"It's not safe to rely on sunscreens," said Marianne Berwick, an epidemiologist at the Memorial Sloane-Kettering Cancer Centre in New York. "Your genetic disposition to these cancers is the most important factor." A large, population- based study she had done found the melanoma risk for people with many moles was six times higher than that of someone with only a few. The risk for someone with the type of colouring which goes with greatest sensitivity to sunshine - red or fair hair, light-coloured eyes - was almost six times that for a Caucasian with dark hair and eyes.
Lotions certainly stop burning but do not appear to stop melanomas. For a white person in northern Europe, the lifetime risk of contracting one of these cancers is around one in 100. About a fifth of victims are killed by them. They are rare and more lethal than the two other types, known as non-melanoma. The main cause of all three is ultra-violet rays in sunshine, which damage DNA.
Dr Berwick examined 10 epidemiological studies into melanoma and sunscreen use. Six indicated that the more sunscreen one used, the more likely one was to develop the cancer. Two suggested the opposite conclusion, and the other two showed no link at all.
She concluded there were so many problems and "confounding factors" with such studies that it was unsafe to draw any conclusions. For instance, people who burnt easily might use lotions to stay out in the sun for longer - thereby preventing the sunburn which would otherwise have made them get out of the sun. But this would mean exposing themselves to more than they should, and running a higher risk of melanoma.
Oxford University chemist John Noland said he had grave doubts about an ultra-violet blocking ingredient used in a few lotions, called Padimate- O, or octyl-dimethyl PAB. When sunlight is shone on it, it releases chemicals which can damage DNA. He found this to be the case for isolated DNA and for the DNA inside skin cells when these are cultured in a test tube. He believes it is possible that when the chemical is rubbed over the skin, some could penetrate the cells.
"I would not use a sunscreen containing this chemical," he said.
"Unfortunately, EU regulations do not require manufacturers to state that it is an ingredient in writing on the bottle, but this should be a requirement." He said some products state they are "PAB- free", some admit to containing it and many give consumers no hint. The experts say people should use a lotion which claims to block both ultra-violet B and ultra-violet A radiation. But the best advice is to limit your exposure to strong sunshine, especially if you have sensitive skin.
Professor Jouni Uitto, of Philadelphia's Thomas Jefferson University, told the meeting his research group had genetically engineered a breed of mouse which can give insights into how sunlight wrinkles and ages human skin.
The mice contain the human gene which switches on the production of elastin, a key structural protein which gives tissue their springiness. As the skin ages, the normal arrangement of orderly elastin fibres is transformed into large, haphazard clumps, causing the skin surface to become leathery and wrinkled. He has shown ultra-violet radiation activates this gene in the mouth, leading to large-scale elastin production.