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Sunbeds and skin cancer – the evidence stacks up

by Jess Kirby | Analysis

10 June 2010

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Sunbeds increase the risk of skin cancer – the evidence has been overwhelming for some time, and that’s why we supported the new law to protect children from them. The International Agency for Research on Cancer (IARC, part of the World Health Organisation), who provide gold-standard judgments on the state of the evidence about all sorts of things that could cause cancer, recently upgraded sunbeds to the highest risk category, to say that there’s ‘sufficient’ evidence they increase cancer risk.

Now, a new paper – one of the strongest so far – adds to this evidence and directly addresses some of the weaknesses of previous studies.

The evidence

In 2006, IARC reviewed all the existing evidence on sunbeds and cancer and concluded that if people had first used a sunbed before the age of 35, the risk of melanoma was raised by 75 per cent. But the review didn’t find strong evidence for a ‘dose-response’ effect, where the heaviest users have higher risks than light users. This is important, because with most things that could cause cancer, the higher the exposure, the greater the likelihood of developing the disease.

The new study, published in the journal Cancer Epidemiology, Biomarkers and Prevention, deals with this problem. The researchers, based at the University of Minnesota, looked at over 2,200 people with and without skin cancer. They found out about their history of using sunbeds, asking them whether they had used them, how often, and what type they used. They also found out all about their lifestyles, including asking about skin type, hair and eye colour, freckles and moles, whether they used sunscreen, and whether they had a family history of skin cancer. This helped the researchers to work out if it was the sunbeds causing any differences in risk, and not one of these other factors.

Higher risk

In some ways, the results weren’t that surprising. There was a 74 per cent higher risk of melanoma among people who had ever used a sunbed, in comparison with those who hadn’t (that’s odds of 1.74 to 1) – which is consistent with the kind of risk that other studies have shown.

But where this study really makes its mark is that the heaviest sunbed users, no matter how that’s measured – by amount of time, number of sessions or number of years – had the highest risks of melanoma. And this trend held out across lower exposures too – the more people used sunbeds, the higher their risk was.

Even if people hadn’t burned on a sunbed, they still had a 59 per cent higher melanoma risk (that’s odds of 1.59 to 1). Getting burned while using a sunbed was “fairly common” in this study, and if people did burn, their risk was more than doubled. Again, the more often people had burned, the higher their risk.

While previous studies have found that the biggest risks of melanoma were in people who started using sunbeds young, this study found that the overall, or ‘cumulative’ amount people were exposed to was most important, and that exposure at any age was still dangerous. So, as the authors write, “early age exposure is most likely a marker for cumulative exposure… not that younger individuals are at increased susceptibility to the effects of UV radiation”. That’s to say, when people started using sunbeds at younger ages, they were likely to spend more time on a sunbed during their lives, and that was what increased their risk.

And looking at whether different types of tanning device could be more or less dangerous in terms of cancer risk, they found increased melanoma risks across all types of device, and across all time periods, allowing them to see any changes as the technical specifications of sunbeds changed over time. This led the researchers to remark that “no device could be considered ‘safe’”.

Remembering past exposures

There is one problem with using this kind of “case-control” study, where researchers compare people who already have cancer with healthy people. These studies rely on finding out about a person’s history and it can be difficult to accurately remember what someone did many years ago. So people may under- or over-estimate how much they used sunbeds. Also, if people have previously heard about a connection, for example in the media, it might unconsciously sway their answers. It’s a common problem, which is called recall bias.

One study, carried out in Norway, aimed to find out how much of a problem this was for melanoma risk factors, by asking people about their behaviour both before and after they were diagnosed with melanoma, and looking for a difference between the two reports. They didn’t find a consistent pattern, with more evidence of recall bias for certain risk factors, and not so much for other ones. And sunbed use was one factor where they didn’t find much evidence of recall bias. Nonetheless, it’s a possibility, and something we should bear in mind when looking at the results.

Adding to the pile of evidence

This study stacks even more evidence onto the already large pile linking sunbed use to a higher risk of melanoma. And it addresses a lot of these previous studies’ limitations.

As the researchers so succinctly say, “our results add considerable weight to the IARC report that indoor tanning is carcinogenic in humans and should be avoided to reduce the risk of melanoma”.