A method for the determination of UVA protection for normal skin.

Although the UVB portion of the electromagnetic spectrum (290 to 320 nm) is responsible for most of the harmful effects of sunlight on the skin, wavelengths in the UVA region also contribute to photodamage. A simple and rapid clinical test, the sun protection factor determination, is available to assess the sunburn protective effect of a sunscreen, primarily a UVB effect. However, no practical test has been proposed to measure a sunscreen's UVA protection. We described a method for the calculation of UVA protection in normal subjects. The determination of UVA protection involves three steps: (1) the UV absorbance spectrum of the sunscreen on skin is determined spectrophotometrically; (2) a convolution spectrum is calculated by multiplying the solar spectrum with the Commission Internationale de l'Eclairage UV Hazard Spectrum; and (3) the sunscreen transmission spectrum is then incorporated into the convolution spectrum to obtain the UVA effectiveness ratio, which can be expressed as the UVA protection percentage. Because the UVA protection percentage value is based both on normal erythemic risk and on standard sunlight, the protection for any product can be easily measured. The procedure is simple, and values generated can be reproduced in other laboratories.