Long-term exposure to air pollution and the incidence of multiple sclerosis: a population-based cohort study

Background — Evidence of the adverse neurological effects of exposure to ambient air pollution is emerging, but little is known about its effect on the development of multiple sclerosis (MS), the most common autoimmune disease of the central nervous system.

Objectives — To investigate the associations between MS incidence and long-term exposures to fine particles (PM_2.5), nitrogen dioxide (NO₂), and ozone (O₃).

Methods — We conducted a population-based cohort study to investigate the associations between long-term exposures to PM_2.5, NO₂, and O₃ and the incidence of MS. Our study population included all Canadian-born residents aged 20–40 years who lived in the province of Ontario, Canada from 2001 to 2013. Incident MS was ascertained from a validated registry. We assigned estimates of annual concentrations of these pollutants to the residential postal codes of subjects for each year during the 13 years of follow-up. We estimated hazard ratios (HRs) and 95% confidence intervals (CIs) for each pollutant separately using random-effects Cox proportional hazards models. We conducted various sensitivity analyses, such as lagging exposure up to 5 years and adjusting for access to neurological care, annual average temperature, and population density.

Results — Between 2001 and 2013, we identified 6203 incident cases of MS. The adjusted HR of incident MS was 0.96 (95% CI: 0.86–1.07) for PM_2.5, 0.91 (95% CI: 0.81–1.02) for NO₂, and 1.09 (95% CI: 0.98–1.23) for O₃. These results were robust to various sensitivity analyses conducted.

Conclusions — In this large population-based cohort, we did not observe significant associations between MS incidence and long-term exposures to PM_2.5, NO₂, and O₃ in adults in Ontario, 2001–2013.