Just when you thought you were safe from the scorch...turns out you’re still baking and you don’t even know it. According to a new Science study, that cancer-causing damage continues for hours after sun exposure ends. Surprisingly, this delayed reaction is linked to the skin pigment melanin, which we normally think of as protecting us from the burn. Yikes, time to slather on some after-hours sunblock.

Within less than a second after exposure, ultraviolet radiation can cause breaks in the DNA of cells called melanocytes, which make melanin. These lesions can lead to skin cancer-causing mutations. We used to think of melanin as a protective pigment that blocks UV radiation, but there’s also been some evidence suggesting that melanin is associated with skin cell damage.

So, Yale’s Douglas Brash and colleagues fried mouse and human melanocytes under a UV lamp. Lesions appeared in those melanin-producing cells immediately, and the damage continued for more than three hours after exposure to UVA radiation, the type that’s found in sunlight and tanning beds. In particular, this type of DNA damage is called cyclobutane dimer (CPD): That’s when two “letters” of DNA attach, bending the genetic material and preventing its information from being read correctly. Cells without melanin, on the other hand, generate CPDs only while being exposed to UV radiation. 

“If you look inside adult skin, melanin does protect against CPDs. It does act as a shield," Brash says in a news release. "But it is doing both good and bad things." It has protective as well as carcinogenic effects. And that shouldn’t happen, he adds.
 
Then the team tested the extent of damage caused by sun exposure by preventing routine DNA repair in mouse cells. Half of the CPDs in the melanocytes, they found, were those that were created in the dark (or in the shade or indoors), called “dark CPDs.” The UV light seemed to produce reactive oxygen and nitrogen that work together to excite an electron in melanin—a process called chemiexcitation. The energy that’s released causes the same damage to DNA in the dark as sunlight would outdoors in the daytime. 

The findings might spur preventive tools to quench the energy transfer. But until then, "I'd give people the same advice they've heard before," Brash tells The Washington Post. "Stay out of the sun between 10 and 2, wear a hat, wear sunscreen."