Micro-organisms on human skin 611 9O 80 70 60 50 40 30- 20- I0- 0 500 - 400- -- 300 - -- 200 - I00 - -- o 90o 8oo 700 6oo 5o0 400 300 200 IOO o - (4) -- -- (5) -- (3) -- I00 500 I000 2000 3000 200 Volume (ml) Figure 3. Recovery of aerobic bacteria from axilla over an area of 14q• (mm). Figure 4. Recovery of aerobic bacteria from axilla over an area of 2q• (mm). Figure 5. Recovery of aerobic bacteria from axilla over an area of 5q• (mm). Table I. Observed washing efficiencies of different diameter sprayheads Diameter of sprayhead (mm) Percentage washing efficacy 50 ml 100 ml 2 69 89 5 58 78 14 18 25

612 E. M. Staal and A. C. Noordzij As can be seen from the Table I, the micro-organisms, which are directly hit (2 mm sprayhead) are loosened very fast and almost quantitatively from the skin. The sur- rounding areas under the larger sprayheads release the micro-organisms at a slower rate that will depend on the distance between that area and the middle of the spraychamber. The relationship between this relative efficacy and the distance towards the middle can be calculated from the above mentioned figures. These relative efficiency figures are shown in Table II. Table II. Calculated efficiencies of micro- organism recoveries Relative percentage Area washing efficacy Inner Outer diameter diameter (mm) (mm) 50 ml 100 ml 0 2 69 89 2 5 56 76 5 14 12.6 17.5 These data are shown graphically in Fig. 6. We believe the best interpretation of the diagram shows that with 100 ml wash fluid the relative efficiency right in the middle of the spraychamber is about 100•o. 90 • 80 70 60 50 40 50 20 I0 0 2 5 14 Sampling area diameter (ram) I00 ml 50 ml Figure 6. Percentage recovery of total aerobic bacteria from axilla. In the tape stripping experiments of Kligman (2) and with the electron microscope studies by Montes and Willborn (25) it appeared that most of the bacteria are located on the upper layer of the epidermis. The high washing efficiency of the 2 mm sprayhead supports this statement. Through its high efficacy the 2 mm sprayhead with a wash volume of 100 ml is an ideal combination for the quantification of the skin population.