Relevance of UV filter/sunscreen product photostability to human safety.

Photostability or photo-instability of sunscreen products is most often discussed in undesirable terms with respect to human safety. The health risks, specifically associated with sunscreens, photostable or photo-unstable, include phototoxic/photoirritation or photoallergic responses and, longer-term, an increased risk of skin cancers or photoageing. The aims of this paper are to define photostability/photo-instability and objectively assess the acute and chronic toxicological consequences from the human exposure to UV filter/sunscreens and any probable photo-degradation products. The reported prevalence of photoirritation and photoallergic responses to sunscreens is rare compared with adverse events, for example, skin irritation or sensitization, produced by cosmetics or topically applied drugs and do not directly implicate potential photo-degradation products of UV filters. Moreover, for at least one photo-unstable combination, octyl methoxycinnamate and avobenzone, the long-term benefits to humans, i.e., reduction in skin cancers, seem to outweigh any potential adverse consequences attributed to photo-degradation. Sunscreen products are formulated to achieve maximum efficacy which, by necessity and design, incorporate measures to support and promote photostability since all organic UV filters have the potential to photo-degrade. Current performance measures, in vivo SPF and in vitro UVA, conducted under standardized conditions, in part account for photostability. The concerns expressed when considering human exposure to potential photo-unstable UV filters or sunscreen products may not manifest as health risks under conditions of use. Still, improvement in sunscreen product photostability continues to be a key strategic objective for manufacturers.