Sun protection factor: meaning and controversies

The Sun Protection Factor (SPF) is the most important data to quantify the effectiveness of a sunscreen, being universally accepted. The method is based on determining the minimum erythematous dose (MED), defined as the smallest amount of energy required for triggering the erythema, in areas of protected and unprotected skin. The SPF value is then calculated as the ratio between the MED of protected and unprotected skin. The first publication of a method for determining the SPF was presented in 1978 by the U.S. FDA agency, followed by other publications of FDA and other international regulatory agencies. Although considered the reference method for quantification of sunscreen efficacy of topical products, there are controversies in literature about the method for determining the SPF and the implications of the real conditions of use in the protection achieved in practice by users.