Sun-loving naturists, look away now: if you're soaking up the rays this summer with the aim of getting a perfect all-over tan, you're likely to be disappointed.
Different parts of the body go brown at different speeds, according to researchers at Edinburgh University, so achieving that idealised image of beauty is not going to happen.
Academics funded by the Medical Research Council say their findings explain why certain people find it difficult to get an even, consistent tan. The main problem, it seems, is people's bottoms, which take a lot longer to go brown than other parts of their anatomy.
Ninety-eight volunteers were given six doses of ultraviolet radiation on their backs and their bottoms from a tanning light, to see whether the effect was the same in both placesWhen the researchers examined the participants a week later, once the resulting redness had disappeared, they found that their backs had turned significantly browner than their buttocks.
"The research shows that instead of thinking that you have got one skin, or one skin type, in fact each of us has lots of different skin regions, each of which responds differently to UV light and so take longer than others to go red and then tan," said Jonathan Rees, a professor of dermatology, who led the study.
"If you shine sunshine on different parts of your body, the difference in how red they go varies by a factor of five for redness and two for tanning, depending on the body site."
The bum is not the only obstacle, though. Previous research by Rees has confirmed what sun worshippers already knew: that the upper back is much more likely to tan than the legs, and that the outsides of the arms go brown far quicker than the insides.
"What is burning for one body site is not for another. And the degree of UV protection that develops following ultraviolet radiation exposure may vary site by site," Reed added.
The study also found that those who had no freckles on their skin tanned more easily than those with freckles.
Rees and his team looked into why different types of skin cancer occur in different parts of the body. Usually they develop on ears, faces and the backs of hands and, among balding men, on the top of the head, rather than on the limbs. Melanoma, the less common but more deadly form, women often get on their calves but men on the shoulders. Different skin thicknessess may explain these differences, Rees said.
The findings show that most advice about how long it is safe to spend in the sun is worthless, because different body parts are more sensitive to the sun's damaging effects than others, he said.
