Titanium dioxide
nanoparticles have been used increasingly in sunscreens in the last
decade to protect the skin because the tiny particles directly absorb the radiation from sunlight,
especially in the UVB range. But because the articles are so tiny —
generally about 100 nanometers across, compared with about 3,000 to
9,000 nanometers for a speck of dust — some scientists have raised
concerns about whether they might do harm by seeping through the skin
and into the bloodstream.
Concerns grew when studies in mice showed that when injected under the skin, titanium dioxide caused inflammation .
In addition, the International Agency on Cancer Research, part of the
World Health Organization, decided in 2006 to classify titanium dioxide
as a potential human carcinogen, based mostly on inhalation studies in
animals, though the group called the evidence “conflicting at best.”
But research has largely dismissed such concerns about absorption, and most experts say that sunscreens containing nanoparticles can be safely used.
More recently,
concerns have focused on the possibility that these nanoparticles could
promote skin aging. Nanoparticles tend to heat up a little when they
absorb UV radiation, says Paul Westerhoff, a professor at Arizona State
University’s School of Sustainable Engineering and the Built
Environment, and some research suggests that this process could cause
oxidative damage to the skin, particularly with one form, called anatase, when it was delivered in uncoated particles.
Dr. Westerhoff says
most manufacturers now coat the nanoparticles with silicon or aluminum
as a buffer against such harm and as a way to increase light
absorption. But, researchers say, there’s some evidence that those
coatings can break down over days or weeks, which may be an issue for
sunscreens stored too long.
“I’m not saying that titanium dioxide is bad,” Dr. Westerhoff said. But he noted recently in Environmental Science and Technology
that the products have not been thoroughly studied and are minimally
regulated. The only Food and Drug Administration rule for sunscreens is
that the titanium dioxide concentration be less than 25 percent (most
are 2 percent to 15 percent). “I’m just saying we need to figure out if
we should worry.”
