J. Cosmet. Sci., 62, 327–329 (May/June 2011) 327 Water immersion does not alter the minimal erythema dose MICHAEL CASWELL, CARYL WOOD, and ELIZABETH MALY, Consumer Product Testing Company, Inc., 70 New Dutch Lane, Fairfi eld, NJ 07004. Accepted for publication February 17, 2011. Synopsis To determine the water resistance of a sunscreen, the COLIPA method requires the determination of two minimal erythema doses (MEDs): a static MED (MEDUS) and a wet MED (MEDUW) (1). The MEDUS is used in calculating the static SPF the MEDUW is used in calculating the SPF after water immersion. Herein, we report that in the 107 subjects examined, the mean MEDUS (21.0±0.55 mJ) is not different from the mean MEDUW (21.0±0.61 mJ). This shows that water immersion does not alter the minimal erythema dose and strongly suggests that the determination of two MEDs is unnecessary and that one should be eliminated. Eliminating one of the two MED determinations would increase the benefi t/risk ratio of the COLIPA sun- screen water-resistance effi cacy testing without harm to effi cacy. INTRODUCTION Any clinical trial should maximize the benefi t/risk ratio. One method is to reduce the risk to the subjects in the clinical trial. Another method is to increase the benefi t for the sub- jects or for a greater population. To that end, sunscreen effi cacy testing maximizes the benefi t because millions of consumers receive the benefi t from the subjects participating in the sunscreen testing. To determine the water resistance of a sunscreen, the COLIPA method requires the deter- mination of two minimal erythema doses (MEDs): a static MED (MEDUS) and a wet MED (MEDUW) (1). The MEDUS is used in calculating the static SPF the MEDUW is used in calculating the SPF after water immersion. The World Health Organization (2) and the US Department of Health and Human Ser- vices–National Toxicology Program (3) have declared that UV radiation is a carcinogen. Thus, clinical trials that involve UV radiation could increase their benefi t/risk ratio by reducing the risk of exposure to UV radiation. Reducing the number of unprotected MED determinations would reduce the risk of sunscreen effi cacy testing during the COLIPA water resistance testing. Gambichler et al. reported no difference between MEDUS and MEDUW in 12 subjects (4). Herein, we report that in the 107 subjects tested, the mean MEDUS is not different from Address all correspondence to Michael Caswell.