Can adding zinc oxide change how sunscreen ingredient mixtures hold up under ultraviolet light?

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Sunscreen safety and efficacy are generally evaluated using the properties of the individual chemicals in a formulation. However, sunscreen photostability—how well it holds up under light—has been shown to depend strongly on the mixture of chemicals present.

To better understand how formulation influences stability and establish a foundation for investigating zinc oxide additives, the researchers formulated five different small-molecule ultraviolet-filter (UV-filter) mixtures with a Sun Protection Factor (SPF) of 15. The mixtures contained active ingredients approved in either the United States or the European Union and were designed to represent formulations of actual products on the market.

In a laboratory experiment, the researchers evaluated the mixtures’ photostability and toxicity, with and without zinc oxide, after two hours of UV exposure. Changes in UV absorbance were minimal for all five small-molecule mixtures without zinc oxide. The presence of either micro- or nano-sized zinc oxide caused significant photodegradation of the small molecules, and the degraded mixtures showed higher levels of toxicity in embryonic zebrafish assays.

The study suggests that care is needed when formulating sunscreens that combine zinc oxide and small-molecule UV filters, to avoid unintended consequences during use.