Background/purpose: Worldwide, sunscreen sun protection factor (SPF) testing is based on 30-year-old technology. At the time the SPF test came into being, the highest SPFs available were in the 6-8 range. The SPF test is reasonably accurate for SPFs up to 15, but is much less reliable for measuring SPFs of 30 and higher. The method we propose addresses two primary reasons for this unreliability: 1) difficulties in applying products uniformly and 2) the subjectivity and variability of perception in evaluating and grading responses to UV doses.
Methods: Our proposed SPF substantiation method differs from the current SPF test in that sunscreen-protected test sites receive the same UV dose in four uniformly spaced sub-sites, which are graded as passing if no response is seen or failing if any response is seen. The response may be tanning, erythema, or a combination of both. To demonstrate the method, two commercial products with labeled SPFs of 30 and 45 and the P2 sunscreen standard were tested at two different laboratories.
Results: The SPF 30 product and SPF 15 standard were shown to be correctly labeled. However, it is questionable as to whether the SPF 45 product provides protection against 45 minimal erythema doses.
Conclusions: Our proposed SPF substantiation method is not dependent on subjective evaluation of responses, accounts for non-uniform product application, and provides a conservative estimate of sunscreen protection. The method consists of a systematic repetition of identical tests that are considerably more rigorous than the current methods that are based on single data points per test subject. While the current SPF test remains necessary and valuable as a dose ranging tool, we propose that this SPF substantiation method supersede the old method for final SPF label determination.