Wednesday June 1 2011
The classification does not mean mobiles definitely cause cancer
Several newspapers have today reported that mobile phones may cause cancer, with the Daily Mail saying that after years of contradictory claims health chiefs have finally delivered ‘an authoritative verdict’ on the matter.
The news comes after the International Agency for Research on Cancer (IARC), a division of the World Health Organization (WHO), classified mobile phone use as a possible cause of cancer. After examining a body of evidence on mobile phone use, the IARC yesterday announced it will now classify mobile phone signals as ‘possibly carcinogenic’ due to some study results suggesting a link to some types of brain cancers.
However, the classification means that the link is far from certain, with the IARC saying there is only ‘limited evidence’ of a link to brain tumours in humans, and that the results supporting a link may be due to other factors distorting study data. The IARC also said there is inadequate evidence to support a link to other cancer types.
Overall, this classification should not be taken to mean that there is a definite link between mobile phone use and cancer, only that some initial (possibly anomalous) study results have highlighted a relationship that needs further robust scientific investigation.
How are mobile phones now classified?
The IARC classifies different substances and exposures according to whether they are likely to cause cancer. The IARC had classified mobile phones as belonging to Group 2B on their scale, which means there is a possibility they cause cancer in humans.
Within the IARC scale, there are five categories of risk:
- Group 1: there’s extremely strong evidence that an agent causes cancer. Smoking and asbestos are in this category.
- Group 2a: an agent is ‘probably carcinogenic to humans’. The evidence in animal studies is 'sufficient' but 'limited' in humans.
- Group 2b: an agent is ‘possibly carcinogenic to humans’. There is limited evidence in humans that it causes cancer and the evidence from animal studies is ‘less than sufficient’. This is the new classification for mobile phones. Cancer Research UK consider Group 2B to mean that, ‘there is some evidence for a risk but it’s not that convincing’.
- Group 3: an agent is ‘not classifiable as to its carcinogencity to humans’. This means that the evidence is inadequate and limited in humans and animals.
- Group 4: an agent is probably not carcinogenic to humans.
What has prompted the classification?
The classification was based on an assessment of the potential carcinogenic hazards from mobile phones made at IARC meetings in May 2011. These were attended by a working group of 31 scientists from 14 countries that discussed and evaluated the evidence on any potential association between cancer and exposure to mobile phones, as well as other radio-frequency electromagnetic fields. This included examining the evidence for occupational exposure to radar and microwaves and environmental exposure associated with transmission of radio and TV signals.
They found that there was ‘limited evidence’ of an association between mobile phones and two types of brain cancer, glioma and acoustic neuroma. The IARC classified limited evidence as when there is a credible interpretation for an observed association between an exposure and cancer but that chance, bias or confounding cannot be ruled out with reasonable confidence.
For other types of cancers the IARC reported the available evidence as too ‘inadequate’ to draw any conclusions from, meaning that available human studies are of insufficient quality, consistency or statistical power to permit a conclusion, or that there are no studies in humans available.
Dr Jonathan Samet, chair of the IARC working group, commented, 'The conclusion means there could be some risk and therefore we need to keep a close watch for a link between cell phones and cancer risk.'
It is important that additional research be conducted into the long-term, heavy use of mobile phones, he added.
How big could the potential risk be?
The group did not quantify the potential risk but said that one study of past mobile phone use showed a 40% increased risk for glioma brain tumours among heavy users (with a reported average of 30 minutes daily over a 10-year period).
To put this 40% risk increase into context, the latest incidence figures from Cancer Research UK indicate that a man has a lifetime risk of developing a brain tumour (any type) of 1 in 133, and women have a 1 in 185 risk. Gliomas (of which there are four subtypes) are said to account for about half of all brain tumours. Therefore, a 40% increase in risk would be on the top of a relatively low baseline risk that any person has of developing a brain tumour.
So do mobile phones definitely cause cancer?
No. The IARC’s classification means there is some evidence linking mobile phones to some types of brain cancer but that this evidence is too weak to draw strong conclusions.
Experts point out that there have been a relatively small number of studies on mobile phones and cancer. Most of these are case-control studies. They compare people who already have cancer (cases) with healthy people (controls), and ask them about how they used their phones in the past.
So far, only one study (in around 420,000 Danish people) has actually used the preferable method of following a group of healthy people in the long term to see if their use of mobile phones affected their future risk of cancer. This study found no evidence for an association between tumour risk and mobile phone use among either short-term or long-term users.
Cancer Research UK said that while a small number of studies had found associations between mobile phones and brain cancer risk, most had found no evidence of a link between brain cancer (or any other type of cancer) and mobile phone use for at least 10 years.
In many of the studies results do not reach statistical significance. For example, only one study out of 14 looking at short-term use found that mobile phones significantly affect the risk of cancer. Pooled estimates, representing the combined results from numerous studies, suggested that mobile phones do not affect the risk of cancer.
Cancer Research also said that the studies conducted so far had several weaknesses that undermine their reliability. For example, mobile phone technology has changed considerably over the past few decades, and it is not clear if studies based on use of old models will also apply to new ones.
It’s also difficult to assess someone’s exposure to mobile phone radiation, and studies often rely on questionnaires asking participants to accurately remembering their mobile phone use over years or decades, which could undermine reliability.
Also, if mobile phones increase the risk of brain cancer, the rates of this disease should theoretically be skyrocketing since mobile phone use has risen dramatically over the last few decades, but studies have found no such trends. However, brain cancers can take many years to develop so it is possible that trends would only start rising after more time.
How might mobile phones cause cancer in the body?
So far experts are uncertain about the biological mechanisms by which mobile phones might increase the risk of cancer. Cancer Research UK point out that phones give off microwave radiation, but that the levels involved are millions of times less energy than, say, an X-ray, and are not thought powerful enough to damage our DNA.
How can I reduce my exposure to mobile phone radiation?
The WHO has advised that until further research is undertaken, people should try to reduce their mobile phone exposure by using hands-free devices or by texting instead.
The Department of Health says that although there is no immediate concern, current advice is that children and young people under 16 should be encouraged to use mobile phones for essential purposes only and to keep calls short. The body and nervous system is still developing in the teenage years and limiting mobile phone use is a precaution, it says.
For specific ways to reduce exposure see Health A-Z: advice on mobile phones.