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

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 UVB) and before and after UVnat. If theAUC Index (AUCI), defined as AUCI = AUCafter/AUCbefore, was > 0.80, the sunscreen was considered photostable.Three sunscreens were unstable after 90 min of UVnat; in the UVA range the AUCI was between 0.41 and 0.76. In the UVB range one of these sunscreens was unstable with an AUCI of 0.75 after 90 min. Three sunscreens were photostable after 120 min of UVnat; in the UVA range the AUCI was between 0.85 and 0.99 and in the UVB range between 0.92 and 1.0. One sunscreen showed in the UVA range an AUCI of 0.87 after UVnat but an AUCI of 0.72 after UVart. Five of the sunscreens were stable in the UVB region.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.Sunscreens give good protection against sunburn, actinic keratosis and squamous cell carcinoma. The results for preventing cutaneous malignant melanoma (CMM) and basal cell carcinoma are less conclusive [1-3]. One explanation for this can be that UVA radiation (320–400 nm) plays a role for induction of CMM [4] and that it is mainly in the UVA range the photodegradation of the sunscreen occurs. In the present work, commercially available sunscreens, containing organic chemical and/or inorganic chemical filters, have been exposed to natural UV (UVnat) as well as to artificial UV (UVart) in order to study their photostability.Previous studies have shown that some sunscreens lose part of their protection when exposed to UV radiation [5-10]. Severa