Background: There are considerable data to suggest that protection from solar ultraviolet (UV) radiation will reduce the risk of acute and chronic skin damage in humans. Whereas the Sun Protection Factor (SPF) provides an index of protection against erythemally-effective solar UV, largely confined to the UVB (290-320 nm) and short wavelength UVA (320-340 nm) region, there is currently no agreed to measure of broad-spectrum protection against long wavelength UVA (340-400 nm).
Objective: The objective of these studies was to assess the potential of in vitro UV substrate spectrophotometry and subsequent calculation of the Critical Wavelength value as a measure of broad-spectrum UV protection and as a routine, practical procedure for classification of sunscreen products.
Methods: The spectral absorption of 59 commercially available sunscreen products and multiple experimental formulae with one or more UV filters was measured. Sunscreen product was applied at a rate of 1mg/cm2 to a hydrated synthetic collagen substrate, pre-irradiated with a solar simulator and then subjected to UV substrate spectrophotometry. Multiple determinations from 5 independent samples per product were used to calculate the critical wavelength value, defined as the wavelength where the integral of the spectral absorbance curve reached 90% of the integral from 290 to 400nm.
Results: We found that a recognised longwave UVA active ingredient such as titanium dioxide (TiO2), zinc oxide (ZnO), or avobenzone (AVO) is a necessary but insufficient product requirement for achieving the highest proposed broad-spectrum classification, i.e., critical wavelength 370 nm. Although SPF and critical wavelength are largely independent of each other, UVA absorbance must increase commensurate with SPF in order to maintain the same critical wavelength value. Substrate spectrophotometry and the calculation of critical wavelength can readily account for sunscreen photostability by UV pre-irradiation. Finally, there is also a strong positive relationship between critical wavelength and of a currently available in vivo measure of UVA protection.
Conclusions: Determination of critical wavelength using UV substrate spectrophotometry provides a rapid, inexpensive and reliable measure of broad-spectrum protection, that is largely independent of SPF, yet ensures long wavelength UVA protection commensurate with SPF. The procedure provides a routine, sensitive means of differentiating and classifying sunscreen products and, importantly, obviates the need to subject volunteers to acute exposures of high-dose, non-terrestrial UV, the health risks of which are still poorly understood.
Diffey BL, Tanner PR, Matts PJ, Nash JF. In vitro assessment of the broad-spectrum ultraviolet protection of sunscreen protection of sunscreen products. Journal of the American Academy of Dermatology, 2000; 43: 1024-1035.
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