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 in the journal Photochemical & Photobiological Sciences.
The research team 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?
The researcher says that 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.
What the public thinks about sunscreen safety has caused manufacturers, often based on limited
data, to use lots of some ingredients while limiting others.
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.
Scientists 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.
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.
The researcher says that the zinc-oxide-induced photodegradation products caused significant
increases in defects to the zebrafish they used to test toxicity.
That suggests zinc oxide particles are leading to degradants whose introduction to aquatic
ecosystems is environmentally hazardous.
The researcher says that 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.