After two hours, sunscreens that include some zinc oxide can lose effectiveness, become toxic

Date:
October 14, 2021
Source:
Oregon State University
Summary:
Sunscreen that includes zinc oxide, a common ingredient, loses
much of its effectiveness and becomes toxic after two hours of
exposure to ultraviolet radiation, according to scientists.



FULL STORY ========================================================================== Sunscreen that includes zinc oxide, a common ingredient, loses much
of its effectiveness and becomes toxic after two hours of exposure to ultraviolet radiation, according to a collaboration that included Oregon
State University scientists.


==========================================================================
The toxicity analysis involved zebrafish, which share a remarkable
similarity to humans at the molecular, genetic and cellular levels,
meaning many zebrafish studies are immediately relevant to people.

Findings were published today in Photochemical & Photobiological Sciences.

The research team, which included College of Agriculture Sciences faculty
Robyn Tanguay and Lisa Truong and graduate fellow Claudia Santillan,
sought to answer important but largely neglected questions regarding the massive global sunscreen market, predicted by market data firm Statista
to be worth more than $24 billion by the end of the decade.

The questions: How stable, safe, and effective are sunscreen ingredients
in combination rather than as individual compounds -- which is how they
are considered for Food and Drug Administration approval -- and what
about the safety of any chemical products that result from reactions
caused by exposure 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.



==========================================================================
What the public thinks about sunscreen safety has caused manufacturers,
often based on limited data, to use lots of some ingredients while
limiting others, she said. For example, oxybenzone has effectively been discontinued because of concerns that it harms coral reefs.

"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 said.

Scientists including the University of Oregon's James Hutchinson and
Aurora Ginzburg and the University of Leeds' Richard Blackburn made
five mixtures containing the UV filters -- the active ingredients in
sunscreens -- from different products available in the United States and Europe. They also made additional mixtures with the same ingredients,
plus zinc oxide at the lower end of the commercially recommended amount.

The researchers then exposed the mixtures to ultraviolet radiation
for two hours and used spectroscopy to check their photostability --
i.e., what did sunlight do to the compounds in the mixtures and their UV-protective capabilities? The scientists also looked at whether the
UV radiation had caused any of the mixtures to become toxic to zebrafish,
a widely used model organism that goes from egg to swimming in five days,
and found that the UV-exposed mixture without zinc oxide did not cause
any significant changes in the fish.



========================================================================== "There have been several studies that showed sunscreens can quickly react
under UV exposure -- the specifically intended setting for their use --
so it's pretty surprising how little toxicity testing has been done
on the photodegradation products," Truong said. "Our findings suggest
that commercially available small-molecule-based formulas, which were
the basis for the formulas we studied, can be combined in different
ingredient ratios that minimize photodegradation." But scientists saw
big differences in photostability and phototoxicity when zinc oxide
particles were added -- either nanoparticles or the larger microparticles.

"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," Santillan said. "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." Tanguay said she was surprised that all five small-molecule mixtures were generally photostable but not surprised that adding zinc
oxide particles led to toxicity upon UV irradiation.

"As a team at Oregon State that specializes in studying nanoparticle
toxicity, these results were not a shock," she said. "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." The National Science Foundation and the National Institutes of Health supported this research.

========================================================================== Story Source: Materials provided by Oregon_State_University. Original
written by Steve Lundeberg. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Aurora L. Ginzburg, Richard S. Blackburn, Claudia Santillan,
Lisa Truong,
Robyn L. Tanguay, James E. Hutchison. Zinc oxide-induced changes
to sunscreen ingredient efficacy and toxicity under UV irradiation.

Photochemical & Photobiological Sciences, 2021; 20 (10): 1273 DOI:
10.1007/s43630-021-00101-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211014100220.htm

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