Should we reapply sunscreen throughout the day?

Several studies have focused on the necessity of reapplying sunscreen throughout the day in order to maintain effective sun protection.

• WULF & al. have demonstrated that two consecutive applications of sunscreen significantly increased the amount of sunscreen used, thereby reducing the areas of skin not covered by the sunscreen (forgotten areas), as well as areas where it was thinly applied, compared to a single application. Therefore, a double application of sunscreen during sun exposure is beneficial in compensating for the initial under-application, thus allowing for better sun coverage.

• In a study involving 20 office workers, they were instructed to apply one gram of SPF 50 sunscreen to their face once in the morning. A global reduction of 28.2% in sun protection was observed at the end of the 8-hour day during the study period among the workers, after applying an adequate amount of sunscreen in the morning. This finding suggests that it may not be necessary to reapply sunscreen.

• In another study, WULF & al. sought to examine the persistence of sunscreens after a single application of 2 mg/cm2 on the backs of 24 volunteers over an 8-hour period of outdoor physical activity, exposure to a hot environment, swimming and wiping, essentially simulating a day at the beach. At the end of the study period, the sunscreens still provided about 43% of the initial protective effect. They thus concluded that a single application is sufficient to reduce the erythema caused by UVB rays, although this assumes that the recommended quantity is applied.

• WULF & al. have even demonstrated that sunscreens accumulate in the skin when applied in the recommended quantities (at a dose of 2 mg/cm2) three times a day for five consecutive days, which resulted in a significantly higher SPF. Thus, applying a sunscreen for several consecutive days prior to UV ray exposure can provide a basic level of skin protection, which may help to prevent severe sunburns obtained during sun holidays. However, further research is needed to substantiate these findings.

The effect of perspiration on sun protection and vice versa is another crucial factor to consider when applying sunscreen during physical activity. A controlled, randomised clinical study on the face and arm, conducted with 24 participants, revealed no significant difference in skin surface temperature or sweat rate between treated sites (application of an SPF 70 sunscreen product designed to withstand water and sweat) and untreated control sites after a physical exercise session. As perspiration is a critical process in skin cooling and thermal regulation, this study underscores the safety of using a water or sweat-resistant sunscreen during outdoor recreational activities, in order to maintain protection.

Recommendation : If the recommended amount of sunscreen has been applied in the morning, a new application is only necessary after activities that could potentially "disrupt" the protective film formed by the sunscreen, such as water activities, excessive sweating, physical activities and rubbing. To date, there is no clear evidence mandating a specific reapplication frequency in the absence of these activities over an 8-hour period. Photoprotection only decreases slowly throughout the day.