Background The declared sun protection factor (SPF) is based on the use of a sunscreen layer of 2 mg cm−2. However, only around a quarter (0·5 mg cm−2) of this amount is applied by sunbathers. Theoretical calculations have suggested that the effective SPF is related to sunscreen quantity in an exponential way but this was not confirmed in vitro and has not been studied in vivo.
Objectives To investigate the relation between SPF and sunscreen amount in vivo.
Subjects and methods On the backs of 20 healthy volunteers, five areas of 34 cm2 each were marked. One area was phototested to determine the ultraviolet (UV) sensitivity. Four areas were treated with a sunscreen SPF 4 in different amounts: 0·5, 1, 2 and 4 mg cm−2. Thirty minutes after sunscreen application a phototest was conducted on each area. The effective SPF was calculated 22–26 h after irradiation using the UV dose needed to produce just perceptible erythema (minimal erythema dose) on protected and unprotected skin.
Results In all areas the mean SPF was significantly different from an SPF of 1 (no protection) (P ≤ 0·0001) and the SPFs of the areas with the various amounts of sunscreen differed significantly from each other (P ≤ 0·0008). The relation between the sunscreen amount applied and the SPF provided was most likely to follow exponential growth (r2 = 0·903).
Conclusions This study indicates that the relation between SPF and sunscreen quantity follows exponential growth. Application of 1 mg cm−2 or 0·5 mg cm−2 makes the SPF fall as the square or fourth root, respectively, and 4 mg cm−2 results in an almost squared SPF.