Abstract
The efficacy of a sunscreen tends to be associated with its sun protection factor (SPF) value, a figure determined in a test that relies on the independence of the SPF value to both UV radiation dose and irradiance. We probe these assumptions when photoinduced product degradation is present, and we estimate that the theoretical limit for their validity is when the sunfilter active molecule relaxation time is faster than ∼10 ns. While such threshold relaxation time should be compatible with the expected ultrafast relaxation mechanisms of sunfilter molecules (picoseconds), recent research on sunfilter photodynamics has identified the existence of much longer-lived molecular states. Such long lifetimes could compromise sunscreen performance and make the SPF value very different in natural sun irradiance conditions than in the solar simulated conditions typically used in SPF determination tests.
Highlights
The efficacy of a sunscreen tends to be associated with its sun protection factor (SPF) value, a figure determined in a test that relies on the independence of the SPF value to both UV radiation dose and irradiance
The sun protection factor (SPF) value has become a standard to benchmark UVB protection for sunscreens.[1−4] Routine SPF tests implicitly assume a linear response between the transmitted and irradiated doses because the individual patients’ SPFs are averaged and those falling outside a confidence interval are either dismissed or the averaged SPF value is deemed not valid
SPF tests rely on the so-called reciprocity law, where the SPF value is assumed to be independent of the irradiance used for measuring it.[2,5,6]
Summary
Eqs 4−6 are a simplification of the real dynamics taking place when a sunscreen is irradiated, they represent a more general model than the “linear” assumption made for N being constant in eq 1. Because our purpose is to identify the order of magnitude of molecular relaxation times (τr) for the appearance of SPF nonlinearities, as opposed to modeling the dynamics of specific sunfilter molecules, we assume flat absorption curves (i.e., σ and φ not changing with wavelength) and the same σ for all processes of Figure 1 While these assumptions are unlikely for a single sunfilter molecule, they are quite representative of broad band sunscreen products, which contain several sunfilters together achieving an overall flat absorbance spectrum in the 290−400 nm wavelength range.[33] On this basis the expected transmitted irradiance E under the linear approximation (Nr, Ni ≪ N0; Na ≈ N0) is given by. 2000 erythematic effective mJ·cm−2 for 40 erythematic effective mJ·cm−2 transmitted through an SPF50 sunscreen
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