324 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS EFFECTS OF SKIN PROTECTION AND OCCLUSION Seven normal healthy male volunteer subjects (21-36 years) had 5% DC in WSP ap- plied to the flexor aspect of each forearm for 24 hours, using Finn chambers. After 24 hours, the Finn chambers were removed and one site covered with woven cotton gauze fastened in place with adhesive plaster. The gauze was renewed twice daily after readings had been taken. The other site was left unprotected. In a second group of six normal volunteers (23-46 years), the above experiment was repeated instead of gauze being used to protect one of the sites, Finn chamber occlusion was used, the chamber being renewed twice daily after fluorescence readings had been taken. During these experiments, volunteers were not allowed to take a bath. EFFECT OF KERATOLYTIC AGENT After 5 % DC in WSP was applied to four sites on the back of depilated guinea pig skin (under occlusion) for 24 hours, experimental sites were treated daily with 0.1 ml/4 cm 2 of a preparation containing 6% each of salicylic acid, sulphur, and resorcinol (in WSP), and readings were taken at the same time each day. RESULTS PENETRATION OF DC INTO THE STRATUM CORNEUM Figure 2 shows the relationship between DC application time and the number of tape strippings required for fluorescence to disappear. The number of strippings increased with increasing application time. In 3-hour DC applications, fluorescence disappeared 25 20 "• lo -.,, .5 Z 0 I I I I 3 6 12 24 Application time (hr) Figure 2. Depth of penetration of dansyl chloride in WSP under occlusion, with time. Results are shown as mean and SD (n = 7).

MEASURING TURNOVER TIME 325 after an average of 9 strippings, indicating that about half of the stratum corneum was stained. However, in 24-hour applications, an average of 20 strippings was required and the glistening layer appeared in every subject when the fluorescence disappeared. This means that whole stratum corneum was completely stained after 24 hours' applica- tion. This result was confirmed by the fluorescent microscopic observation of the skin tissue taken by snip biopsy (Figure 3). The equality of fluorescence intensity in each stratum corneum layer was checked by progressive stripping of a fully stained stratum corneum. As depicted in Figure 4, the fluorescence intensity declined almost linearly with strippings. From this result, stratum corneum appeared to be uniformly stained with dansyl chloride. FOREARM STRATUM CORNEUM TURNOVER TIME Figure 5 shows changes in the fluorescence intensity of skin after DC application. As can be seen, the decrease in fluorescence was linear against time. The turnover time of stratum corneum was obtained by reading the time at which the declining fluorescence intersected with the background fluorescence of a nearby area. Table I shows the turn- over time of forearm stratum corneum obtained from 14 volunteer subjects. The turn- over time was 12.9 days for those in their twenties and 15.1 days for those in their thirties and forties, suggesting that it increases slightly with age. Figure 3. Cryostat section of dansyl chloride stained skin immediately after 24-hour application (phase contrast under UV lighting).