Evaluation of sunscreen efficacy over time and re-application using hybrid diffuse reflectance spectroscopy.

Background: In 2007, the FDA added requirements for sunscreens to be labeled "re-apply at least every 2 hours" based on very limited data. This study used hybrid diffuse reflectance spectroscopy (HDRS) to evaluate the persistence of protection by 80 minutes water-resistant sunscreen formulation with and without re-application, and with and without sweat-inducing activity over 6 hours.

Methods: Sunscreens were applied to subject's foreheads and backs, and they remained at rest or exercised to induce sweating in a heated environment. Efficacy of a sun protection factor (SPF 50) very water-resistant sunscreen was measured with HDRS instrumentation and ultraviolet (UV) photography to determine the sunscreen protection over time.

Results: The sunscreen maintained SPF 50 efficacy over 6 hours for the non-active group with a single application, and for 2 hours for the active group, dropping slowly to SPF 30 level after 6 hours of sweating. Re-application of sunscreen gave additive SPF, with two applications resulting in SPF >100 and three applications approximately SPF 150. UV photography was insensitive to the differences in protection detected with HDRS instrumentation.

Conclusions: Sunscreen efficacy is maintained over time in the absence of sweating or rub-off. After two hours of sweating, an 80 minutes water-resistant sunscreen does not need to be re-applied "at least every 2 hours."