The influence of the amount of sunscreen applied and its sun protection factor (SPF): evaluation of two sunscreens including the same ingredients at different concentrations

Background: To estimate labeled sun protection factor (SPF) for sunscreen, the amount of product applied on volunteers, according to food and drug administration (FDA) and International protocols, is 2 mg/cm2. However, different studies have shown that consumers actually apply much less product when exposed to the sun. Previous studies have reported contradictory findings in an attempt to correlate the amount applied in relation to SPF. The objective of the present study was to estimate the influence of the quantity of sunscreen applied in the determination of SPF, according to the FDA methodology. Subjects and methods: Forty volunteers were included in two groups (SPF 15 and 30). The selected sunscreen was then applied in four different quantities (2, 1.5, 1.0 and 0.5 mg/cm2). All areas were irradiated with a solar simulator. After 24 h, the minimal erythemal dose (MED) and SPF were determined. Results: In both groups, we observed that the SPF decreased when the amount of sunscreen applied was decreased. The differences between the 2 mg/cm2 area and the others were significant in both groups (P