EFFECTIVENESS OF SUNSCREENS 175 The implications of this observation to standardized testing using solar simulators and to uniform labeling of sunscreening products are such that we immediately undertook a comparative study to confirm that the difference resulted from the change in spectrum. EXPERIMENTAL TEST MATERIALS 4% PABA standard sunscreen formulation prepared by Westwood Pharmaceutical Co., Buffalo, N.Y. 8% HMS standard sunscreen formulation prepared by Plough, Inc., Memphis, Tenn. (See reference 1.) SUBJECTS Only fair-skinned volunteers with skin types I, II, and III, having suitable areas of clear skin on their backs, were selected according to the Proposed Rules for Sunscreen Testing (1). Informed consent was obtained from each volunteer prior to testing. METHODS This was a paired study. Twelve volunteers were used in testing each of the standard sunscreen formulations with one of the formulations being applied to two comparable sites on the back of each volunteer. Determinations of SPF values were made with both the 2-mm WG-305 and the 1-mm WG-320 filter configurations of the same solar simulator on each volunteer, using the methods set forth in the FDA proposed rules (1). On the day prior to the test, each of the filters was used to administer a series of five UV light exposures of increasing duration to 1-cm 2 areas of unprotected skin closely adjacent to the appropriate test site to determine each volunteer's unprotected MED for that filter configuration of the solar simulator. These exposures were incremented according to the formula t x 1.25 n where n = the number of the exposure in the series and t = the duration of the first exposure. On the day of the test, one of the standard sunscreen formulations was applied uniformly at 2 /21/cm 2 to two areas of 50 cm 2, one on either side of the volunteer's back. A similar series of five exposures using each of the filters was given to the appropriate protected test site after a 15-min drying period. The duration of the exposures to the protected sites was determined from the unprotected MED for the appropriate filter and the anticipated protectiveness (SPF) of the test sunscreen formulation applied, incrementing in the same manner as above. The SPF value assigned to each formulation and filter configuration was based on the mean of the individual SPF values obtained in this test.

176 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS RESULTS The results of this study (Table I) show that in the case of the 4% PABA standard sunscreen the SPF values obtained differed significantly between the two filter configurations (P 0.005 by paired student "t" test). A higher SPF value was obtained for the 4% PABA standard sunscreen when tested with the WG-305 filter, i.e., when the incident UV spectrum contained shorter wavelengths of UV-B radiation. The SPF value obtained with the WG-305 filter configuration was not significantly different from the SPF value obtained with that same filter configuration in 1977. Table I Results of SPF Value Determinations SPF Values Obtained (Mean + s.d.) 1977 Study WG-305, 2 mm WG-320, 1 mm Sunscreen Tested (n = 13) (n = 12) • (n = 12) • 4% PABA Standard 6.51 _+ 1.04 5.72 _+ 0.99 4.56 + 1.12 8% HMS Standard 3.75 -+ 0.62 3.48 _+ 0.56 3.70 + 0.76 •Both filters were tested on the same individual for each of the standard sunscreens in the present study. In the case of the 8% HMS standard sunscreen, the SPF values obtained for the two filter configurations did not differ significantly nor did either result differ significantly from SPF values obtained for the 8% HMS standard suncreen formulation in the 1977 study. DISCUSSION Human erythemal sensitivity to UV radiation increases with decreasing wavelength through the UV-B region (3-6). The short wavelength UV-B content of natural sunlight reaching the earth's surface varies considerably with latitude, altitude, time of day, and with atmospheric conditions (7,8), thus risk to sunburn varies with these conditions (9). Our results suggest that the protectiveness of sunscreen formulations may also vary with such changes in the solar spectrum. The SPF value for the 4% PABA standard formulation tested increased when the amount of incident short wavelength UV-B was increased. Kaidbey and Kligman (10) tested formulations containing PABA or PABA esters and reported differences of the same nature but greater magnitude when efficacy values obtained using a bank of fluorescent suniamps were compared with values obtained using a xenon arc solar simulator with a 2-mm WG-320 cut-off filter. The spectral output of fluorescent .suniamps extends to shorter wavelengths than does the solar spectrum however, and does not conform to the proposed FDA specifications for solar simulators. This effect of increasing protectiveness probably does not occur with all sunscreening agents. Indeed, the HMS formulation's SPF value decreased slightly, but not significantly, when determined using the WG-305 filter configuration which passed the greater amount of short wavelength UV-B light. We have not yet tested other sunscreening agents in this manner. In the interest of increasing the accuracy of product labeling and thus the protection