Linear and exponential sunscreen behaviours as an explanation for observed discrepancies in sun protection factor testing.

Background In vivo testing of sun protection factor (SPF) values can show considerable interlaboratory variability. We studied the underlying reasons and clinical implications. Methods Following the ISO 24444:2010 SPF testing method, seven contract research organizations (CROs) tested eight sunscreens marketed as SPF50 or SPF50+ and the reference SPF15 sunscreens P2 and P3 and SPF43 P6. We analysed differences in the products and CRO testing methods with regard to SPF variability. We tested the erythema prevention capacity of five of the products in subjects exposed to high doses of natural sunlight in Mauritius. Results Sun protection factor values varied dramatically between different CROs for some, but not all of the sunscreens. Those with the largest variability had an SPF50+, and their SPF values differed from a maximum of 62.4 to a minimum of 5.5. These products did not share a common sun-filter composition, and some CROs used low and others high irradiation dose regimens. When comparing these two regimens, test products fell into two categories: (i) they either behaved similarly (linear) or (ii) they behaved differently (exponential). In the outdoor clinical study, exponential and linear sunscreens did not differ in their photoprotection capacities. Conclusion Differences in reported SPF values depend on the linear vs exponential behaviour of such products if subjected to low- vs high-dose test regimens. Under real-time exposure to natural sunlight, exponential and linear sunscreens did not differ in their erythema prevention capacity. Laboratory SPF testing of exponential sunscreens bears the risk of underestimating their in-use SPF.