Thursday November 13 2014
The link between mobile phones and cancer is not proven
"Do mobile and cordless phones raise the risk of brain cancer?" asks the Mail Online.
There are now more mobile phones than people in the UK, so you would expect the commonsense answer to be a resounding "no". But, as we never get tired of saying, it's a bit more complicated than that.
The Mail Online reports on the latest study looking for evidence of a link between mobile and cordless phone calls and brain tumours. This large Swedish study found more than 25 years' use of mobile phones trebled the (very small) risk of glioma, the most common type of brain tumour.
The study matched healthy volunteers with people who had been diagnosed with a glioma, and asked them to estimate the amount of time they had ever spent using mobile and cordless phones. This ranged from less than one year to around 25 years.
The researchers found:
- any mobile phone use increased the risk of glioma by a third
- using 2G phones for 15 to 20 years doubled the risk
- 3G phone (smartphones) use for 5 to 10 years gave four times the risk (the research was carried out before the launch of 4G phones)
However, some of these results were based on very small numbers and so may not be reliable. And this type of study cannot prove that mobile phones cause brain tumours.
It has not taken into account other factors, including exposure to chemicals or occupational hazards, despite collecting this information. Even so, it could not account for every possible confounder.
It is also rather unlikely that the estimates for the extent of mobile phone usage are accurate. So, it remains unclear whether there are long-term cancer risks associated with mobile phone use.
Where did the story come from?
The study was carried out by researchers from the University Hospital in Örebro, Sweden and was funded by Cancer- och Allergifonden, Cancerhjälpen, the Pandora-Foundation for Independent Research, and the Berlin and Kone Foundation, Helsinki, Finland.
It was published in the peer-reviewed medical journal Pathophysiology, and appears to be available on an open-access basis.
The Mail Online has reported the story reasonably accurately, and put the findings into context, citing a previous large study looking at the risk of mobile phone use and brain cancer.
What kind of research was this?
This was a case-control study that aimed to see if there was an association between mobile phone use and the development of a type of brain tumour called glioma.
In this study, cases (people who have a glioma) were matched with controls (people of the same age without brain tumours). The researchers then looked at a variety of factors that each group had been exposed to.
This is a type of epidemiological study, which can identify potential risk factors for developing a brain tumour. However, this kind of study cannot prove that any of these factors directly caused the brain tumour.
What did the research involve?
The researchers contacted all adults aged 20 to 80 who were newly diagnosed with a brain tumour in central Sweden from 1997-2003, and all cases throughout Sweden aged 18 to 75 from 2007-09.
They recruited 1,498 (89%) people – 879 men and 619 women. The majority (1,380) had a glioma. The researchers matched each case by age and gender at random using the Swedish Population Registry to obtain a control group of 3,530 people.
A questionnaire was sent to all cases and controls to determine their exposure to mobile phones and cordless desktop phones. As mobile phones have changed during this timescale, the type of mobile phone exposure was recorded, including:
- first generation – output power 1 Watt, 900 MHz
- second (2G) generation – pulsed output power of tens of microWatts (mW), 900 or 1800 MHz
- third generation (3G) – output power tens of mW, amplitude modulated
The questions asked about:
- preferred ear for using a mobile or cordless phone
- number of years of exposure and average daily use
- overall working history
- exposure to different chemicals
- smoking habits
- X-ray exposure to the head and neck
- hereditary traits for cancer
If any of the answers were unclear, a follow-up telephone interview was conducted by someone who was not informed if the person was a case or a control.
The researchers performed statistical analyses to take socioeconomic status into account.
What were the basic results?
Any mobile phone use increased the risk of glioma by a third (odds ratio [OR] 1.3, 95% confidence interval [CI] 1.1 to 1.6).
More than 25 years' use of mobile phones trebled the risk of glioma (OR 3.0, 95% CI 1.7 to 5.2). This was based on 29 cases and 33 controls.
For the longest possible periods of use of newer mobile phones:
- 2G phone use for 15 to 20 years doubled the risk of glioma (OR 2.1, 95% CI 1.5 to 3.0)
- 3G phone use for 5 to 10 years gave four times the risk of glioma (OR 4.1, 95% CI 1.3 to 12) – this was based on 12 cases and 14 controls
Use of cordless phones also increased the risk (OR 1.4, 95% CI 1.1 to 1.7), with the greatest risk seen in people who had used cordless phones for 15 to 20 years (OR 1.7, 95% CI 1.1 to 2.5). This was based on 50 cases and 109 controls.
The odds of glioma increased significantly for every 100 hours of use and for every year of use.
First using a mobile or cordless phone before the age of 20 increased the odds of glioma more than first use at older ages.
How did the researchers interpret the results?
The authors report that this study gives further support to their previous research, in which they concluded that gliomas "are caused by RF-EMF [radiofrequency electromagnetic field] emissions from wireless phones, and thus regarded as carcinogenic, under Group 1 according to the IARC [International Agency on Research on Cancer] classification, indicating that current guidelines for exposure should be urgently revised".
This case-control study found mobile phone use is associated with an increased risk of the commonest type of brain tumour, glioma. But this type of study cannot prove that mobile phone use caused the brain tumours, as it cannot account for confounding factors.
Indeed, despite collecting data on variables such as exposure to chemicals and occupation, this information was not taken into account during the statistical analyses.
A further limitation of the study was that the extent of mobile phone use was estimated retrospectively up to a 25-year time period.
It is highly unlikely these estimates would be accurate because of factors such as memory recall, and patterns of mobile phone usage have changed substantially over the years.
There is also the possibility of cases having recall bias after receiving a brain cancer diagnosis and therefore overestimated their mobile usage.
Additionally, many of the calculations were based on very small numbers, which reduces the reliability of the findings.
This study does not prove that mobile phones cause brain cancer, and the long-term effects of mobile phone use remain unclear.
What is clear is that brain tumours are relatively uncommon. While this is a good thing, it means that "proving" what, if any, environmental factors cause them is likely to require a great deal of long-term research effort.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.