Holidaymakers are advised to slap on plenty of suncream to reduce their risk of skin cancer. But now scientists are saying an ingredient in some bottles could possible harm the skin.
Early research from Missouri University suggests that zinc oxide, when exposed to sunlight, 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.
The researchers suggest this in turn could potentially increase the risk of skin cancer.
Lead author Dr Yinfa 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, and those free radicals can kill cells," Ma said.
However, the authors cautioned people from drawing conclusions about the safety or dangers of sunscreen based on this preliminary research.
For instance Dr. Ma said he needed to conduct further tests to see whether zinc oxide truly does generate free radicals as he suspects.
He advised people to continue to take usual precautions in the sun.
"I still would advise people to wear sunscreen," he said.
"Sunscreen is better than no protection at all."
Dr Ma and his team studied how human lung cells immersed in a solution containing nano-particles 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 hours, half of the lung cells in the zinc oxide solution died. After 12 hours, 90 per cent 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 nano-particles 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.
Besides sunscreen, zinc oxide is used in many commercial products, including plastics, paints, ointments and sealants.
–Daily Mail, London