Long-term exposure to three common air pollutants — fine particulate matter, nitrogen dioxide, and ozone — were not found to be “convincingly” linked to incidence of multiple sclerosis (MS) in a large population study conducted in Canada.

The study, “Long-term exposure to air pollution and the incidence of multiple sclerosis: A population-based cohort study,” was published in the journal Environmental Research.

A person’s genes are known to affect MS risk, as do environmental conditions. Among the latter, exposure to ambient air pollution is thought to possibly heighten inflammation and oxidative stress — an imbalance between the body’s production of potentially harmful reactive oxygen species and its ability to contain them.

Recent studies have suggested that pollutants in the air can cross the blood-brain barrier, entering the brain and damaging the central nervous system, and to progress certain neurological disorders. In fact, the study points to associations made in previous research to such diseases as Parkinson’s, dementia, and cardiovascular and pulmonary disorders.

It notes that very few studies have looked into possible links between airborne environmental pollutants and MS incidence, despite the high level of MS across North America.

Among the most harmful types of air pollutant are fine particulate matter — microscopic particles resulting from natural or human causes — and gases such as nitrogen dioxide and ozone.

The effects of ambient air pollution on MS are still controversial. Clinical data has linked daily increases of such pollutants to elevated daily rates of MS relapse, while other studies have found no association between MS and long-term exposure to these pollutants.

Researchers investigated the association between MS incidence and long-term exposure to three common airborne pollutants — fine particulates with a diameter of 2.5 micrometers (PM2.5), nitrogen dioxide (NO2) and ozone (O3) — in a large study of residents of Ontario.

They collected information on 2.82 million Canadian-born residents of the province, ages 20 to 40, from 2001 to 2013. Over this time, 6,203 of these people were given a validated MS diagnosis.

Annual concentration of pollutants was estimated and assigned to province areas using individuals’ residential postal codes over the study’s 13 years.

Results, the researchers wrote, found “no convincing evidence associating MS incidence with exposures to PM2.5, NO2, or O3. These results were robust to all sensitivity analyses conducted.”

This finding was in keeping with earlier studies on MS and air pollutants done in the United States and in Ontario in 2017 and also finding no long-term association, the study noted.

An exploratory analysis, however, found a possible and “borderline association” between ozone exposure and MS in women, although such an  association varied across regions in Ontario.

Overall, “in this population-based cohort study, we found no convincing evidence associating MS incidence with exposures to PM2.5, nitrogen dioxide or ozone,” the researchers wrote.

“To our knowledge, this represents the largest study to date assessing the association between the development of MS and exposure to ambient air pollution,” they concluded.

The researchers, however, noted their study had some limitations. Namely, it considered each the three pollutants individually for their impact on MS incidence, rather than looking at the mix, and it lacked information on personal exposure to pollutants (such as time spent by an individual outdoors).