Although the cells were outwardly normal, the mutations that occurred could be the first stages of cancer.

The researchers, led by Peter Campbell, a cancer geneticist at the Wellcome Trust Sanger Institute in England, examined the eyelid skin of four middle-aged adults - three were Western European and one was of South Asian descent - and found that hundreds of ostensibly normal cells had mutations linked to cancer, a number "way higher than we'd expect", Campbell said. Clusters of these mutant cells, called clones, appeared in every 0.1 square inch of skin, with thousands of DNA mutations in each cell.

Although it is not known if the same rate of mutations occurs in sun-exposed skin elsewhere on the body, or in people of different ethnic backgrounds, or even how many of the mutations would progress to cancer, it is not a finding to dismiss lightly.

Douglas Brash, a biophysicist at Yale University School of Medicine who has studied ultraviolet damage to cells for more than 40 years and wrote a commentary on the British study, described the new findings as "a canary in a coal mine" and a warning to take the effects of ultraviolet radiation, whether from sunlight or tanning beds, more seriously.

It is especially important, he said, "to be very conscientious about protecting young children", who are more susceptible than teenagers and adults to ultraviolet-induced mutations.

"A lot of damage occurs when people go to the beach," Brash said.

"While the body does a great job of repairing the damage and gets 99.9 per cent of things right, every once in a while, you do get a mutation that may make a cell resistant to death, allowing it to form a clone that can become a cancer."

Complicating the matter is that many people don't understand the meaning of the SPF rating listed on modern sunscreens - only 43 per cent of 114 people surveyed at a dermatology clinic in Illinois, in the US last summer. The rating, which stands for sun protection factor, is meant to reflect how well a product protects against sunburn, which should also reduce the risk of skin cancer and sun-induced skin ageing. And less than a third of people use sunscreen regularly, according to the US Centers for Disease Control and Prevention.

Dermatologists have long cautioned people not to rely too heavily on sunscreens, even products with the recommended SPF rating of 30 or higher. Few people who do use sunscreen use enough to achieve the level of protection measured in the laboratory, and few reapply it often enough. A better plan is to stay out of the sun, especially around midday, and cover the skin when sun exposure is unavoidable.

In its latest analysis of 1000 sunscreens, the US Environmental Working Group rated sunscreens made with zinc oxide or titanium oxide high because "they provide strong sun protection with few health concerns" and "they don't break down in the sun". These substances are also more protective than other sunscreen ingredients against UVA rays.

Some consumers erroneously believe that choosing a more expensive sunscreen with a sky-high SPF number like 70 or 100 will provide complete protection. However, the Food and Drug Administration has not determined that an SPF of more than 50 has any added benefits.

New York Times