Zinc oxide and titanium dioxide nanoparticles in sunscreen have
come under increasing scrutiny, as should be the case, given that
we know so little about the behavior of nano-sized versions of
materials like zinc oxide and titanium dioxide that have been used
for years in bulk form.
Because very small particles of metal oxides gave a transparent
look to a sunscreen that was difficult to achieve with bulk sizes,
many companies quickly grew to embrace the new technology. To this
day these companies argue that nanoparticles are safe because there
is no evidence that nanoparticles (defined as less than or equal to
100 nm) penetrate past the epidermal barrier. The argument begs its
own refutation, as it implies that evidence to the contrary could
pose a problem, and now more studies are emerging which indicate
that nanoparticles do penetrate past the epidermis.
A
test by Dr. Brian
Gulson et al from Macquarie University demonstrated that tracer
zinc showed up in blood and urine samples taken from test subjects,
indicating that zinc oxide nanoparticles were indeed being
absorbed. Another
study from the
Institute of Clinical Pharmacology, Department of Pharmacology,
Huazhong University of Science and Technology, Wuhan, China, found
that TiO2 particles penetrated past the epidermis in hairless mice,
subsequently reaching different tissues and causing pathologic
changes to organs, most notably the skin and the liver.
While size is certainly an important factor to consider in risk
assessment studies, the physical properties of the material itself
must also be taken into consideration. A nanoparticle and
nanostructured surfaces conference cited a
study showing that titanium
dioxide particles in sunscreens “catalyzed DNA damage both in vitro
and in human cells” when exposed to sunlight. The conference raised
the question of “the suitability of photoactive TiO2 as a sunscreen
component without further studies.”
Zinc oxide applications on the other hand, even when absorbed,
result in relatively harmless biological pathways and endpoints. To
quote Dr. Gulson: “Maybe with young children where you’re applying
it seven days a week, it could be an issue but I’m more than happy
to continue applying [20% zinc oxide sunscreen] to my
grandchildren.”
Nanoparticles are now found in everything from cosmetics to
clothing and even food, and as regulations now stand, consumers
cannot depend upon labels to inform them of what they are getting.
The rule of thumb used by environmental scientists is that the
smaller the particle the riskier it is to use it. More studies need
to be done on many of these nanomaterials, particularly when
physical properties, such as photoreactivity in the case of TiO2,
indicate that there might be problems.