710 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS cated as sunburn protectives, and the testimonial data and the possible re- actions are relevant to the present problem. With the collaboration of several practicing dermatologists, Hopkins, Fitzpatrick, and I (1) studied the reactions of a group of patients to a placebo and to a probably inef- fective dose of 8-methoxypsoralen. In spite of the fact that we were deal- ing with an extremely potent photosensitizing agent, the statistics indi- cated that the results had been overwhelmed by a variety of enthusiastic placebo reactions. Individuals who had the greatest problem with sun- burning and poor tanning claimed the most beneficial results from the placebo. It is easy to understand these placebo reactions in terms of the social pressures of our day. Our modern culture has revived ancient sun- worship but unfortunately contains many individuals whose ancestors for at least 10,000 years and perhaps 100,000 years have undergone selection for adaptation to a cloudy, foggy or glaciated environment (2). Thus, the girl with the peaches and cream complexion inherited from her beautiful grandmother is now expected to have a healthy "tanned" look and to be able to tolerate daylong ultraviolet exposure at seaside or poolside. Toler- ance to the environmental factor of ultraviolet radiation is much more of a social problem than adaptation to high altitude, cold or ability to hold one's breath under water without drowning. Another source of error in the evaluation of the alleged protective effect of ingested 8-methoxypsoralen was the variation between individuals in their interpretation of the definition of sunburn. Some subjects appeared to evaluate sunburn in terms of redness and some in terms of subjective sensations. Since the 8-methoxypsoralen produced a delayed, protracted erythema without much pain or tenderness, many of the subjects inter- preted the results as protection. Their conclusion tended to be, not that they had had a photosensitization reaction, but that they had a potentially severe sunburn from which they had been spared the subjective discomfort of their previously experienced sunburns. They thought of the enhanced erythema in such terms as "If I got this red on the pill, just think how bad I would have been without it !" QUALITATIVE EVALUATION OF SUNSCREEN AGENTS The usual manner of testing sunscreen preparations seems straightfor- ward enough. Different preparations are applied to different areas of the skin and the test and control areas exposed to sunlight or a suniamp. Ab- sence or reduction of erythema in test areas compared to control areas is presumptive evidence of protection. The most extensive studies of methods and problems in testing topical sunburn preventives have been reported by Blum (3, 4). His conclusions are expressed in mathematical terms which have discouraged me and, probably, many others from following directly in his footsteps. The prob-

MEASUREMENT OF THE PROTECTIVE EFFECT OF SUNSCREENS 711 leans revolve around the fact that the skin is not just a photocell, and the protective agent is not a clear solution in a cuvette of a spectrophotometer. The skin consists of nonhomogeneous layers, absorbing and scattering pigments, particles, and cell surfaces. Attenuation is the term applied to cover the effects of absorption and scatter in each layer. The expressions for the attenuation coefficient in multiple nonhomogeneous layers are mathematically very complicated. Both scatter and absorption are functions of wavelength, which greatly complicates practical testing be- cause of the polychromatic nature of sunlight compared to the limited wavelengths put out by mercury arc and other artificial sources of ultra- violet radiation. The rapidly changing vagaries of sunlight, except in the Chamber of Commerce versions of Florida, California and Arizona, are well known in practical testing. Blum has shown (3, 4) that with polychromatic light, such as a carbon arc lamp, there is a systematic variation of the protective capacity of sun- screens with the erythema threshold. This is attributed to the fact that absorption and scatter are functions of wavelength and that the variations of erythema threshold are probably due to the variations of optical proper- ties of thickness of the stratum corneum. Whatever the explanation, the lower the threshold, the greater the protection--which is probably a good thing from the standpoint of practical protection. Another problem is that at different wavelengths the slope of erythema response curves differ (5). In general, the longer the wavelengths producing the erythema, the steeper the erythema slope in relation to the minimal erythema dose. At the 2537 A. output of the germicidal lamp, it is virtually impossible to get an erythema much more than twice as intense as the minimal per- ceptible. At 3100 A. the degree of erythema rises very rapidly with doses of ultraviolet above that required to produce a minimal perceptible ery- thema. The erythema reaction through a sunscreen will therefore be in- fluenced by the spectral composition of the light getting through the preparation. EXTF.•SmN OF THE T•4•. RF.Q.U•RF.D TO PRODUeF. A PEReE PTIB LE ]5•RYTH EMA In contrast to the qualitative method described and criticized above, this method consists of giving a series of exposures through each sunscreen preparation and determining the prolongation of exposure required to pro- duce a minimal perceptible erythema 24 hours later. This method has the advantage of producing meaningful numbers for comparing different prep- arations. Instead of a series of exposure times, it is possible to use a single exposure time and various neutral density filters, moving screens or rotating shutters to produce a different total radiation. This is possible