METHOD FOR TRANSEPIDERMAL WATER LOSS 115 O-• 5.0 4.0 m m mm •mmm m• ecipr0cl of Kelvin Temperature X10 z Figure 4. Natural logarithm of TEWL vs reciprocal of absolute temperature x 1000. emollient materials on TEWL using a silastic membrane. There was a good correlation between the TEWL rate for this in vitro model and the TEWL rate for an in vivo human skin model. They found that the decrease of TEWL rate that resulted from emollient application was inversely proportional to the polarity of the material applied. In other words, the more polar the material applied to the membrane, the greater the TEWL rate. Mineral oil is the least polar substance that we tested and it inhibited TEWL the most. At steady state, the effect of castor-oil-treated skin on TEWL (Figure 6) was similar to that of sesame-oil-treated skin (Figure 5). Glycerin, which acts to bind water to the skin, produced an interesting effect. During the first two hours glycerin acted as an occlusive agent, since TEWL values were close to that of mineral-oil-treated skin. After four hours, however, the glycerin-tissue com- posite was observed to become saturated with water. At the six-hour time point, TEWL rates were almost double those of untreated skin. This is comparable to the results obtained by Reiger and Deem (30) who found an increase in the TEWL rate upon application of 25% glycerin in water. Reiger and Deem showed that hurnectants, in general, increase TEWL rate at low relative humidities.

116 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ß Untreated o Mineral Oil A Sesame Oil [] Glycerin 500 400 $oo 200 - 100 - 0 i 2 5 4 5 6 7 24 Time (hours) Figure 5. TEWL rates vs time values for untreated skin and skin treated with mineral oil, sesame oil, and 25% glycerin in water. Bars refer to standard error of measurement. A two-week conditioning study was carried out with mineral oil, the most effective of the occlusive agents tested. This test provided us with some insight into the mechanism of mineral oil as a moisturizer. Results of this study are reported in Table II. No significant differences were found between treated and untreated ear skin samples. It was concluded that for mineral oil to act to decrease TEWL rate in this procedure, it must be able to bind to the skin. The protective effects of mineral oil were experimen- tally removed during sample preparation. Thus we conclude that mineral oil acts to decrease TEWL by imparting a physical barrier to the transport of water from the membrane. Conditioning the skin with mineral oil appeared to be an ineffective treat- ment modality. CONCLUSIONS Previously developed in vitro techniques for measuring TEWL are tedious and time- consuming. Most of these in vitro methods employ a gravimetric technique for measure- ment. Steady state TEWL rates cannot be achieved in less than 24 hours. Blank's in vitro method (30) of using tritium-labeled water to measure water loss was combined