Here's a warning to sunbathers everywhere. Scientists have found that the skin damage caused by UV rays does not stop once you get out of the sun.
Researchers say that much of the potentially cancer-causing damage wrought by ultraviolet radiation from sunlight or tanning beds occurs up to three to four hours after exposure, thanks to chemical changes involving the pigment melanin.
But there is some good news. The researchers said it may be possible to develop sunscreen that protects against this type of damage. Melanoma, closely linked to UV exposure, accounts for most skin cancer deaths.
The role of melanin, responsible for our skin, eye and hair colour, in promoting DNA damage was a surprise because melanin was previously known to play a protective role by absorbing much of the UV energy before it penetrates the skin.
"But the unusual chemical properties of melanin that make it a good UV absorber also make it susceptible to other chemical reactions that just happen to have the same end result as the UV," said Douglas Brash, a therapeutic radiology and dermatology professor at the Yale School of Medicine whose study appears in the journal Science.
The researchers revealed this aspect of melanin in experiments involving human cells in a lab dish as well as lab mice and mouse cells in a dish.
Damage after UV exposure
UV exposure can cause DNA damage that may spur carcinogenic mutations in melanin-producing cells called melanocytes.
The researchers exposed mouse and human melanocytes to radiation from a UV lamp. The cells experienced DNA damage immediately but the damage also continued for hours. In fact, half of the damage occurred in the hours after exposure.
After a type of chemical reaction called chemiexcitation, also witnessed in bioluminescent creatures including fire flies, energy gets transferred to DNA to potentially cause mutations.
"People should be aware of the chemistry initiated in the skin after the UV exposure so that they can take proper care of themselves whenever going out in the sun or to the beach," said Yale School of Medicine researcher Sanjay Premi.
"We'd like to find new ingredients for sunscreens that will block these reactions," Brash added. "But in the meantime, I tell people to enjoy the sun but just don't lie on the beach between 10 a.m. and 2 p.m., and wear a hat. Sunscreens are useful, too, so long as they block both UVB and UVA," two kinds of ultraviolet rays.