According to scientists at Missouri University of Science and Technology,
zinc oxide can undergo a chemical reaction when exposed to UV rays for a prolonged period of time. Free radicals, unstable molecules linked to certain types of cancer, can be released during this reaction.
"Zinc oxide may generate free radicals when exposed to UV (ultraviolet) sunlight," explains Dr. Yinfa Ma, Curators' Teaching Professor of chemistry at Missouri S&T. "Those free radicals can kill cells."
In a laboratory setting, lung cells were placed in a solution of zinc oxide and subjected to UVA light. After three hours, half the cells died. After twelve hours, a staggering 90 percent had died.
Dr. Ma admits that further study is needed to better understand the chemistry at work, as well as the potential health risks. After all, the cells used in the study came from lungs, not skin. In the meantime, Ma still thinks people should wear sunscreen.
"I still would advise people to wear sunscreen," he says
. "Sunscreen is better than no protection at all."
Zinc oxide is often used in sunscreen to "scatter" the rays, reflecting them off the body. In addition to being used in sunblock, it is also found in certain paints and plastics, as well as topical ointments.