14 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS from one to another depending on skin color. The UV sensitivity of human subjects was about 2 times as high as that of guinea pigs. Next, the SPF value of 9 model sunscreen preparations with guinea pigs and human subjects by using natural sunlight was determined. The results are summarized in Table III. The SPF values of guinea pigs were almost the same as those of human subjects. Moreover, the relationship between the SPF values of guinea pigs and that of human subjects was investigated (Figure 3). By least-squares analysis, a correlation coefficient 10- _ _ _ _ _ r=0.9936 Y----1.4260X--1.267 , SPF (Guinea pigs) Figure 3. Correlation between the SPF values of guinea pigs and those of human subjects determined with natural sunlight. of 0.9936 was obtained between the SPF values of guinea pigs and that of human subjects, which was statistically highly significant. Regardless of species difference in the MED value, a close relationship in the SPF value was found between the two subjects. From the data presented, it is apparent that guinea pigs can be used as a reliable substitute for human subjects in evaluating the effectiveness of sunscreen preparations. Next, investigation was made as to what extent the SPF value determined with guinea pigs using natural sunlight is compatible with the SPF value determined with human subjects using natural sunlight. From our results, a regression equation that correlates the SPF value of guinea pigs and that of human subjects was obtained. The equation can be shown as:

ANIMAL MODEL FOR EVALUATION OF SUNSCREENS 15 Predicted human SPF value = 1.426 x SPF (guinea pig) -- 1.267 Next, studies were undertaken as to what extent the SFP value of human subjects may be predicted from the SPF value of guinea pigs on the basis of the regression equation. The results of such an analysis are shown in Table IV. Table IV Correlation Between the Predicted SPF Value from the SPF of Guinea Pigs and Actual SPF Value of Human Subjects With Natural Sunlight x Model SPF of SPF of Predicted 2 Sunscreen Preparation Guinea Pigs Human Subjects SPF Value B 2.21 2.10 L.88 D 1.76 1.40 1.24 F 2.10 1.83 1.73 G 2.68 2.40 2.55 H 3.19 3.15 3.28 I 5.24 5.56 6.21 J 2.79 2.50 2.71 K 7.68 9.99 9.68 L 4.14 5.00 4.64 1At 95% confidence level, predicted and actual human SPF values can not be shown to be statistically different. 2predicted SPF value = 1.426 x SPF of guinea pigs - 1.2670 As compared with the SPF values in the third column (human subjects) with values in the last column (predicted SPF value), the predicted human SPF value correlated very closely with the actual SPF value in human subjects. This indicates that the human SPF value can be predicted closely from the SPF value of guinea pigs. The advantages of this method are as follows: (1) There was a good correlation between the SPF values of guinea pig (our method) with natural sunlight and that of human subjects with natural sunlight (FDA's method). (2) New sunscreens and active ingredients can be tested where safety has not yet been established. (3) The test condition can approximate more closely the actual means in which sunscreen preparations will be used by the consumer, regardless of season or place. (4) Exposure times can be freely arranged and even highly effective sunscreen preparations can be determined. (5) Ethical problems, cost, and labor in human studies can be reduced. COMPARISON OF LIGHT SOURCE: ARTIFICIAL SUNLIGHT AND NATURAL SUNLIGHT Comparison was made of the efficacy value obtained by using artificial sunlight in the laboratory with the efficacy value obtained by using natural sunlight in the field.