The sun can induce cancerous DNA damage in skin cells even after it sets, according to a new study (Science 2015, DOI: 10.1126/science.1256022).

An international team of scientists has found that radical species generated by ultraviolet light can cause damage to DNA long after irradiation stops. They do this with the aid of compounds derived from melanin, a pigment known to shield mammals from burns and other harmful UV effects. The melanin-assisted process creates lesions known as cyclobutane pyrimidine dimers in DNA, which can lead to mutations that cause melanoma, a type of skin cancer.

These lesions are identical to the well-known genetic alterations caused when UV light directly strikes DNA. “But at least half of the dimers you get come in the dark, rather than during your trip to the beach,” says Douglas E. Brash of Yale University, who led the new study.

These chemically mediated lesions can take hours to evolve, Brash explains, whereas direct DNA damage occurs within picoseconds of exposure. Using techniques including mass spectrometry and high-performance liquid chromatography, the researchers identified a previously unrecognized process that could play a significant role in melanomas.

UV light stimulates enzymes that produce superoxide and nitric oxide, Brash says. These radicals react to create peroxynitrite, which can oxidize and fragment melanin polymers. Peroxynitrite further reacts with these fragments, creating unstable compounds. These molecules degrade into electrically excited species able to dump their energy into DNA and damage it.

“There’s no other way to explain how you get the damage after you turn the light off,” says John-Stephen Taylor, a chemist at the University of Washington in St. Louis, who was not involved in the study. In a written perspective, Taylor calls the new report a “brilliant piece of detective work” (Science 2015, DOI: 10.1126/science.aaa6578).

The researchers hope their work will help create topical treatments or “evening after” sunscreens to block this chemical damage, Brash says. In the meantime, people can protect themselves by wearing sunscreen and staying out of tanning beds.

“Basically, don’t fry yourself,” he suggests.

This article is reproduced with permission from Chemical & Engineering News (© American Chemical Society). The article was first published on February 19, 2015.