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Reapplication May Not Be Necessary for Water Resistant Sunscreen

A recent study showed that an 80-minute, water-resistant sunscreen does not necessarily need to be reapplied every 2 hours.

“In 2007, the FDA added requirements for sunscreens to be labeled ‘re‐apply at least every 2 hours’ based on very limited data,” the researchers said.

They assessed the persistence of protection of an 80-minute, very water-resistant SPF 50 sunscreen formulation with and without replication over 6 hours. Participants applied the sunscreen to their foreheads and backs and were randomly assigned to either a non-active group or an active group. In the active group, participants exercised in a heated environment. The efficacy of the sunscreen overtime was measured using hybrid diffuse reflectance spectroscopy (HDRS) and UV photography.
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After 6 hours, the researchers found the sunscreen maintained SPF 50 efficacy among non-active participants with a single application. In the active group, the sunscreen maintained SPF 50 efficacy for 2 hours and dropped slowly to SPF 30 after 6 hours of sweating. Reapplication of sunscreen gave an additive SPF, with 2 applications resulting in SPF greater than 100 and 3 applications resulting in approximately SPF 150, the researchers said.

However, UV photography was found to be insensitive to the differences in protection detected by the HDRS instrument, they added.

“Sunscreen efficacy is maintained over time in the absence of sweating or rub‐off,” the researchers concluded. “After two hours of sweating, an 80-minute water resistant sunscreen does not need to be re‐applied ‘at least every two hours’.”

Reference

Ruvolo E, Aeschliman L, Cole C. Evaluation of sunscreen efficacy over time and re-application using hybrid diffuse reflectance spectroscopy [published online February 6, 2020]. Photodermatol Photoimmunol Photomed. doi:10.1111/phpp.12535

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