Can Sunlight Continue To Damage Skin When It's Dark?

Most of the damage to skin from ultraviolet radiation happens within a few hours of sun or tanning bed exposure. But can skin damage continue after it becomes dark? 

A team of Yale-led researchers have determined that is the case. The study is published in the journal Science.

The researchers stated that much of the damage that ultraviolet radiation does to skin occurs hours after sun exposure. They explained that exposure to UV light damages the DNA in melanocytes the cells that make the melanin that gives skin its color. This damage is a major cause of skin cancer, the most common form of cancer in the United States.

In the past, experts believed that melanin protected the skin by blocking harmful UV light. But there was also evidence from studies suggesting that melanin was associated with skin cell damage.

For the study, mice and human melanocyte cells were exposed to radiation from a UV lamp.

The Findings 

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 melanocytes not only generated CPDs immediately but continued to do so hours after UV exposure ended. Cells without melanin generated CPDs only during the UV exposure.

This finding showed that melanin had both carcinogenic and protective effects.

"If you look inside adult skin, melanin does protect against CPDs," said Douglas E. Brash, clinical professor of therapeutic radiology and dermatology at Yale School of Medical, and one of the study co-authors. "It does act as a shield. But it is doing both good and bad things." 

Next, the researchers 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. It was discovered that the UV light activated two enzymes that combined to "excite" an electron in melanin. The energy generated from this process - known as chemiexcitation - was transferred to DNA in the dark, creating the same DNA damage that sunlight caused in daytime. Chemiexcitation has previously been seen only in lower plants and animals.

The researchers felt that news of the carcinogenic effect of melanin was disconcerting, but also pointed out that the slowness of chemiexcitation may allow time for new preventive tools, such as an "evening-after" sunscreen designed to block the energy transfer.

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