Sunscreen stability

MARIA BURKE

Combining chemical sunscreen mixtures with zinc oxide may limit protection against UVA, according to a new study. Sunscreen mixed with zinc oxide was also found to cause toxic effects when tested in zebrafish embryos.

Researchers from the University of Oregon, US, and the University of Leeds, UK, created five different SPF 15 chemical sunscreens that included small-molecule UV filter ingredients (Photochemical & Photobiological Sciences, doi: 10.1007/s43630-021-00101-2). The team then mixed one of the five sunscreens with zinc oxide (6%), which is often combined with small-molecule UV filters in hybrid sunscreens - those containing chemical and mineral components. After two hours of UV exposure, they found that the UVA protection factor was reduced by 84.3-91.8%, compared with the original sunscreen, which showed a 15.8% loss. They suggest the ZnO degrades other UV absorbers in the mixture.

‘We still recommend consumers use sunscreen but suggest they should be careful to avoid mixing sunscreen with zinc oxide, whether intentionally with hybrid sunscreens that combine small-molecule UV filters with zinc oxide, or incidentally by mixing sunscreen with other products containing zinc oxide, such as make-up containing SPF,’ says Richard Blackburn of the University of Leeds.

In further studies, the team found that combining zinc with other sunscreen ingredients led to developmental changes in zebrafish embryos, a model organism for biomedical research. They placed embryos in solutions of the sunscreen mixtures for five days. Those exposed to the ZnO-containing sunscreen showed more changes to their normal development including under-developed fins and shorter body length. There were minimal changes in embryos placed in the five non-mineral mixtures.

‘The paper is interesting and thought-provoking,’ says Oliver Jones, Professor of analytical chemistry at RMIT University, Melbourne, Australia. ‘[It] highlights the importance of knowing your chemistry when you mix materials that have previously been used separately into a new product.’ However, it does have limitations, some of which the authors acknowledge. For example, he notes they did not test any commercial sunscreens, which tend to have more complex formulations. ‘It is quite possible that if the experiment was repeated with off-the-shelf sunscreens the results would not be the same. There was also no analysis of the hybrid sunscreens after UV exposure to determine what the reaction products might have been.’

Winston Morgan, reader in toxicology at the University of East London, UK, suggests it would have been more useful for the study to have looked at human skin cells, rather than zebrafish embryos. ‘The degradation of the protection is of interest but it’s hard to assess the toxicity from this study alone,’ he adds. ‘Much further research is required and we next need to do in vivo animal studies.’