HIGH-SPF SUNSCREENS 291 guide, and the photocurrent was recorded from 290 to 400 nm in steps of 5 nm. A micropipette was then used to dispense an amount of sunscreen equivalent to 2 mg/cm 2 onto the epidermis. The sunscreen was "spotted" at several positions on the epidermis, and a light, circular rubbing motion with a gloved finger was used to give as uniform a layer as possible. The sunscreen was allowed to dry for 20 minutes, and the photo- current was again measured in 5-nm steps between 290 and 400 nm. For each wave- length, the ratio of the photocurrent recorded before and after application of the sun- screen was calculated this gave the monochromatic protection factors, PF0•), which were used in the following expression (5) to give the SPF: 400 Z EOQ½0Q 290 SPF - 400 [ 1 ] 290 where E0•) is the spectral irradiance of sunlight expected for a clear sky at noon in midsummer for a latitude of 40øN (solar altitude 70ø), and ½0•) is the effectiveness of radiation of wavelength )•nm in producing delayed erythema in human skin (7). For the physical sunscreens containing 4.4, 6.9, 7.8, and 8.6% TiO2, measurements were made on four samples of epidermis from one subject (epidermis A) and a single epider- mal sample from each of six other subjects (epidermis B, C, D, E, F, and G). In the case of the sunscreen containing 12% TiO2, measurements were not made on epidermal sample A. For the organic chemical sunscreens, measurements were made on a single epidermal sample from each of six subjects (epidermis H, I, J, K, L, and M). RESULTS AND DISCUSSION Figure 1 shows absorption spectra normalized to equal area for the five physical sun- screens. It can be seen that the spectra have similar shapes, the small differences between spectra probably being due to different size distributions of the TiO2 particles within the sunscreens (8,9). Figure 2 shows the corresponding absorption spectra for the five organic chemical sunscreens. It can be seen that the spectra for C5, C! 5, C25, and C30 are similar, all four products having the active ingredients methylbenzylidene camphor and butyl methoxydibenzoylmethane in common. However, the spectrum for C50 shows that this sunscreen has relatively little absorption in the long UV-A region. Table I summarizes the SPFs measured on each of the five physical sunscreens. For the products P8, P15, P25, and P35, the variance in SPFs measured on four samples of epidermis from a single volunteer (A) was compared with that obtained from SPFs measured on epidermis from the six other volunteers (B-G). In every case there was no significant difference (p 0.05 Stairnov test). We conclude from this analysis that the variability in the measured SPFs is primarily associated with the experimental technique (most probably the application of sunscreen to the epidermis), rather than to variability in epidermal architecture between subjects. The mean SPFs for a 2-mg/cm 2 application thickness of the physical sunscreens are given in the final column of Table I and, with the exception of product P35, are in close

292 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 0.075 - 0.060 o.o45 0.030 0.015 0.000 • • • • • • • • • • • 290 300 310 320 330 340 350 360 370 380 390 400 Wavelength, nm Figure 1. The absorption spectra of physical sunscreen products normalized to equal area. agreement with those claimed by the manufacturer. Furthermore, the coefficients of variation for each product are generally smaller than those expected from in vivo assay of products of similar SPF (3), indicating that in vitro assay using excised human epidermis as a substrate is an extremely reliable technique. Therefore, while an in vivo SPF was not available for P35 +, we can infer from Table I that we expect the product to have an SPF of around 36 at an application thickness of 2 mg/cm 2. It should also be noted that the SPFs we obtained for each product increased with increasing TiO 2 concentration (p 0.0001 Spearman coefficient of rank correlation). In particular, P35 +, which contained 12% TiO2, offered significantly higher protection (SPF 36) than P35, which contained 8.6% TiO 2 (SPF 23) (p -- 0.02 Mann-Whitney U test). Table II gives the SPFs measured on epidermis from six volunteers (H-M) for the five organic chemical sunscreens. It can again be seen that, with the exception of product C50, the mean SPFs are in close agreement with those claimed by the manufacturer. The fact that the SPF of C50 is significantly less than 50 is not surprising since this product offered relatively little protection against UV-A radiation. UV-A contributes between 15 % and 25 % of the erythemal dose from sunlight, depending on latitude, season, time of day, and atmospheric conditions (10), and hence in the extreme case of a sunscreen that absorbs no UV-A radiation, the maximum SPF that can be achieved is only 6, irrespective of the concentration of UV-B absorbers. It is unlikely that a sunscreen offering as low a UV-A protection as C50 could, in practice, provide an SPF as high as 50. One reason for the discrepancy between the SPF we measured for C50 and that claimed by the manufacturer is the difference between the ultraviolet spectrum of natural sun-