Re-applying sunscreen while outdoors or at the beach is always a good idea.
However, researchers have now warned people against mixing mineral and chemical sunscreens, as it may limit protection against the UVA rays.
For the study, scientists 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 approved for use in the European Union and United States.
They found these chemical, or non-mineral, sunscreens (without zinc oxide) had minimal changes in UVA absorption after being exposed to UV for two hours, indicating these formulations reliably protect the skin.
However, one of the mixtures that contained zinc oxide had degraded other UV absorbers in the mixture. They found the UVA protection factor was reduced by between 84 and 91 per cent in the sunscreen mixed with zinc oxide particles, while the original sunscreen without zinc oxide only showed a 15 per cent loss in UVA protection factor, after UV exposure for two hours.
“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,” study co-author Professor Richard Blackburn shared.
Full study results have been published in Photochemical & Photobiological Sciences.