Scientists have found that the skin damage caused by UV rays does not stop once you get out of the sun. Researchers said on Thursday 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. The researchers said it might 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 UV-exposure experiments involving human cells in a lab dish as well as lab mice and mouse cells in a dish. The cells experienced DNA damage immediately but the damage also continued for hours.