ANTIBACTERIAL AGENTS IN HUMAN SKIN 373 The appropriate test formulation (0.1 ml) was applied once each morning for 5 consecutive days to the dorsal surface of the left hand of each subject on an area measuring 3.5 cm in diameter. The method of application was as described for the animal studies. Excess material was rinsed off with tap water 3 min after application. The surface skin counts were determined immediately and 15 min, 4 hours, and 24 hours after each application of the test material. No washing of the test sites was allowed for 4 hours following application of the test material. Applications were made daily for 5 days. Relationship of Surface Counts to Total Radioactivity in and on the Shin The activity (in microcuries) in and on the skin of each subject treated with the labeled soap suspension was determined by counting the surface of the skin using a Nuclear Radiation Detector and converting surface counts to total activity using the graph (Fig. 1) constructed from data obtained in the animal studies, and assuming analogy between human skin and rat skin. RESULTS Elficiency of Surface Counting From Fig. 1, a surface count of 30,000 cpm is equivalent to 0.0565 vc/cm 2 of skin. If all of the radioactivity were on the surface and if counting were 100% efficient, the detectable surface counts may be cal- culated using the following conversion formula: cpm a •c X 2.22 X 1060 6.06 c 1.33 a 0.0565 X 2.22 X 106 X 6.06 cpm = 1.33 cpm = 5.72 X 105 of radioactivity in and on the skin detectable by surface counting (% efficiency) 30 X 10 • 5.72 X 105 X 100 = 5.25 a Does not include a background of 65 cpm. b 1 /•c = 2.22 X 10 • disintegrations per min. * The effective surface area of the Nuclear Radiation Detector used was 6.06 cm 2. a 1.33 is an aliquot adjustment. After combustion of a skin sample, the carbon dioxide formed was collected in 20 ml of basic scintil]ator solution and 15 ml of the solution was counted in a liquid scintillat,•on counter.

374 JOURNAL OF THE SOCIETY OF COSMET1C CHEMISTS Table II Counting Efficiency •c/cm 2 in and on Skin Surface Counts % Efficiency 0.0202 7,500 3.65 0.0320 15,000 4.63 0.0565 30,000 5.25 Mean 4.51 Use of this value and others calculated similarly for different levels of total radioactivity gave the results shown in Table II. Persistence and Accumulation of Bacteriostats on the Shin From Fig. 1, the surface counts for male and female subjects and for the composite groups taken four times each day for a 5-day period were converted to microcuries of total radioactivity in and on the skin. Since there were no significant differences between male and female subjects, only the composite data are presented (Table III). These data are shown graphically in Fig. 2. Analysis of variance shows the differences between days and time periods within any day to be significant (p 0.0l). Table I I I Accumulation and Persistence of Labeled Germicides on Human Skin After Repeated Daily Applicationsa (Mean Values for Five Male and Five Female Subjects) Time of Increase Determination After 5 Post-Dosing Day I Day 2 Day 3 Day 4 Day 5 Days Immediate 25.0 b 28.0 29.4 30.8 32.3 7.3 15 Minutes 25.0 28.4 31.3 33.3 32.3 7.3 4 Hours 22.2 24.1 26.2 27.0 27.3 5.1 24 Hours 11.7 14.3 15.4 15.4 ... 3.7 Read across for accumulation and down for persistence. All values are given in •uc X 10a/cm 2 of skin. DISCUSSION Inspection of Table III indicates an apparent gradual leveling out of the amount of sorbed material by day 4, the values for that day being about 25% greater than on day 1. It is likely that this represents equilib- rium conditions related to the quantity of antibacterial agents applied per unit area of skin. Of interest is the decrease of activity during any