The aim of a study published in PLoS One was to investigate how much two consecutive sunscreen applications increased the quantity of sunscreen applied and decreased the skin area left without sunscreen (missed area) compared to a single application.

In this study, 31 healthy volunteers wearing swimwear were enrolled. All participants applied sunscreen, two consecutive times in a laboratory environment and had pictures taken in black light before and after each application. A standard curve was conducted establishing a link between change in picture darkness and quantity of sunscreen. The quantity of sunscreen at selected skin sites as well as the percentage of missed area was determined after each application.

It was observed that the darkness of the skin increased with increasing amounts of sunscreen applied—as sunscreens absorb black light. Participants had missed a median of 20% of their available body surface after a single application while after double application they had missed 9%. The decrease in missed areas was significant for the whole-body surface and for each of the body regions separately. Moreover, median participant had applied between 13% and 100% more sunscreen at the selected skin sites after double application than after single application.

Therefore, a double application was recommend, especially before intense sun exposure.

Source: PLoS One. 2018 Mar 28;13(3):e0193916. doi: 10.1371/journal.pone.0193916.