SPF describes protection against UVB under standardised test conditions. It does not measure every aspect of sun protection and is not a fixed allowance for spending longer in the sun.
UVA protection is assessed separately. Read both the SPF and UVA information on the pack, as well as the directions and water-resistance instructions.
The NHS guidance recommends at least SPF30 with suitable UVA protection, enough product, and frequent reapplication. Use shade and clothing too.
The SPF number also says nothing about environmental performance. AETHIC’s SPF50 life-cycle assessment is a separate comparison against two reference SPF50 formulations. It reported lower impact in 11 of 16 measured EF 3.1 categories, with trade-offs in other categories.
Coral experiments in the patent and the 2017 sea-urchin study address different research questions. The latter tested a finished SPF40 formulation, so its results should not be assumed to cover every current SPF50 or SPF50+ product. See the research summary for sources and limitations.