As
vacationers prepare to spend time outdoors this summer, many of them will pack
plenty of sunscreen in hopes it will protect their bodies from overexposure,
and possibly from skin cancer. But researchers at Missouri University of
Science and Technology are discovering that sunscreen may not be so safe after
all.
Cell
toxicity studies by Dr. Yinfa Ma, Curators’ Teaching Professor of chemistry at
Missouri S&T, and his graduate student Qingbo Yang, suggest that when
exposed to sunlight, zinc oxide, a common ingredient in sunscreens, undergoes a
chemical reaction that may release unstable molecules known as free radicals.
Free radicals seek to bond with other molecules, but in the process, they can
damage cells or the DNA contained within those cells. This in turn could
increase the risk of skin cancer.
Ma
also found that the longer zinc oxide is exposed to sunlight, the greater the
potential damage to human cells.
“Zinc
oxide may generate free radicals when exposed to UV (ultraviolet)
sunlight,” Ma says, “and those free radicals can kill cells.”
Ma
studied how human lung cells immersed in a solution containing nanoparticles
of zinc oxide react when exposed to different types of light over numerous time
frames. Using a control group of cells that were not immersed in the zinc oxide
solution, Ma compared the results of light exposure on the various groups of
cells. He found that zinc oxide-exposed cells deteriorated more rapidly than
those not immersed in the chemical compound.
Even
when exposed to visible light only, the lung cells suspended in zinc oxide
deteriorated. But for cells exposed to ultraviolet rays, Ma found that
“cell viability decreases dramatically.”
When
exposed to ultraviolet long-wave light (ultraviolet A or UVA) for 3 hrs, half
of the lung cells in the zinc oxide solution died. After 12 hrs, 90% of the
cells in that solution died, Ma found.
Why
does zinc oxide, an ingredient used in sunscreen to help block harmful UV rays,
cause cells to deteriorate when exposed to sunlight? According to Ma, when the
zinc oxide nanoparticles in the solution absorb the UV rays, the reaction
releases electrons, which in turn may produce unstable free radical molecules
in the zinc oxide solution. Those free radical molecules then bond with other
molecules and act as parasites, damaging the other molecules in the process.
Ma’s
research on zinc oxide’s effect on cells is still in the early stages, so he
cautions people from drawing conclusions about the safety or dangers of
sunscreen based on this preliminary research.
“More
extensive study is still needed,” Ma says. “This is just the first
step.”
For
instance, Ma plans to conduct electron spin resonance tests to see whether zinc
oxide truly does generate free radicals, as he suspects. In addition, clinical
trials will be needed before any conclusive evidence may be drawn from his
studies.
In
the meantime, Ma advises sunbathers to use sunscreen and to limit their
exposure to the sun.
“I
still would advise people to wear sunscreen,” he says. “Sunscreen is
better than no protection at all.”
Besides
sunscreen, zinc oxide is used in many commercial products, including plastics,
paints, ointments, and sealants.
