Evaluating the factors influencing sun protection factors (SPF): Pooling data from multiple studies involving two reference ISO 24444:2019 sunscreen products (P2 and P8)

Background Standardized methods for sun protection factor (SPF) testing are still beset with endpoint and method‐driven issues, and can be influenced by multiple factors. The purpose of this analysis is to explore the factors influencing the results of sun protection factor (SPF) testing in human subjects according to the ISO 24444:2019 standard. Intrinsic factors, such as baseline skin color, age and gender, the minimal erythemal dose on an unprotected area (MEDu), as well as environmental factors such as season/weather influences, are considered for analysis. Methods Datasets generated for two reference products (P2 and P8) during the conduct of 50 such studies using the ISO standard 24444:2019 for the testing of SPF products, from a single testing center located in Bucharest, Romania between April 2021 and December 2022, were retrieved and compiled. Overall, the data for 334 subjects was available, with 276 observations for the reference P8, and 171 for P2. Results No effects due to gender or age were detected. Seasonal changes, the individual typology angle (ITA°) and MEDu were found to have an influence on the outcome of the SPF values. Conclusions This study adds new original data about the impact of intrinsic and extrinsic factors on SPF variations pertaining to ISO reference sunscreen P8 (SPF 50+). The findings suggest that some factors will inevitably impact the results between two SPF experiments for the same product and SPF testing laboratory. The interconnections between the sources of this variation are discussed. The findings of this research help to identify and characterize factors that contribute to SPF testing variability.