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  • A recent study suggests that mixing mineral and chemical sunscreens may limit the protection against UVA.
  • Additionally, it found that sunscreen mixed with zinc oxide may be rendered less effective.
  • The study suggests that chemical reactions between zinc oxide in mineral sunscreens, and some of the chemicals in chemical sunscreens, may mean sunscreens become ineffective in 2 hours.

One thing we can all agree on is that sunscreen is always a good idea.

The American Academy of Dermatology says that wearing sunscreen to all exposed skin that offers broad-spectrum protection, water resistance, and SPF 30 or higher is one of the easiest ways to protect your skin from the sun. Sunscreen is an everyday essential. Unprotected sun exposure can cause damage in less than 20 minutes.

But a recent study published in Photochemical & Photobiological Sciences on October 14 suggests that mixing mineral and chemical sunscreens may limit the protection against UVA, and that sunscreen mixed with zinc oxide maybe rendered less effective.

What does this mean for protecting your skin? We reached out to experts to find out.

There are two types of sunscreens out there: chemical and mineral. They’re both effective at protecting your skin, but with some key differences.

Chemical sunscreens use chemical formulas to protect the skin. SPF is the most commonly used chemical and includes ingredients like oxybenzone, avobenzone, octinoxate, and octisalate.

Mineral sunscreens physically block out UV radiation with specific ingredients, like titanium dioxide or zinc oxide. You may see these sunscreens also called “physical sunscreen,” as their ingredients physically block the sun.

Each has its own set of benefits and negatives, but many people mix the two in order to achieve maximum protection. But this is where the study suggests we may be doing more harm to our skin than good.

The study suggests that chemical reactions between zinc oxide in mineral sunscreens, and some of the chemicals in chemical sunscreens, may mean sunscreens become ineffective after just 2 hours of sun exposure.

“On its own, zinc oxide is an effective and harmless UV blocker,” said Professor Richard Blackburn, PhD, professor of sustainable materials at the University of Leeds School of Design, and leader of one of the portions of the study, in a statement. “Our research raises concerns about how the individual formulation ingredients react with each other during use and this isn’t currently tested by the industry.”

The study found that once exposed to sunlight for 2 hours, the zinc oxide destroys the UVA protection provided by the other ingredients. This becomes harmful because people believe they’re double protected and will stay out in the sun longer, even though they’re actually being harmed by UV rays.

Additionally, the study found that zinc oxide causes degradation of other UV absorbers, and the protection provided by sunscreens was reduced significantly. The UVA protection factor was reduced by 84.3 percent and 91.8 percent in the sunscreen mixed with zinc oxide particles, while the original sunscreen without zinc oxide only showed a 15.8 percent loss in UVA protection after 2 hours of UV sun exposure.

The authors also exposed zebrafish embryos — a common model organism for research of this kind — to the sunscreen mixtures. Those exposed to sunscreen mixed with zinc oxide showed increased changes to their normal development, including under-developed fins and shorter than normal body length.

While this research is new, some experts think it’s still too early to sound the alarm about mixing sunscreens.

“This one study does not make me believe that the chemicals degrade,” said Dr. Michele Green, a cosmetic dermatologist in New York City. “One thing to remember is that all sunblocks tell you to reapply after 90 minutes. I tell all of my patients to reapply after 90 minutes. This has to be done anyway.”

Dr. Mary Stevenson, assistant professor of dermatology at NYU Langone Health, said the study brought up interesting points.

“There may be some mechanism in which something is breaking something down, but I think we need more information,” Stevenson said. “I think we need to look at this carefully. This information is very novel in being added to the literature, but it brings a bigger list of questions.”

Stevenson added that the first thing scientists and researchers look at is whether or not the results from a study can be replicated. Is this reproducible data, meaning do we have repeatable trials?

The authors of the study note that they’re not able to replicate exact commercial sunscreens as they did not have access to information on additives, fragrances, or precise measurements of ingredients. Therefore, the impact of these ingredients when mixing products is still unknown.

“I would have to see more studies before I think I would change my practice,” said Stevenson. But, she added, “if you want to use one amazing sunscreen, I always support those with zinc or titanium.”

The important takeaway, added Green, is that people should not stop using sunscreen.

“This leaves a lot of questions unanswered, but I don’t want people to panic and not use sunblock,” she said. “You still do need it. If you want a take-home, then trust that pure zinc oxide is a 100 percent UV blocker.”

And always reapply, reapply, reapply.