3 mg/cm 2, which implied that the calculated minimum thickness of film whose occlusivity is not susceptible to the coating amount was 30 microns for both oils. The occlusivity of liquid paraffin at 15 min after the application increased with the coating amount of the oil however, the occlusivity at 60 min after the application was shown to be constant beyond the coating amount larger than 2 mg/cm 2. This was because the liquid paraffin, due to its fluidity, continued its spreading on the skin surface over a period of time and the thickness of the resulting film was nearly the

348 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS lOO PETROLATUM SOLID PARAFFIN LIQUID PARAFFIN AFTER 15 MINUTES i I I I 0 I 2 3 • 5 AM0JNT CF 0IL(MG/C ) lOO 50 PETROLATUM SOLID PARAFFIN LIQUID PARAFFIN AFTER 60 MINUTES 0 I 2 3 4 5 AM0JNT CF 0IL(MG/CM 2) Figure 1. Effect of coating amount of oils on occlusivity. same beyond 60 min after the application even if the coating amount of the oil increased. It was found that the physical forms of oils strongly influenced the extent of the occlusivity as shown in Figures 2 and 3. The occlusive effect was most evident immediately after the application and it decreased with time for both liquid paraffin and solid paraffin when applied to the skin in their solution form. Different occlusive patterns attributable to the difference in the nature of oils were observed in the neat and emulsion forms. Solid paraffin in the neat form had no significant occlusive effect at any time, but liquid paraffin in the neat form occluded water as efficiently as liquid paraffin in the solution form. In the case of the emulsion form, the occlusivity immediately after the application of the samples could not be determined because of the evaporation of water contained in the emulsion. It required more than 30 min to