Sunlight continues to damage people’s skin and increase the risk of cancer for hours after they leave the beach and head indoors, a study shows.
Scientists at Yale University, in the US, discovered it was the supposedly protective pigment melanin that was causing the damage.
The team say the findings may lead to better sunscreens that can prevent the extra damage.
British experts said that the findings “reinforce” current advice.
When UV radiation pummels our skin cells, it can cause mutations in the DNA.
Melanin, the pigment behind a tan or natural skin tone, is the body’s defence as it absorbs the radiation.
What scientists did not know previously was what happens to all the energy that the melanin has absorbed.
The Yale team showed, in the journal Science, that the high-energy version of melanin supercharges a series of chemical reactions.
A cocktail of superoxides and peroxynitrites culminate in a “very high-energy molecule breaking apart and releasing the energy that was holding it together”, said lead researcher Prof Douglas Brash.
He told the BBC News website: “It’s what happens in fireflies when they [use the energy to] produce light and glow, except the energy is just transferred to the DNA.”
In laboratory tests, the whole damage in skin was still taking place four hours after UV exposure was stopped.
“Half or more of this kind of DNA damage is not happening on the beach, it’s on the car on the way home,” Prof Brash said.
The team hope they can develop a sunscreen that combines the usual protection with absorbing any energy from the melanin.
Dr Bav Shergill, of the British Association of Dermatologists, said: “This research serves to reinforce current advice on sun protection, which is something I welcome.”
He said that sunscreen should be used with a sun protection factor (SPF) of at least 30 and good UVA protection, and that people should stay out of the sun between 1100 and 1500.
He added: “The researchers note that the time it takes between sun exposure and the damage being completely done gives a window of opportunity in which new preventative tools could work.
“This is an interesting concept. Whilst this does open future avenues for treatments, that could be a long way off. So in the meantime the public should focus on traditional sun protection methods.”