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Study Shows Why Mixing Your Sunscreens Might Not Be a Good Thing to Do - ScienceAlert

(Laura Olivas/Getty Images)

Sunscreens that are safe and effective on their own might not work as well when mixed together. In certain combinations, new research suggests they might even create toxic byproducts.

Zinc oxide is increasingly marketed as a safe, 'chemical-free' alternative to other sunscreens on the shelf, but that depends on what it's served with.

If this inorganic sunblock is put under or on top of other organic sunscreens, researchers say most of the ultraviolet rays from the Sun aren't blocked as well as either product on its own.

Researchers found that the mix would degrade the organic UV- filters, reducing their effectiveness and generating potentially toxic products. 

In the current study, the potential toxicity of sunscreens was only tested on zebrafish, not humans, but these creatures are genetically very similar to ourselves. What's more, they are representative of many other fish in the sea, which could be harmed by the sunscreen we choose to use.

Until we know more, researchers say we should avoid layering these products on top of one another - not only for our own health but the health of the environment around us.

This certainly isn't to say we should stop using sunscreen altogether. For all the concerns we might have over potential risks, we can be certain that high exposure to the Sun's UV rays has seen too many people develop deadly skin cancers.

"We still recommend consumers use sunscreen," says materials scientist Richard Blackburn from the University of Leeds, "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."

Despite the fact that a number of other studies have shown sunscreens can quickly react under UV exposure, very few researchers have examined whether this reaction releases toxic byproducts. 

The chemicals approved for sunscreen have only been determined to be non-toxic on their own, as a pure individual chemical, not a blend of chemicals.

To better understand how the chemicals approved for sunscreen in the United States and Europe react when combined, researchers tested the ingredients found in five commercial sunscreens with a sun protection factor of 15.

The team then tested what happened to the effectiveness and safety of these chemical formulas when combined with the mineral zinc oxide, and exposed to the Sun for two hours.

The results suggest that if even a little bit of zinc oxide is mixed with non-mineral sunblocks, a person's protection against ultraviolet A rays (which make up the vast majority of sunlight) is reduced by over 80 percent and possibly up to 92 percent.

Without any zinc oxide added, a mixture made up of non-mineral sunscreens lost only 15.8 percent of their UVA protection.

This suggests small-molecule-based sunblocks don't degrade very much in the Sun, although the study did not test the commercial products directly, only the main UV-blocking ingredients. 

Preservatives and other chemicals in sunblock on the shelf may therefore change the results and should be further investigated, researchers say.

Until then, the authors suggest people avoid mixing zinc oxide with other sunscreens. Even if makeup has SPF, it could reduce the effectiveness of the product - and possibly also its safety.

When zebrafish in the embryo were exposed to various chemical and zinc oxide mixtures for five days in the Sun, researchers noticed higher levels of toxic byproducts in the embryo when zinc oxide was involved.

The zebrafish didn't die from these products, but they did often show morphological defects.

"These results suggest that zinc oxide particles may increase sunscreen toxicity in ways not currently recognized," the authors write.

"We fear that the increasing ubiquity of UV-filters (in particular metal oxide particles), coupled with the lack of studies on sunscreen phototoxicity, especially as formulated products, is likely to result in products that have unintended consequences and regrettable chemical substitutions," they add.

Unfortunately, there are not many federally approved alternatives to zinc oxide in the US that can filter the Sun in the same way. In Europe, on the other hand, there are other inorganic ingredients that are approved for use as a sun filter, and these don't seem to break down and produce as many toxic chemicals as zinc oxide.

To better test the potential toxicity, the solutions were mixed into dimethyl sulfoxide – an agent that made them more water-soluble. While sunscreens don't contain this chemical, the fate of any chemical with potentially hazardous implications for us or wildlife should be a cause for ongoing study.

Especially since dangerous sunscreens have slipped by our notice before. A UV-filtering chemical compound contained in 3,500 brands of sunscreen, for instance, has been shown to disrupt the growth of ocean corals and leave them especially vulnerable to bleaching events for years, and yet these ingredients continue to be used regularly.

In recent years, more people have grown concerned about the chemicals in their sunscreen, but research and regulation of these formulas are a long way behind.

And it's not just sunscreen, either. There are numerous cosmetic products that we put directly on our skin every day that have yet to be properly safety tested. Even worse, some have been found to be potentially toxic or possibly carcinogenic in Europe but are still commercially available in the US.

Filling that vast gap in knowledge is a daunting task, made even more challenging by the fact that chemicals could very well become toxic with even a slight change in a formula or a mixing of multiple products.

"Overall," the authors of the sunblock paper conclude, "much more work studying sunscreen formula photostability and phototoxicity is needed to guide design and mass production of safe and effective formulations."

The study was published in Photochemical Photobiological Sciences .