The scientist Stephen Hawking lived with amyotrophic lateral sclerosis (ALS), the most common type of motor neurone disease, for 55 years. He was one of the longest-surviving people with the condition.

However, most people with motor neurone disease are not as lucky. It often progresses quickly, and many pass away within two to five years of diagnosis. There is still no cure. Genetics account for only about 10% of cases, and the rest of the causes are still largely a mystery.

A new study in the journal Jama Neurology showed one possible contributor: air pollution, both for the risk of developing motor neurone disease and for how it progresses.

In the study, my colleagues and I examined air pollution levels at each of the 10,000 participant’s home address for up to ten years before diagnosis. We focused on two common types of outdoor pollutants that are widely linked to health harms: nitrogen dioxide and particulate matter.

Particulate matter is made up of tiny airborne particles (far thinner than a human hair). It is usually grouped by size: PM2.5 (less than or equal to 2.5 micrometres), PM10 (less than or equal to 10 micrometres), and the in-between fraction PM2.5-10 (between 2.5 and 10 micrometres).

We found that being exposed to air pollution over the long term, even at the fairly low levels typically seen in Sweden, was linked to a 20–30% higher chance of developing motor neurone disease. What’s more, the pattern still held up when we compared siblings, which helps rule out a lot of shared factors like genetics and growing up in the same environment.

We also observed that people with motor neurone disease who had been exposed for years to higher levels of PM10 and nitrogen dioxide faced a greater risk of death or of needing a machine to help them breathe.

These pollutants are typically produced by nearby road traffic. Taken together, the results suggest that pollution generated close to home, especially from local vehicle emissions, may have a stronger effect than particulate matter carried in from farther away, which tends to account for much of the broader day-to-day variation in particulate matter levels.

Stephen Hawking survived for 55 years with ALS. Koca Vehbi/Shutterstock.com

Doctors regularly keep tabs on how well patients are managing everyday functions across a few key areas. These include bulbar function (speech, saliva control and swallowing), fine motor function (handwriting, cutting food, dressing and personal hygiene), gross motor function (turning in bed and adjusting bedding, walking and climbing stairs) and breathing (shortness of breath, difficulty breathing when lying flat, and signs of respiratory failure).

The participants in our study were assessed about every six months after diagnosis. We then looked at how quickly the disease was getting worse overall and within each of these domains. Patients whose decline was faster than that of 75% of other patients were labelled as having faster progression.

We found that long-term exposure to air pollution was associated with higher odds of having faster progression overall, particularly affecting motor and respiratory function, but not bulbar function.

Broader implications

The reasons for these differences are not yet clear. One possibility is that different parts of the nervous system vary in their vulnerability to pollution-related injury. It could also be because air pollution has consistently been linked to chronic lung diseases, reduced lung function and infections, all of which have been associated with poorer outcomes in ALS.

We accounted for many factors that could influence both air pollution exposure and motor neurone disease risk, including personal and neighbourhood income, education, occupation and whether participants lived in urban or rural areas. Our study did not have data on smoking habits or indoor air pollution exposure. However, there is no evidence suggesting that people with and without motor neurone disease differ significantly in these factors in ways that would explain our findings.

These results bring us closer to understanding motor neurone disease and may eventually help with earlier diagnosis and better treatment. But there’s a wider message here. We’re all exposed to air pollution, and the evidence keeps mounting that it harms our health in serious ways. Cleaning up our air could do far more good than we realise.