The measurement of the sun protection factor (SPF) is the usual method in the examination of the effectiveness of sunscreen. The declared SPF is based on the use of a sunscreen layer of 2 mg/cm². However, only around a quarter (0.5 mg/cm²) of this amount is generally used in real life. Theoretical calculations have suggested that the effectiveness of SPF is related to sunscreen quantity in an exponential way but this was not confirmed in Asian skin.

This study was performed to investigate the change in SPF values when less than the recommended amount of sunscreen was applied.

A board divided into 10 areas measuring 7 × 4 cm was placed on the back of 15 healthy volunteers. Low- and high-SPF standard reference sunscreens, and two types of sunscreen (SPF 30 and 35) were each applied on 4 areas, 0.5, 1.0, 1.5, and 2.0 mg/cm², respectively, and were left to dry for 20 minutes. The irradiation was conducted at a distance of 50 cm using a template (1 × 1 cm) placed directly on the skin with 10 windows allowing ultraviolet (UV) radiation to pass through with a dose increment of 20%. Erythema was evaluated 20 to 24 hours after exposure to UV radiation.

Sunscreen showed its expected SPF value when 2.0 mg/cm² was applied. The SPF values of the different amounts were significantly different from each other and decreased when less was applied (P < .05). The relation between the amount of sunscreen applied and the SPF provided was most likely to follow exponential growth.

Spectral differences between our solar simulator and the UV sources of commercial laboratories are likely to be important. In addition, differences in sunscreen application techniques may have influenced the ultimate SPF values.

This study concludes that to get the expected SPF value, it is important to apply the UV protective sunscreen precisely in the amount of 2.0 mg/cm² on Asian skin as recommended by the Food and Drug Administration. In addition, it was difficult to predict the SPF values when the usual amount of 0.5 mg/cm² was applied