It is already known that without proper protection, sunlight can cause major damage to the skin. Now scientists have discovered that a lot of this damage can occur hours after sun exposure.

According to US scientists, exposure to ultraviolet radiation (UV), either from the sun or tanning beds, damages the DNA in melanocytes. These are the cells that make melanin - the pigment that gives skin its colour.

This type of damage is known to be a major cause of skin cancer, which is the most common type of cancer in Ireland. Around 9,000 new cases are detected every year.

Until now, it was thought that melanin protected the skin by blocking out harmful UV light. However some studies also indicated that melanin may be linked with cell damage in the skin as well.

The scientists from Yale University appear to have discovered how this works. They found that UV light causes a type of DNA damage known as cyclobutane dimer (CPD). However, they were surprised to discover that the body's melanocytes not only generated these CPDs immediately when exposed to UV light, this process continued hours after the UV exposure had ended.

In other words, melanin appears to have both protective and carcinogenic effects.

"If you look inside adult skin, melanin does protect against CPDs. It does act as a shield. But it is doing both good and bad things," noted Prof Douglas Brash of the Yale School of Medicine.

The scientists also discovered why this happens. UV light appears to activate two enzymes in the body, which when combined, ‘excite' an electron in melanin. The energy created from this process is known as chemiexcitation and it is transferred to DNA in the dark.

In other words, it is creating the same DNA damage in the dark that UV light caused in the daytime.

Chemiexcitation has only previously been detected in certain plants and animals.

Details of these findings are published in the journal, Science.