Sunscreen protection in the ultraviolet A region: how to measure the effectiveness

Products containing ultraviolet (UV) radiation absorbing or scattering ingr edients provide varying degrees of protection from sunlight (or other UV so urces), thus minimizing the deleterious effects on the skin. The "sun prote ction factor" (SPF) of sunscreen products has become a well recognized indi cator of protection against sunburn induced predominantly by ultraviolet B radiation (UVB: 290-320 nm), A similar system of denoting sunscreen protect ion from ultraviolet A (UVA: 320-400 nm) radiation has not been universally recognized. A variety of test methods have been proposed, both in vitro an d in vivo each with specific virtues and shortcomings. Regulatory agencies and industry have been reviewing the available methods over the past decade in an effort to develop consumer meaningful claims and appropriate substan tiation methods. This article reviews these test methodologies in vitro and in vivo as well. as the biological background that establishes the need fo r UVA protection, and the UVA content of solar radiation and its variabilit y.