Sun protection factors

The absorption of ultraviolet light by a film of sunscreen product on the surface of the skin is considered from a theoretical standpoint. It is apparent that Beer-Lambert's Law would predict protection factors in excess of the actual factors by many orders of magnitude. The possible reasons for this are discussed in detail, and the relevance of it to in vitro and in vivo measurements of protection factors, and also the in-use situation, are discussed. The implications for formulators of sunscreen products are also discussed.
The methods of measuring protection factors are reviewed. These include instrumental methods, based on both solutions and thin films, model systems, animal methods and human volunteer studies. The current situation with respect to proposed standard methods for measuring sun protection factors, particularly the American FDA method and the German DIN method, is reviewed.