Sunscreens: the impervious path from theory to practice

The preparation of commercial products able to protect the skin against damage from solar radiation requires safe and photo-stable stable UV-absorbing molecules with high extinction coefficients, prepared with solvents of the appropriate polarities and polarizabilities in formulas which allow the uniform spreading of the UV-absorbing substances. The products should also maintain the ingredients on the top of the skin and provide efficient scavenging activities against singlet oxygen and other directly or indirectly generated reactive oxygen species. Because of the high doses of UV used to test high SPF, simple and reproducible methods are needed to estimate the protection afforded by the product before risking the induction of severe burns to the volunteers, as can occur when the value of the product's SPF is smaller than the expected one. This paper describes several aspects of the preparation of commercial sunscreens. It also points out several of the hidden hypotheses which, when not tested, might lead to the overestimation of the protection factor of a product.