Air Pollution and the Risk and Progression of Multiple Sclerosis: A Systematic Review and Meta-Analysis

Air pollution has been linked to several neurological conditions, including stroke and neurodegenerative diseases. Evidence regarding its association with multiple sclerosis (MS) remains conflicting, limited by small sample sizes.

PubMed, Embase, Scopus, and Cochrane controlled register of trials (CENTRAL) were searched on September 9, 2025 without any language or time restrictions. The inclusion criteria were observational studies that evaluated the association between exposure to air pollutants and MS development or severity. Hazard ratios (HRs) with 95% confidence intervals (CIs) were pooled as effect estimates using the fixed or random effects model. Exposures included PM2.5, PM10, nitrogen dioxide (NO₂), carbon monoxide (CO), sulfur dioxide (SO₂), and ozone (O₃). Outcomes were MS risk and severity, including relapses, disability progression measured with the Expanded Disability Status Scale (EDSS), and contrast-enhancing lesions (CELs) development.

Twenty-two studies comprising 16,585,206 participants were included. Long-term exposure to PM2.5 (HR = 1.21; 95% CI: 1.07-1.38), PM10 (HR = 1.20; 95% CI: 1.02-1.42), and CO (HR = 3.85; 95% CI: 1.34-11.08) were associated with increased MS risk. Short-term exposure to PM2.5 (HR = 1.20; 95% CI: 1.01-1.43), PM10 (HR = 1.20; 95% CI: 1.01-1.45), NO₂ (HR = 1.13; 95% CI: 1.02-1.25), and O₃ (HR = 1.15; 95% CI: 1.04-1.27) were associated with relapses. Short-term exposure to PM2.5 (HR = 2.20; 95% CI: 1.05-4.60) and PM10 (HR = 1.02; 95% CI: 1.01-1.03) were linked to CELs, while PM10 (HR = 1.31; 95% CI: 1.04-1.65) was associated with disability progression.

Long-term air pollution exposure was associated with higher MS risk, and short-term exposure with greater disease severity. Reducing air pollution may be a key strategy to protect brain health in MS.