A more reliable index of sunscreen protection, based on life table analysis of individual sun protection factors.

The efficacy indices of two representative sunscreen preparations were determined using life table analysis of individual sun protection factors (SPF), obtained by exposing a group of subjects to ultraviolet radiation (UVR) doses from 4 to 20 multiples of the control minimal erythema dose (MED). The ultimate efficacy index is expressed as the UVR dose under which a selected percentage of the population is safely protected by a sunscreen. This method takes into account several sources of bias in the current test procedure, in particular the considerable variability in individual responses. It also takes into account subjects in whom the MED of the protected skin is either higher or lower than the range of challenge doses to which they were exposed, thereby increasing the effective sample size. This method provides a more precise and comprehensive index of sunscreen protection, and enables statistical comparison of different preparations, or of the effect of other factors on their efficacy.