"Workers exposed to electromagnetic fields in their jobs could be at risk of developing motor neurone disease," the Daily Mail reports. A Dutch study found a link between occupational exposure to low-frequency magnetic fields and increased risk of dying from the most common type of motor neurone disease – amyotrophic lateral sclerosis (ALS).
ALS is a condition that causes progressive damage to the nervous system, resulting in widespread loss of bodily functions, and is usually fatal within a few years. It is a rare condition affecting around two in every 100,000 people in the UK – the causes of ALS are unclear.
It's been linked to a variety of work-related factors, including pesticides, solvents, metals such as lead and mercury, electrical shocks and, as in this study, extremely low frequency (ELF) magnetic fields. These fields are generated by electrical currents. People who work with appliances that use a lot of electricity – such as welders or electrical power supply installers – are exposed to these fields more than most.
The study looked for links between any of these work-related factors and the chances of getting ALS. They found men who'd had a job with high exposure to ELF magnetic fields had double the risk of ALS, compared to people who'd only had normal, background exposure. None of the other work-related factors showed a link to ALS.
While the increased risk of ALS was statistically significant, the overall risk of ALS was still tiny in the exposure group. Of the 58,279 men in the study only 88 died of ALS. With figures as low as these, there is always the risk than any identified link is actually down to chance.
Where did the story come from?
The study was carried out by researchers from Utrecht University and Maastricht University in the Netherlands and was funded by the Netherlands Organisation for Health Research. It was published in the peer-reviewed journal Occupational Environmental Medicine.
The Daily Mail and Daily Express ran very similar stories, quoting the same experts and the same statistics. Their stories give a broadly accurate overview of the study and its potential implications.
What kind of research was this?
This was a prospective cohort study. Cohort studies follow large groups of people to see what happens to them over time. While this can be a good way to spot patterns and links between factors, cohort studies cannot prove that one factor (such as extremely low frequency (ELF) magnetic fields) causes another (such as amyotrophic lateral sclerosis (ALS)).
What did the research involve?
Researchers used data from a cohort of 58,279 men and 62,573 women, originally designed to investigate cancer risk in people in the Netherlands. The group, aged 55 to 69, answered questions about the jobs they'd held, their education and lifestyle. They were tracked for around 17 years.
After the study ended, researchers compared information about the people who had died of ALS, with information about a randomly-chosen subgroup of 4,166 from the original group. They looked to see whether people who'd died of ALS were more likely to have been exposed to various occupation risk factors.
The factors studied were:
- electrical shocks
- ELF magnetic fields
The researchers used a job categorisation index to estimate people's exposures, rather than asking people directly. A job categorisation index is a statistical model which estimates the likely rates of exposure to certain factors based on an individual's occupation.
This modelling was intended to reduce biases, such as recall bias. This index was used to categorise people into ever having high exposure, only having low exposure, or only having background exposure. They also looked at the effect of total exposure – ie the intensity of exposure in a job, multiplied by the amount of time people had been in that job.
They adjusted their figures to take account of these potential confounding factors: smoking, body mass index (BMI), education level and physical activity. They carried out sensitivity analyses (a statistical technique used to try to account for uncertainties in data) to look for any effect of people without full data about their exposure, and to exclude people who'd never had paid work.
Results for men and women were studied separately. Very few women (less than 2%) had experienced high exposure to any of the workplace factors studied, so the researchers presented only the results for men.
What were the basic results?
Of the 58,279 men in the study, 88 died of ALS – of whom 76 had occupational data available for analysis.
- Men who'd ever had a job that exposed them to high levels of ELF magnetic fields were twice as likely to have died of ALS as people who'd only had background exposure (hazard ratio [HR] 2.19, 95% confidence interval [CI] 1.02 to 4.73).
- Men who'd had the highest exposure over time were almost twice as likely to have died of ALS than people who'd had no workplace exposure (HR 1.93, 95% CI 1.05 to 3.55).
None of the other workplace factors studied showed any increased risk.
How did the researchers interpret the results?
The researchers said their study: "offers further support for an association between occupational exposure to ELF magnetic fields and an increased risk of ALS mortality [ie death from ALS]."
They said the other factors studied "showed some weak non-significant associations" but no evidence that increased exposure led to increased risk.
The study found an increased risk of ALS for men with high exposure to ELF magnetic fields, but that doesn't mean magnetic fields are a direct cause of ALS. While figures such as a doubling of risk suggest a big increase, the overall risk of ALS remains low, at 0.009 per hundred people per year in this study.
We should also be cautious because the rarity of the disease means – even with a big group of people – there's room for error. The margin of error on the possible increased risk from ELF magnetic fields comes close to the point where the result could be down to chance. This point is reinforced by the fact that the lower range of the measured risk value (CI 1.02) barely passed the cut-off point for statistical significance.
The study has some strengths; including its size, its prospective nature, and its ability to adjust for known risk factors. It also used standardised tools to assess exposure to risk factors at work, rather than relying on people's memories of their exposure. Researchers were able to look at people's exposure over time, as well as one-off exposure from a single job.
But cohort studies can't point to a single factor as a cause of a disease. ALS remains mysterious. While the disease is inherited in about 5% of cases, the cause for other people is unclear. It's likely there is more than one single cause, including both genetic and environmental factors. It's possible that ELF magnetic fields are one factor affecting environmental risk.