Clothing reduces the sun protection factor of sunscreens

Background Individuals are recommended to wait for 20 min following sunscreen application before dressing. However, this is probably seldom done in daily life, and therefore we investigated how dressing earlier than 20 min after application affected the sun protection factor (SPF). Objectives To determine the SPF of a sunscreen applied at different amounts at 4, 8 and 20 min before dressing. Methods An organic sunscreen was used on the backs of 22 healthy volunteers. Before SPF testing, participants wore a cotton T-shirt for 60 min after the test areas had been uncovered for 4, 8 or 20 min after sunscreen application. The SPF was also tested on unclothed skin. Results The median SPF was 11.7 (2 mg cm(-2)), 5.7 (1 mg cm(-2)) and 3.3 (0.5 mg cm(-2)) for unclothed skin, and 8.1 (2 mg cm(-2)), 4.8 (1 mg cm(-2)) and 2.2 (0.5 mg cm(-2)) following an interval of 8 min before dressing. The SPF was similar for time intervals of 20 and 8 min when the amount was 1 mg cm(-2) (P = 0.48) and 2 mg cm(-2) (P = 0.56). For 0.5 mg cm(-2) there was no difference between skin clothed after 20 min and unclothed skin (P = 0.19), nor between skin clothed after 4 min and after 8 min (P = 0.28). Conclusions When sunscreens are applied at amounts of 1 and 2 mg cm(-2) the time between sunscreen application and dressing can be as little as 8 min. When less sunscreen is used the SPF is insensitive to the length of time between application and dressing.