Photoprotection by sunscreen depends on time spent on application

To be effective, sunscreens must be applied in a sufficient quantity and reapplication is recommended. No previous study has investigated whether time spent on sunscreen application is important for the achieved photoprotection.

To determine whether time spent on sunscreen application is related to the amount of sunscreen used during a first and second application.

Thirty-one volunteers wearing swimwear applied sunscreen twice in a laboratory environment. Time spent and the amount of sunscreen used during each application was measured. Subjects' body surface area accessible for sunscreen application (BSA) was estimated from their height, weight and swimwear worn. The average applied quantity of sunscreen after each application was calculated.

Subjects spent on average 4 minutes and 15 seconds on the first application and approximately 85% of that time on the second application. There was a linear relationship between time spent on application and amount of sunscreen used during both the first and the second application (P < .0001). Participants applied 2.21 grams of sunscreen per minute during both applications. After the first application, subjects had applied a mean quantity of sunscreen of 0.71 mg/cm² on the BSA, and after the second application, a mean total quantity of 1.27 mg/cm² had been applied.

We found that participants applied a constant amount of sunscreen per minute during both a first and a second application. Measurement of time spent on application of sunscreen on different body sites may be useful in investigating the distribution of sunscreen in real-life settings.