Enhancing Sunscreen Efficacy for Realistic Application

By: Elsa Jungman, University of Paris XI, and Howard I. Maibach, MD, University of California

Posted: June 30, 2010, from the July 2010 issue of Cosmetics & Toiletries.

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UV exposure was assessed on the backs of three athletes involved in an Iron Man competition. The athletes wore a UV spore detector and an SPF 25 water resistant sunscreen. Despite the UV filter, however, erythema was visible at the race’s end. It is possible that the sunscreen was washed off by the sea water and/or sweating; it also was not indicated whether the athletes reapplied sunscreen during the race.¹³

Sunscreen Reapplication
Sunscreen reapplication during sun exposure is recommended every 2 hr by the FDA to maintain total protection. Bodekaer et al. examined the persistence of a single sunscreen application during an 8-hr day of sun exposure, physical activity and water bathing.¹⁴ Physical and organic sunscreens at 2 mg/cm² were tested on the backs of 24 volunteers. During the 8 hr, subjects wore tee shirts, exercised on bicycles, then entered a room held at approximately 30°C and took a 20-min bath. As a result, the SPF level after 4 hr was reduced by approximately 40%, and 56% after 8 hr for organic and inorganic sunscreens. This demonstrates the necessity of reapplying sunscreen to keep total photoprotection under real life conditions.

Although some sunbathers reapply sunscreen, less is known about the optimal application time. Diffey developed a mathematical model to estimate how the time of sunscreen reapplication affects the UV exposure of skin.15 His work mathematically showed the benefit of early reapplication of sunscreen; a single reapplication of an SPF 15 sunscreen after 20 min during 6 hr of sun exposure was found to be better than reapplication 2 hr or 3 hr later. The reapplication was independent of the SPF rating or UV intensity.

Conclusions
Since studies have shown that consumers apply far less sunscreen than that used in SPF evaluations, the SPF achieved is often half of what is labeled.² Sunscreens are also spread onto small areas and many anatomic sites are missed.^{4, 5} Thus, it is recommended that consumers be educated to understand how to appropriately apply sunscreens.¹⁶

In addition, it was found that consumers typically apply more sunscreen when it is packaged in a large-mouthed jar.⁶ It is therefore recommended that manufacturers observe consumer behavior when designing sun product packaging. Offering sunscreen in a jar could permit consumers to better judge the amount applied.