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

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.