Sunshine is not the main cause of the most dangerous form of skin cancer, according to researchers, who say some warnings about the perils of sunbathing are scaring people unnecessarily.

Scientists, whose research appears in the current issue of the journal Nature Genetics, claim the number of moles on your skin is the most important factor in the risk of getting melanoma. This reignites the debate over whether official health warnings about avoiding the sun are overstated.

The authors of the research paper maintain sunshine causes only a small proportion of melanoma cases. They believe health warnings would be more useful if they focused on people who have more than 100 moles, and taught them to check regularly the moles for changes in shape, size or colour.

Melanoma can be treated – for instance by the early removal of a suspicious mole – but it is the most serious type of skin cancer, as it can spread to other organs in the body. The cancer can start in an existing mole or on normal-looking skin, and can occur in people who have no moles but have fair skin and freckles.

Melanoma is relatively rare, accounting for 10% of all skin cancer cases, but it is responsible for most skin-cancer deaths. About 1,500 people die every year in England and Wales of malignant melanoma.

An international team of researchers from Queensland, Australia, Montreal, Canada and Philadelphia, America, led by King’s College London, identified two genes which dictate how many moles someone will have, and their risk of getting skin cancer. Tim Spector, professor of genetic epidemiology at King’s College London, said: “The number of moles you have is one of the strongest risk factors for melanoma – stronger than sunshine. This paper shows that we found two important genes that control the number of moles you have. Those genes also give you an extra risk of melanoma.”

Dr Veronique Bataille, a researcher at King’s College, London, and dermatologist at West Hertfordshire NHS Trust, argues that we have overemphasised the risk of sun exposure. Bataille said: “As a dermatologist working in the melanoma field for nearly 20 years, I feel quite strongly that there is always an overemphasis on sunshine. You often read that nearly all melanomas are caused by sunshine – which is not supported by the evidence. The more research we do, the more we realise that sunshine is a small part of the puzzle.

“In any population you study across the world, if you are ‘moley’ it is a very steady risk factor for melanoma, and it doesn’t make any difference whether you live in Glasgow or Sydney or LA.”

Bataille added: “Let’s keep sunshine in the picture because it does make you age and causes you wrinkles – we have never denied that. But let’s move away from scaring people by saying they are going to die because they go in the sun.”

Sara Hiom, Cancer Research UK’s director of health information, said: “This study confirms Cancer Research UK’s advice that people with lots of moles – as well as those with red hair or fair skin – are more at risk of the most dangerous form of skin cancer and should take extra care in the sun.

“The research does not in any way contradict the bulk of scientific evidence, which shows that most skin cancers are caused by overexposure to ultraviolet [UV] rays.”

She points out that malignant melanoma rates in the UK have more than quadrupled in the past 30 years – coinciding with a rise in “sun holidays” and the increased availability of artificial UV.

The scientists partly attribute the rising incidence of skin cancer to better screening picking up borderline changes in the skin that were unlikely to cause harm.

The worldwide incidence of melanoma in populations of European descent has risen rapidly over the past 30 years, more so than any other cancer. Globally, the World Health Organisation estimates 132,000 new cases of melanoma per year.

Discussing their work, the researchers even claim our “obsession” with the dangers of exposure to the sun has contributed to serious vitamin D deficiency. A shortage of sunlight – the most important source of vitamin D – can lead to an increase in deaths from other cancers, osteoporosis, depression and premature ageing.

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