If you have more than ten moles on your right arm, you could be at a higher risk of skin cancer, doctors have warned.
People who have at least 11 moles on their arm are highly likely to have more than 100 across their body, British research found.
Although most moles are benign, they can turn cancerous, and having more than 100 moles is known to raise the risk of malignant melanoma, the most deadly form of skin cancer, up to tenfold.
Fuelled by the rise of tanning salons and package holidays to sunny locations, melanomas are now five times more common than in the 1970s and claim more than 2,000 lives a year.
Fair skin, red hair and sunburn are known to increase the risk – but the researchers, from King’s College London, say a person’s mole count is much more important.
They say that if doctors can quickly work out who is a ‘moley person’, more cases of the disease could be caught earlier, when it is easier to treat.
Nurses from St Thomas’ Hospital in London counted the number of moles on more than 3,500 healthy women. They divided the body into 17 areas to see which parts were indicative of overall mole count. Men were included in a second study.
The arm was judged to be a particularly good proxy, with men and women who had more than ten moles there being highly likely to have more than 100 overall.
Legs were also a good indicator for women and backs for men, but the arm was chosen because it is easier for GPs to examine. Either arm provides a good indication but the right was chosen to provide a memorable message.
Having lots of moles is a sign your skin cells are very active, which raises the odds of some cells tipping over into cancer. Dermatologist Veronique Bataille hopes the research, published in the British Journal of Dermatology, will help GPs diagnose skin cancer earlier.
She added that patients with moles should keep an eye out for changes in shape or colour, which can be a sign a mole is cancerous.
Dr Claire Knight, of charity Cancer Research UK, added: ‘This study suggests that the number of moles on our arms gives a good indication of how many moles we have on our bodies. This could be helpful because we know that people with lots of moles have a higher risk of melanoma.
‘Other risk factors for melanoma include having red or fair hair, fair skin, light-coloured eyes or having been sunburnt in the past.
‘But less than half of melanomas develop from existing moles. So it's important to know what's normal for your skin and to tell your doctor about any change in the size, shape, colour or feel of a mole or a normal patch of skin.
‘And don't just look at your arms - melanoma can develop anywhere on the body, and is most common on the trunk in men and the legs in women.’
Most people have 20 to 30 moles but it’s not unheard of to have 400 to 500. But they could have their advantages.
Previous research by the same team has suggested that people with lots of moles are biologically younger than those with mark-free skin.
It is thought that moles are linked to the body’s biological clock, with those with more than 100 having bodies that are up to seven years younger than those with less than a quarter of the number.
This could explain why nature has allowed moles to persist, despite their link to skin cancer.