J. Cosmet. Sci., 66, 87–93 (March/April 2015) 87 Choosing an expected sun protection factor value JOHN R. SICA and MICHAEL CASWELL, Consumer Product Testing Company, Inc., Fairfi eld, NJ 07004. Accepted for publication February 8, 2015. Synopsis Sun protection factor, SPF, is a measure of the effi cacy of a topical sunscreen product the higher the SPF, the greater the blockage of ultraviolet-induced erythema. While there are several methods to determine SPF, the Food and Drug Administration (FDA) methods are unique. The FDA methods defi ne the label SPF value as the largest whole integer after subtracting an “A” value from the mean SPF. The A value, composed of the product of the upper 5% point of the t-distribution and the standard deviation (SD), divided by (n), where n equals the number of subjects, has a signifi cant impact on the label SPF value. Two examples explore this impact. Development of strategies to mitigate the impact of A using expected SPF values are explored using historical clinical trial data. A more enlightened choice of expected SPF values is shown to lead to higher label SPF values. INTRODUCTION Sun protection factor (SPF) is a measure of the effi cacy of a topical sunscreen product. A higher SPF means greater protection against erythema from ultraviolet (UV) radiation exposure. SPF is calculated by determining the increase in UV dose to perceptible ery- thema from unprotected skin to sunscreen-protected skin following UV exposure. An SPF of 2 doubles the time to perceptible erythema (e.g., from 10 to 20 min), an SPF of 4 doubles the SPF 2 times (e.g., from 20 to 40 min), an SPF of 8 doubles the SPF 4 times (e.g., from 40 to 80 min), and an SPF of 16 doubles the SPF 8 times (e.g., from 80 to 160 min). Figure 1 shows the nonlinear relationship between SPF and the blockage of UV-induced erythema. While the difference in percent blockage of UV-induced erythema between an SPF 32 and an SPF 64 is only 1.5% (98.4 - 96.9% = 1.5%), the SPF 64 product will allow the user to expose themselves to UV for twice as long (e.g., 320 min vs. 640 min). SPF is an important number because it indicates the effectiveness of the product to block UV-induced erythema. Recently, Garzarella and Caswell (1) published data comparing the SPF of test materials with three test methods, International (2,3) versus Australia/NZ (4), International versus Food and Drug Administration (FDA)-Final Monograph (FM) Address all correspondence to Michael Caswell at MCaswell@cptclabs.com.

JOURNAL OF COSMETIC SCIENCE 88 (5), and Australia/NZ versus FDA-FM. The statistical analysis for the correlation be- tween International versus Australia/New Zealand, International versus FDA-FM, and Australia/New Zealand versus FDA-FM, were 0.94, 0.99, and 0.95, respectively, illus- trating a strong positive correlation between each pair. The difference in least squares mean SPF for each method pair was 0.12, 0.62, and 0.81, respectively, showing no statis- tically signifi cant differences between the mean SPFs obtained using the different testing methods. The authors concluded that “the procedure discrepancies in FDA-FM, Australia/ New Zealand Method, and the International Method are inconsequential either the dif- ferences have no impact on mean SPF value, or, less likely, the differences produce equally and opposite changes in mean SPF, thus cancelling any effects.” Although there is no difference in mean SPF from the various methods, there is a differ- ence in the label SPF, which the consumer sees to make a determination of purchase. In the United States, the FDA regulations dictate the subtraction of an A value from the mean SPF to generate the label SPF. The FDA regulations are the only regulations in which the label SPF differs from the mean SPF. Thus, special consideration must be given when SPF testing for the United States or for other countries that will accept the FDA method. In testing a formulation for sunscreen effi cacy, the fi rst step is to choose an expected SPF value. Typically, a product brief will contain a target label SPF along with several other characteristics of the fi nal product. For successful product development, the label SPF must be achieved, which begins with choosing the correct expected SPF. Several factors need to be considered when choosing an expected SPF for the United States, with the most important being the A factor (5–7). The A value is composed of the product of the upper 5% point of the one-tailed t-distribution and the SD, divided by (n), where n equals Figure 1. Relationship between percentage blockage of UV-induced erythema and SPF value.