Much of the damage that ultraviolet radiation (UV) does to skin occurs hours after sun exposure, and this delayed reaction appears to be due to the skin pigment melanin, a U.S. study said Thursday.

Melanin, which gives skin its color, is usually thought of as a protective pigment, blocking the UV radiation that damages DNA and contributes to skin cancer.

But there was also evidence from studies suggesting that melanin was associated with skin cell damage.

In the new study, Douglas Brash, professor of the Yale University, and his colleagues first exposed mouse and human melanocytes, the cells that make the melanin, to radiation from a UV lamp.

The radiation caused a type of DNA damage known as a cyclobutane dimer (CPD), in which two DNA "letters" attach and bend the DNA, preventing the information it contains from being read correctly.

The researchers were surprised to find that the melanocytes not only generated CPDs immediately but continued to do so more than three hours after UV exposure ended. Cells without melanin generated CPDs only during the UV exposure.

The results, published in the U.S. journal Science, showed that melanin had both carcinogenic and protective effects.

"If you look inside adult skin, melanin does protect against CPDs. It does act as a shield," said Brash. "But it is doing both good and bad things."

The researchers next tested the extent of damage that occurred after sun exposure by preventing normal DNA repair in mouse samples. They found that half of the CPDs in melanocytes were " dark CPDs" -- CPDs created in the dark.

Further research showed that the UV light activated two enzymes that combined to "excite" an electron in melanin, and that the energy generated from this process, known as chemiexcitation, in turn, causes the same DNA damage that sunlight caused in daytime.

Chemiexcitation has previously been seen only in lower plants and animals.

While noting that news of the carcinogenic effect of melanin is disconcerting, the researchers also said that the slowness of chemiexcitation may allow time for new preventive tools, such as an "evening-after" sunscreen designed to block the energy transfer.