Sunscreen labels may overestimate SPF at temperate latitudes
MedWire News: Ultraviolet (UV)B sunscreen sun protection factor (SPF) is likely to be unreliable at temperate latitudes, say researchers who believe this is because UVA light makes a greater contribution to sunburn at temperate latitudes than in the laboratory conditions used to test and assign SPFs.
“Solar UV radiation is divided into shorter wavelength UVB (~295–320 nm) and longer wavelength UVA (320–400 nm) radiation, the vast majority being UVA (typically more than 95%),” explain Young and team.
However, the quantity of a given waveband is considerably less important than its biological efficacy and generally “a given dose of UVB is approximately 1000 times more erythemogenic than the same dose of UVA,” the researchers explain, making UVB the main cause of sunburn and probably nonmelanoma skin cancer.
To test the efficacy of two SPF 6 suncreens, one UVB only and one UVA plus UVB absorbing, Antony Young (King’s College London, UK) and colleagues recruited eight fair-skinned healthy young adults. They were exposed to simulated sunlight, typical of that experienced at a temperate latitude such as that of Southern England, both on skin where sunscreen had been applied and on unprotected skin over 13 consecutive days (0.33 minimal erythema dose/day).
They found that broad spectrum (UVA plus UVB) sunscreen-protected skin had significantly reduced levels of collective erythema compared with skin protected with the UVB sunscreen only in all cases.
The researchers say this is likely to be because “UVA makes a greater contribution toward sunburn at temperate latitudes than under the laboratory conditions in which SPF is tested and assigned.”
They explain that the solar simulated radiation spectra specified by the SPF assessment regulatory bodies “represent the sun at high elevation at low latitudes with more UVB relative to UVA than is found on the equator at noon.”
Young et al add: “The consequence of this extreme spectrum, coupled with the high UVB dependence for erythema, is that SPF is primarily a measure of UVB protection with no quantitative information of UVA.”
They conclude that their data support the use of broad spectrum rather than UVB-only absorbing sunscreens for sun protection, especially in countries at temperature latitudes such as the UK.
The results of this study are published in the Journal of Investigative Dermatology.
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By Helen Albert