The DNA mutations that cause skin cancer may occur hours after sunbathers leave the tanning salon or beach, according to a new study. The findings suggest that some of the deadliest skin cancers may be prevented by an “evening-after” sunscreen that stops deadly mutations in their tracks. Skin cancer affects 1 in 5 Americans. Its most aggressive form, melanoma, claims at least one life every hour. Scientists have long suspected that ultraviolet radiation from sunlight and tanning beds can set off a chain reaction within human cells, mutating DNA and giving rise to deadly tumors. But the entire process supposedly occurs within a millionth of a millionth of a second. With that kind of time frame, the best prevention seems to be sunscreen, which can reduce UV exposure in the first place. Now researchers have found that some of the key mutations behind melanoma can happen even hours after human cells are exposed to UV rays in the lab. Sunlight appears to set off a much longer chain reaction—an electron bounces from proteins to pigments and eventually smacks into melanin, which gives skin cells their color (and melanoma its name). This DNA-mutating process can take several hours, the researchers say, which means that a special sunscreen may be able to jump in after a day at the beach and break the chain before it causes irreparable damage. Prior to this study, no one had ever observed this sort of molecular chain reaction in human cells. The general process (known as chemiexcitation) routinely occurs in plants and gives fireflies their glow. But this is the first time that it has been observed in advanced mammals.