Sunscreens absorb ultraviolet radiation before it can be absorbed in the skin. They are recommended to protect the skin from the major toxicities of sun exposure: sun-burn and skin cancer. Most available agents primarily absorb UVB, although newer preparations also provide protection against UVA. Physical sunscreens (which are generally opaque, like titanium dioxide and zinc oxide) block all ultraviolet radiation.
The frequency of application of sunscreen is guided by the SPF (sun protection factor) of the preparation. This derived value is the ratio of the time of ultraviolet exposure that causes erythema with the sunscreen to the time that causes erythema without the sunscreen. The higher the SPF, the less frequent the needed appli-cation of sunscreen. SPFs of available preparations vary from 2 to 50.