However, other types of sunscreen appear to be much more resilient.
Skin cancer is the most common type of cancer in the US (and several other countries). It's so common that around 1 in 5 Americans will likely develop this form of cancer at some point in their lifetime -- which is why it's so important to wear sunscreen.
However, we're not really good at wearing sunscreen. Many people aren't truly aware of what the "SPF" or Sun Protection Factor on sunscreen products means and most of us are using sunscreen all wrong. To make things even more problematic, a new study casts doubt on products using a common ingredient.
A group of researchers from Oregon State University wanted to investigate how different products affect the sunscreen's ability to do its job after it's exposed to sunlight.
"Sunscreens are important consumer products that help to reduce UV exposures and thus skin cancer, but we do not know if the use of some sunscreen formulations may have unintended toxicity because of interactions between some ingredients and UV light," said Tanguay, an OSU distinguished professor and an international expert in toxicology.
The photostability of sunscreens has been shown to be highly dependent on the mixture of the chemicals present, the researchers explain in the study -- and zinc isn't making sunscreen more stable. The team formulated five different ultraviolet-filter (UV-filter) mixtures with an SPF of 15 and analyzed their effect on zebrafish -- a widely used model organism that shares many similarities with humans.
"With either size of particle, zinc oxide degraded the organic mixture and caused a greater than 80% loss in organic filter protection against ultraviolet-A rays, which make up 95% of the UV radiation that reaches the Earth," said Claudia Santillan, one of the study authors. "Also, the zinc-oxide-induced photodegradation products caused significant increases in defects to the zebrafish we used to test toxicity. That suggests zinc oxide particles are leading to degradants whose introduction to aquatic ecosystems is environmentally hazardous."
"And sunscreens containing inorganic compounds like zinc oxide or titanium dioxide, that block UV rays, are being marketed more and more heavily as safe alternatives to the organic small-molecule compounds that absorb the rays," Tanguay added.
Tanguay mentions that it's remarkable that small-molecule mixtures were stable, but it's unsurprising that adding zinc oxide particles makes sunscreen lose effectiveness and become toxic.
"The findings would surprise many consumers who are misled by 'nano free' labels on mineral-based sunscreens that imply the sunscreens are safe just because they don't contain those smaller particles. Any size of metal oxide particle can have reactive surface sites, whether it is less than 100 nanometers or not. More important than size is the metal identity, its crystal structure and any surface coatings."
However, this doesn't mean that you shouldn't use sunscreen -- quite the opposite, sunscreen is one of our most important allies against skin cancer (and nasty skin burns). In addition, there is still some debate within the scientific community about how toxic zinc oxide is to humans, and a 2018 study found that fears are overblown. The main takeaway is that we should be extra careful about what type of sunscreen we use.
"Overall, much more work studying sunscreen formula photostability and phototoxicity is needed to guide design and mass production of safe and effective formulations. Such stability and toxicity studies should inform any sunscreen reformulations, or changes in UV-filter policy, so that regrettable chemical substitutions are avoided," the study concludes.
The research was published in the journal Photochemical & Photobiological Sciences.