Researchers from the University of Oregon and the University of Leeds created five different SPF 15 chemical sunscreen mixtures that included small-molecule UV filter ingredients that are approved for use in the European Union and United States.
The authors found these chemical, or non-mineral, sunscreens (without zinc oxide) had minimal changes in UVA absorption after being exposed to UV for two hours, suggesting these formulations reliably protect against UVA.
One of the mixtures that was most representative of sunscreens used in both the EU and US, was further tested with the presence of 6% zinc oxide – a mineral often combined with small-molecule UV filters in hybrid sunscreens made of both chemical and mineral components .
After two hours of UV exposure, the sunscreen mixture containing zinc oxide had significant small-molecule photodegradation, where the zinc oxide had degraded other UV absorbers in the mixture.
The authors calculated how much UVA the sunscreen blocked out to determine the UVA protection factor for each mixture. They found the UVA protection factor was reduced by between 84.3% and 91.8% in the sunscreen mixed with zinc oxide particles, while the original sunscreen without zinc oxide only showed a 15.8% loss in UVA protection factor, after UV exposure for two hours.
Professor Richard Blackburn, co-author of the study, said: “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 makeup containing SPF.”
The authors also exposed zebra fish embryos, a model organism for biomedical research, to the sunscreen mixtures in experiments to explore the potential effects of different combinations. They diluted the sunscreen mixtures in 99% water (99:1 ratio) and exposed some to UV radiation. They placed the zebra fish embryos taken four hours after fertilization into the diluted mixtures.
The embryos remained in the solutions for five days and were monitored at 22 different points.
The zebra fish embryos exposed to sunscreen mixed with zinc oxide showed increased changes to their normal development including under-developed fins and shorter than normal body length. The five non-mineral sunscreen mixtures were found to show minimal changes to the zebra fish development.
Zebra fish embryos in the control group who were placed in sunscreens without UV exposure were compared to embryos who were placed in UV-exposed sunscreens. The percentage difference in the physical development of embryos in the experimental group compared to the control was considered “toxicity” of sunscreens.
Only one of the non-mineral sunscreens demonstrated a slight increase of under 10% in toxicity after UV exposure.
Zinc oxide was also tested in isolation and did not significantly increase toxicity, which indicates it is the combination of zinc and other sunscreen ingredients that is linked to developmental changes in zebra fish embryos, according to the authors.
The authors highlight that they were not able to replicate exact commercial sunscreens as they did not have access to information on additives, fragrance and precise measurements of ingredients so the impact of these when mixing products is unknown.
They further caution that the formulation date and any conditions the bottle may be exposed to in everyday use could influence formula stability and so more samples from a range of conditions would be needed to further test their findings.