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Very low frequency electromagnetic field exposure linked to motor neurone disease
Association evident among men exposed through work
Workplace exposure to very low frequency electromagnetic fields may be linked to a doubling in risk of developing the most common form of motor neurone disease—amyotrophic lateral sclerosis, or ALS for short—suggests research published online in Occupational & Environmental Medicine.
The association was evident among men who had been exposed through their work.
ALS is a neurological disease, characterised by progressive degeneration of the motor nerve cells in the brain and spinal cord. There is currently no cure, and those affected usually die within a few years of diagnosis.
Previous research has suggested that ALS might be linked to workplace exposure to extremely low frequency electromagnetic fields, electric shocks, solvents, metals, and pesticides. But flaws in the study design and/or methods of these studies have undermined the strength of the associations found.
In a bid to try and avoid these pitfalls, the study authors relied on data from the Netherlands Cohort Study. This has been looking at diet and cancer and has involved more than 58,000 men and more than 62,000 women, who were all aged between 55 and 69 when they were first entered the study in 1986.
Participants who had died of motor neurone disease (76 men and 60 women) were compared with around 4000 (2411 men and 2589 women) who had been randomly selected for the purposes of the current study.
Their detailed employment histories were converted into workplace exposure to solvents, pesticides, metals, extremely low
frequency magnetic fields and electric shocks, using a validated technique (job exposure matrices).
High levels of electromagnetic field exposure were largely confined to the men, and depended on job type. These ranged from 2-25% among the men; among the women, the equivalent figure was 0-2%.
Participants’ neurological health was then tracked for an average of 17 years to see if any of them succumbed to ALS. During this time, 76 men and 60 women died of ALS.
Occupational exposure to extremely low frequency electromagnetic fields was associated with a heightened risk of developing ALS among the men.
Those whose jobs had exposed them to high levels of extremely low electromagnetic fields were more than twice as likely to develop ALS as those who had never been exposed through their work.
Furthermore, those in the top 30% of cumulative exposure (duration x intensity) were nearly twice as likely to develop the disease.
The other occupational factors assessed were only weakly associated with ALS risk in both men and women, and there was no clear evidence of a linear increase in risk according to the amount of cumulative exposure.
This is an observational study so no firm conclusions can be drawn about cause and effect. Furthermore, the researchers used cause of death (motor neurone disease) rather than ALS incidence, meaning that some deaths might have been misclassified, and although ALS is the most common type of motor neurone disease, it can take different forms.
Nevertheless, the researchers conclude that their findings strengthen the evidence suggesting that ALS may be linked to workplace exposure to extremely low electromagnetic fields.
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Dr Don Higson was formerly a Senior Specialist on nuclear reactor safety at ANSTO
As the Press Release says, an association does not establish a causal relationship but suggests strongly that there may be a link that warrants further investigation.
Another possible explanation is that there might be a third factor linking the apparent cause and the observed effect.
Consideration also needs to be given to possible differences, called “confounding factors”, between the group of people exposed to the electromagnetic fields and the group who were not exposed.
Dr Graham Wheeler is a Bayesian Medical Statistician at the Cancer Research UK & UCL Cancer Trials Centre, University College London
By looking at many different exposures and their association with ALS-related deaths, there is a risk that at least one interesting association is found purely by chance.
Even if there is truly no association between any of the 16 comparisons this study analysed and ALS deaths in men, the chance that at least one of these would incorrectly show a statistical effect is about 55%.
In other words, we’re more likely than not to find something statistically significant – even if it’s not real.
Prof Roel Vermeulen is co-author of the paper
ELF-MF are magnetic fields produced by electrical appliances and the power grid, with a frequency up to 300 Hz.
Jobs with relatively higher ELF-MF levels are for example: electric line installer, repairers and cable jointer, welders, sewing-machine operators, air craft pilots. Essentially jobs where workers are placed in close proximity to appliances that use a lot of electricity.
Prof Malcolm Sperrin is Director of Medical Physics and Clinical Engineering at the Royal Berkshire NHS Foundation Trust, UK
This paper refers to occupational exposure which implies high current applications such as metal fabrication and other industrial processes. It is reasonable that such high currents generate large fields, but the paper does not quantify either the magnitude or frequency of such fields or the duration of the exposure.
Notwithstanding further clarity from the authors this is different from the fields associated with WiFi, domestic power transmission etc. where to date there is no conclusive evidence of an elevated health risk.
Prof Neil Pearce is a Professor of Epidemiology and Biostatistics at the London School of Hygiene & Tropical Medicine, UK
Several previous studies have found that electrical workers are at increased risk of ALS. We don’t know why the risk is higher, but the two most likely explanations involve either electrical shocks, or ongoing exposure to extremely low frequency magnetic fields. However, previous studies have not had good information on exposures, and have not been able to separate the effects of electric shocks from those of chronic exposure to magnetic fields.
This study has much better information on exposure to magnetic fields than previous studies. It shows that the increased risk of ALS in electrical workers is most likely due to magnetic field exposure, rather than to electrical shocks.
In particular, it is well-known that there are genetic causes of ALS, but the occupational and environmental causes of the disease have not received much attention to date. These new findings indicate that occupational and environmental causes of ALS may be more important than was previously thought.
Currently we don’t know what the likely mechanisms behind exposure to low frequency magnetic fields are. However if this finding is real, it is important as it identifies a new, preventable cause of ALS. These findings may also help us to understand the disease better including the mechanisms by which it is caused and could help us find other causes of ALS.
Prof Christian Holscher is at Lancaster University, UK
This study analyses the potential risk factors for ALS such as chemicals, smoking, exposure to metals or electric shocks.
They found that of all factors, only extremely low frequency magnetic fields appear to increase the risk of developing ALS.
One has to take this result with caution, as the patient numbers were very low.
Even if this study does not show an effect in factors such as exposure to chlorinated solvents, it does not mean that there is no contribution of these factors.
It may be that the patient numbers are just too low to show a statistically significant effect.
In addition, the effect of extremely low frequency magnetic fields on ALS development is not clear.
The trend is only just statistically significant, and with such low numbers, it may well be a false positive.
Furthermore, it is not clear what extremely low frequency magnetic fields actually are.
We are surrounded by electro- magnetic fields which are high frequency, e.g. from power lines.
This paper does not define what those low frequency magnetic fields are.
Is the 50Hz field of our electric current included? Probably not, as this is not an extremely low frequency.
It is not clear what conclusion to draw from this study, and further studies with more patients will have to be made before any clear conclusions can be drawn from this.
Prof Paul Pharoah is Professor of Cancer Epidemiology at the University of Cambridge, UK
The study of health risks associated with exposure to things in the environment like chemicals and different types of radiation is important. This paper reports the results of a large study in which about 120,000 middle aged men and women were followed up for 17 years. During this time 76 of the men and 60 of the women died from a form of motor neurone disease called amyotrophic lateral sclerosis (ALS).
Questionnaires were used to estimate the occupational exposure to a wide range of chemicals including solvents, pesticides, metals, extremely low frequency magnetic fields in the people who had developed ALS and these were compared to about 4,000 individuals from the study who had not developed ALS.
A large number of exposures were evaluated. Of these only occupational exposure to extremely low frequency magnetic fields was “significantly” associated with risk of ALS in men, but not women.
However, an association between extremely low frequency magnetic fields and ALS is far from proven by these data.
Even though the study was very carefully conducted there are two main problems with the results. The first is that chance is the most likely explanation for the findings. The researchers evaluated ten different exposures in men and in women. If none of these exposures were associated with a risk of ALS then by chance an apparent association (statistically significant) would be observed in 1 out of 20 exposures. Thus, what was found in this study was no more than would be expected by chance.
If there were a true association between extremely low frequency magnetic fields and ALS it would be very surprising if this was present in men but not women. Thus, the fact that no association at all was found in women is evidence against a true association at all.
A second problem is that, even if the association between extremely low frequency magnetic fields and ALS were not due to chance, association does not mean causation. It is likely that occupational exposure to extremely low frequency magnetic fields is correlated with many other factors that could be the cause of the observed association.
Finally it is worth noting that the absolute risk of ALS in this cohort was very low with about 1 case per thousand individuals followed up over 17 years and only 1 in 10 men and 1 in 100 women in this population were exposed to high levels of extremely low frequency magnetic fields.
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