Paul Gerard McCormick - Comment

21 CFR 201.327(i) incorrectly requires that sunscreen products be SPF tested using a film thickness determined by a mass application rate of the sample to the skin. Since the attenuation of incident solar radiation through a sunscreen layer at a particular wavelength depends directly on the thickness of the layer, not the mass applied per unit area, this procedure is incorrect and has no scientific basis. Constant film thickness cannot be achieved by the specification of application rate in units of mass per unit area unless the specific gravities (SG’s) of all sunscreens are the same. This is clearly not the case. This is a critical point, particularly as the SG’s of the current range of sunscreens can range from 0.86 for a product containing chemical actives and 50% alcohol to 1.25 for an all-mineral 25% zinc oxide product containing 50% water. A simple calculation shows that, using the applied mass loading of 2 mg/cm2, the film thickness will vary from 23.2 microns for the alcohol containing sunscreen to only 16.2 microns for the zinc oxide sunscreen. Thus the new range of all-mineral zinc oxide sunscreens are being tested with film thicknesses of up to 20% less than organic sunscreens with SG values of 1 and 30% less than alcohol containing sunscreens. In the attached paper, (P G McCormick, “Adapting SPF Testing Methods for Mineral Sunscreen Density”, Cosmetics and Toiletries Magazine, 126, 164 (2011)), it is shown that the SPF of the zinc oxide sunscreen would be 46% greater if tested at the same film thickness as a sunscreen with SG of 1. In comparison with an alcohol containing sunscreen the difference in SPF would be correspondingly greater. Requiring sunscreens to be tested using film thicknesses that depend on their specific gravity is clearly wrong and does not provide a level playing field for all products and significantly disadvantages all-mineral sunscreens. The final rule should require the sunscreen be applied at an application rate of 2 μL/cm2.