A commercial sunscreen's protection against ultraviolet radiation-induced immunosuppression is more than 50% lower than protection against sunburn in humans.

Abstract

Ultraviolet radiation (280-400 nm)-induced suppression of cutaneous cell-mediated immunity plays an important part in the development of skin cancer. Sunscreens are widely advocated to protect against skin cancer but if they offer insufficient protection against immunosuppression they may inadvertently increase skin cancer risk. This human study evaluated immunoprotection afforded by a commercial sunscreen preparation (labeled sun protection factor 15) offering primarily ultraviolet B (280-320 nm) protection. Indirectly, it also investigated whether ultraviolet A (320-400 nm) plays a part in ultraviolet radiation-induced immunosuppression. Healthy white-skinned volunteers were used (n=119). Ultraviolet radiation exposures were on previously unexposed buttock skin with an ultraviolet radiation source that complied with European recommendations for sunscreen testing. Ultraviolet radiation dose-response curves for sunburn/erythema and suppression of the contact hypersensitivity response were generated either with or without sunscreen in vivo and protection factors were derived for both end-points. The ultraviolet radiation wavelengths transmitted by the sunscreen were determined in vitro and showed that the sunscreen was primarily an ultraviolet B absorber, with relatively poor absorption in the ultraviolet A region. The sun-screen protected against both erythema and immunosuppression but protection against immunosuppression (IPF=4.9, 95% confidence interval: 2.3-10.6) was less than half that for erythema (Ery-PFg=14.2, 95% confidence interval: 10.2-19.8). Failure of the sunscreen to afford comparable protection against both end-points was probably due to immunosuppression by ultraviolet A, a part of the solar spectrum that does not readily cause sunburn. The sunscreen protected against both end-points, which supports the use of sunscreens to reduce immunosuppression but protection against immunosuppression may be improved if sunscreens are formulated to offer equivalent protection against ultraviolet B and ultraviolet A.