New research suggests that a high level of occupational exposure to extremely low-frequency magnetic fields increases the risk of developing ALS.
Amyotrophic lateral sclerosis (ALS) - sometimes referred to as Lou Gehrig's disease - is a rare illness that progressively and irreversibly kills the neurons responsible for movement. Gradually, patients with ALS become unable to chew, walk, or even breathe.
The ALS Association estimate that more than 5,600 people receive a diagnosis of ALS every year, and approximately 30,000 people in the United States live with the disease.
The majority of people with ALS usually die from respiratory failure within 3 to 5 years after the initial symptoms develop. However, around 20 percent of patients live for more than 5 years, and 10 percent survive for 10 years or longer.
Although medical researchers do not yet know what causes the disease, several hypotheses have been put forth. Some studies have suggested that environmental exposure may be responsible for the illness, and others have proposed a series of risk factors including smoking, physical activity, and educational level.
Previous research has pointed to a variety of occupational exposures that may drive the illness. Some of these include extremely low-frequency magnetic fields (ELF-MFs), electrical shocks, metals, and toxic substances such as solvents and pesticides.
The correlations found in these studies, however, have been questioned due to methodological flaws in the analysis.
A new study aims to succeed where previous research has failed, examining the existing evidence and analyzing the effects of the above-mentioned occupational exposures on ALS mortality.
The research was carried out by scientists from Utrecht University, Maastricht University, and the University Medical Center Utrecht - all in the Netherlands - and the results were published in the journal Occupational & Environmental Medicine.
Studying the link between ALS mortality and occupational exposures
The researchers used the data available from the Netherlands Cohort Study - a large-scale, cohort study that examined diet and cancer risk in more than 58,000 men and 62,500 women.
Participants were between 55 and 69 years old when they enrolled in the study in 1986, and they were clinically followed for more than 17 years.
The researchers collected data on their current and previous professional occupations using a self-administered questionnaire. They then entered these data for a randomly selected subcohort of more than 2,400 men and 2,500 women, together with the data on ALS mortality - 76 male deaths and 60 deaths among women.
Using job exposure matrices - a validated tool commonly used to assess occupational health hazards - the scientists estimated the occupational exposure to metals, electrical shocks, ELF-MFs, and to pesticides and solvents.
Finally, the researchers used Cox regression models to calculate the associations between those who had never been exposed to ELF-MFs, those who had been exposed, cumulative exposure, and ALS mortality.
Men exposed to ELF-MFs more than twice as likely to develop ALS
Calculations were amassed by gender. Additionally, researchers accounted for variables such as smoking, educational level, body mass index (BMI), and patterns of physical activity.
Overall, the study revealed that occupational exposure to ELF-MFs increased the risk of developing ALS in men.
Men who were occupationally exposed to high levels of ELF-MFs were 2.19 times more likely to develop ALS than those who had never been exposed to them. Additionally, those in the top tertile (or the top 30 percent) of cumulative exposure were almost twice as likely to develop ALS.
The study did not find a proportional link between ALS risk and the amount of exposure.
The authors conclude that their study "strengthens the evidence" and provides "further support for an association between occupational exposure to ELF-MF and an increased risk of ALS mortality."
However, they also note the observational nature of the study, which means that it cannot explain causality. A further limitation of the study is that some of the deaths considered in the research might have mistakenly been attributed to ALS.