158 JOURNAL OF COSMETIC SCIENCE The probe was gently positioned onto the skin area. On each site, sixfold determinations were performed. STATISTICS Differences between products were calculated with the two-sided Wilcoxon U-test. Values with p • 0.05 were considered as significant. RESULTS UREA CONCENTRATION OF FOREARM SKIN EXTRACTS OF NORMAL SKIN The urea concentrations of skin extracts of the volar forearms of 30 volunteers comprised a range of 10-1000 nmol/ml. REPRODUCIBILITY OF EXTRACTION We observed no statistically significant differences when we compared samples taken from the same volar sites on the right and left forearms of the same individual. Minor fluctuations were observed for three volunteers, with only a little more fluctuation for one volunteer when comparing the results from a single individual (n = 4) taken on two different days (Figure 1). 800 700 E 600 o l= 500 .,•.J "" 400 o 300 • 200 [] day 1, left -- [] day 1, right [] day 2, left .... [] day 2, right 100 ....... ' I i I• A B C T volunteer Figure 1. Comparison of the urea content of stratum corneum from the right and left forearm at two different days of extraction. Means and standard deviations were calculated from three extracts from each volar forearm. Subjects A-D.

UREA ANALYSIS OF STRATUM CORNEUM 159 REPEATED EXTRACTION FROM THE SAME SKIN AREA To test the completeness of the extraction of urea from the stratum corneum pool of water-soluble substances by our method, we performed repeated extractions at a single skin area (n -- 15). The urea concentrations of the second, third, and fourth extractions decreased from 15 to 8 percent of the initial extraction (Figure 2). INFLUENCE OF SKIN CLEANSING ON UREA CONCENTRATION IN STRATUM CORNEUM EXTRACTS The influence of skin cleansing on the stratum corneum urea amount of four volunteers is given in Figure 3. Skin cleansing was performed with water only or a solution (4%) of sodium lauryl sulphate (SDS) in water. Water only reduced the amount of extractable urea by 76% to 90%. Washing the skin area with a solution of SDS (4%) decreased the amount of extracted urea by 60% to 94% in a similar range. An SDS solution (4%) was compared with a standard shower gel to determine the influence of the detergent concentration (Figure 4). No significant difference was observed, although the concen- tration of detergents was about three times higher in the shower product. A further forearm skin area was treated to test the idea of possible restoration of the urea pool in stratum corneum by supplementation, using a shower product with 10% urea (Figure 4). When comparing data obtained after cleansing with a urea-supplemented product with data for skin treatment with the standard shower gel (see above), we observed a signifi- cantly higher amount of urea in the former skin extracts. The amounts varied in the individuals between 18% and 103%. However, a complete reconstitution of the urea pool by urea-containing products appears to be the exception. lOO 90 80 70 60 50 40 30 20 ..... : ................... 2 3 number of extraction I....7.7.' .... 4 Figure 2. Urea content of skin extracts (in percent of the initial extract) after repeated extractions at the same skin site. Means and standard deviations were calculated from fifteen extracts from different indi- viduals.