The reason, according to a talk given yesterday at the American Chemical Society’s meeting in Washington, is that small differences in the chemistry of the skin pigments in people with different coloured hair affects the production of cancer-causing agents when the skin is exposed to ultra-violet light.

The risk of skin cancer is about two to four times higher for redheads and blonds than for those with dark hair. In 1995 there were 5,626 new cases of melanoma — the most dangerous form of skin cancer — in Britain, but by 2000 this had risen to 6,967. Each year about 1,700 people die from melanoma, the third most common cancer among people aged 15 to 39.

A group led by Professor John Simon of Duke University, North Carolina, isolated the melanin-containing structures, called melanosomes, from human hair and then measured the oxidation potentials of the different melanins. Oxidation potentials are a measure of how likely chemicals are to create highly active oxygen radicals by taking up electrons. Oxygen radicals, in turn, are known to be damaging to DNA, and DNA damage is the trigger for cancer. So if the team could find differences in the oxidation potentials of the melanins, it could explain differences in skin cancer risk.

To measure the potentials the team subjected the melanins to precisely tuned laser light in the right region of the ultraviolet spectrum. The results reported yesterday showed that black pigments were indeed less likely to produce oxygen radicals, and red pigments more likely to.

While the finding may explain the higher risk of redheads, it does not offer them any way to reduce it. They will simply have to take greater care not to become sunburnt.

If the red-haired gene carries this risk, why has it not died out? The answer is that it is indeed much more common in northerly climes, where it may also offer an advantage. One theory is that the red-haired melanins help the skin to create vitamin D in colder climates, thus reducing the risk of diseases such as rickets.

An alternative explanation is that the pale skin that often goes with red and strawberry blonde hair is so attractive that it helps women who possess it to get mates.

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