MEASURING TURNOVER TIME 327 Table I Turnover Time of Stratum Corneum at Forearm Site Turnover time Turnover time Subjects (days) Subjects (days) M, 21 11.3 M, 33 M, 23 11.2 M, 34 M, 23 11.9 M, 36 M, 24 17.4 M, 37 M, 24 8.9 M, 37 M, 26 11.0 M, 46 M, 28 13.8 M, 28 17.9 Mean ñ SD 12.9 --- 3.2 18.8 14.5 15.3 13.1 13.8 15.0 15.1 ñ 2.0 EFFECT OF SKIN PROTECTION Figure 6 shows the change in fluorescence intensity when the skin was protected with gauze. Similar to the non-protected sites, the fluorescence intensity at the protected sites declined linearly, though at a slower rate. The turnover time of the stratum corneum was obtained by extrapolating the line of declining fluorescence to a point which met the background fluorescence. Table III shows the results of stratum corneum turnover times at gauze-protected sites. The mean turnover time was 14.5 days at control sites, and it was prolonged to 23.7 days by gauze protection. Table IV shows the results obtained by Finn chamber occlusion. The mean turnover time at occluded sites was 29.1 days, and it appears that Finn chamber occlusion prolonged turnover time further than gauze. EFFECT OF KERATOLYTICS The effects of salicylic acid, sulphur, and resorcinol were studied using the depilated skin of guinea pig backs. The fluorescence intensity decreased linearly in guinea pig (Figure 7), and the desquamation rate was obtained in the same way as for human skin. The results are shown in Table V. The rate of exfoliation of stratum corneum became faster with all keratolytics in comparison to the controls (WSP application), and the effect of salicylic acid was especially noteworthy. Also, the thickness of epidermis in the skin treated with salicylic acid or resorcinol was similar to that of control skin, though it was thickened by sulphur treatment (Figure 8). Table II Circadian Rhythm of Turnover Rate of Stratum Corneum Decrease (comparator units) Decrease/hour (comparator units) Day (9 AM-6 PM) Night (6 PM-9 AM) 3.00 ñ 1.53 2.51 ñ 1.37 0.33 ñ 0.17 0.17' ñ 0.09 Mean ___ SD (n = 7). *,0.05 p0.1.

328 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 130 Z •00 E o Forearm (9, 21) 0 non-protected skin ß protection by soft gauze O• '•• (} backgrøund -øo%o • ...... -.. 10 15 •0 Days Figure 6. Decline of fluorescence intensity at forearm site of male subject age 21 years. DISCUSSION It is well known that the dansyl chloride fluorescence test is very useful to investigate the turnover time of stratum corneum. However, there have been a few inconveniences in determining the end point by eye observation under Wood's lamp. First of all, the end point can be difficult to determine, as hair follicles may continue to fluoresce after the fluorescing stratum corneum has all been shed. Secondly, the duration of an experi- ment becomes long because observation is continued to the point of extinction, i.e., for the number of days equivalent to the turnover time. Such disadvantages can be reduced by using the instrument mentioned in this study. The turnover time can be obtained accurately from the point of intersection of the declining fluorescence with the back- ground fluorescence of a nearby area and may be easily extrapolated from a few readings even when the fluorescence measurement is not continued to extinction. Stratum cor- neum seems to be stained uniformly with dansyl chloride, judging from the result that Table III Effect of Skin Protection by Soft Gauze on Turnover Time of Stratum Corneum Non-protection Protection • Subjects (days) (days) Ratio M, 21 11.3 16.9 1.50 M, 23 11.9 22.5 1.89 M, 24 17.4 31.5 1.81 M, 24 8.9 17.4 1.96 M, 28 17.9 -- M, 33 18.8 30.3 1.61 M, 36 15.3 -- Mean _ SD 14.5 - 3.8 23.7 -+ 6.9 1.75 - 0.19 *, estimated by extrapolation.