A major active ingredient in many sunscreens damages DNA when exposed to sunlight in a test tube, according to scientists in Northern Ireland. They say that if similar damage occurs within skin cells, it could destroy them or possibly initiate changes leading to skin cancer.

The research is presented in the web edition of the peer-reviewed journal Chemical Research in Toxicology, which is published by the American Chemical Society, the world's largest scientific society. It will appear in the journal's Jan. 18 print edition.

The ingredient in question is PBSA, which is commonly used in sunscreens on sale in the U.S. and Europe. It protects skin by strongly absorbing harmful high- energy UV-B wavelength light. In the process PBSA becomes energized and, in principle, capable of damaging adjacent skin tissue, according to the scientists.

In experiments carried out with DNA outside of cells, the researchers say the light-exposed PBSA damaged the genetic material's guanine base sites. "If such damage were to occur to DNA inside cells," says biochemist R. Jeremy H. Davies, Ph.D. of Queen's University in Belfast, "it could increase the risk of developing skin cancer."

While stressing that there is currently no evidence that PBSA actually enters human skin cells, Davies adds that "this new information regarding the photosensitizing properties of PBSA sounds a cautionary note: it may be safer to replace it with another ultraviolet filter that does not attack DNA."

Davies says his methods for testing PBSA -- which are outlined in the Chemical Research in Toxicology paper, co-authored with Queen's University colleague Clarke Stevenson could be used to evaluate other substances as well. He says such work can help to optimize the range and quality of sunscreens available.

In the meantime, Davies emphasizes his confidence in the safety and efficacy of existing sunscreens. "Almost certainly," he concludes, "the benefits associated with their use far outweigh the risks of short or long-term adverse reactions."

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A nonprofit organization with a membership of more than 155,000 chemists and chemical engineers, the American Chemical Society publishes scientific journals and databases, convenes major research conferences, and provides educational, science policy and career programs in chemistry. Its main offices are in Washington, D.C., and Columbus, Ohio.