Evaluation of seven sunscreens on hairless mouse skin.

The ability of seven sunscreens to protect against ultraviolet (UV)--induced inhibition of epidermal DNA synthesis was evaluated in vivo using a hairless mouse model. There were statistically significant differences among sunscreens in their ability to prevent UV-B (290 to 320 nm) inhibition of DNA synthesis. The protective factor (PF) of a sunscreen was arbitrarily defined as the ratio of the dose required to inhibit DNA synthesis by 50% with and without a sunscreen. The following PF values were determined: Coppertone 4, 4.4; Sundown Extra Protection, 8.4; Supershade 15, 21.0; Eclipse 15, 22.2; Blockout 15, 22.4; and Bain de Soleil 15, 27.6. Zinc oxide ointment protected against any significant suppression of DNA synthesis at all UV-B doses used. There was a relatively good correlation between the PF and the sun protection factor (SPF) claimed for each sunscreen by the manufacturer. However, the PF values determined in mouse skin were generally higher than the SPF values measured in human skin. Further studies are needed to determine if sunscreen substantivity (resistance to removal by water) can be evaluated by this technique.