Derm News: 2007.8(10)

Photostability of commercial sunscreens upon sun exposure and irradiation by ultraviolet lamps

BMC Dermatology 2007;7:1 (Published 26 February 2007)
Helena Gonzalez, Nils Tarras-Wahlberg, Birgitta Stromdahl, Asta Juzeniene, Johan Moan, Olle Larko, Arne Rosen and Ann-Marie Wennberg
ABSTRACT

Background

Sunscreens are being widely used to reduce exposure to harmful ultraviolet (UV) radiation. The fact that some sunscreens are photounstable has been known for many years. Since the UV absorbing ingredients of sunscreens may be photounstable, especially in the long wave region, it is of high interest to determine their degradation during exposure to UV radiation. Our aim was to investigate the photostability of seven commercial sunscreen products after natural UV exposure (UVnat) and artificial UV exposure (UVart).

Methods

Seven commercial sunscreens were studied with absorption spectroscopy. Sunscreen product, 0.5 mg/cm2, was placed between plates of silica. The area under the curve (AUC) in the spectrum was calculated for UVA (320-400 nm), UVA1 (340-400 nm), UVA2 (320-340 nm) and UVB (290-320 nm) before (AUCbefore) and after (AUCafter) UVart (980 kJ/m2 UVA and 12 kJ/m2 of UV radiation, included UVB) and before and after UVnat. If the AUC index (AUCI) (AUCafter/AUCbefore) was higher than 80%, the sunscreen was considered photostable.

Results

Three sunscreens were unstable after UVnat; in the UVA range the AUCI was between 35.9% and 75.5%. In the UVB range one of these sunscreens showed an AUCI of 63.2%. Three sunscreens were photostable after UVnat; in the UVA range the AUCI was between 85.5% and 98.7% and in the UVB range the AUCI was between 91.7% and 100%. One sunscreen showed in the UVA range an AUCI of 87.2% after UVn but an AUCI of 72.5% after UVa. Five of the sunscreens were stable in the UVB region.

Conclusions

The present study shows that several sunscreens are photounstable in the UVA range after UVnat and UVart. There is a need for a standardized method to measure photostability and the photostability should be marked on the sunscreen product.

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