Living near major roads and the incidence of dementia, Parkinson’s disease, and multiple sclerosis: a population-based cohort study

Background — Emerging evidence suggests that living near major roads may adversely affect cognition. However, little is known about its relationship with the incidence of dementia, Parkinson's disease (PD) and multiple sclerosis (MS).

Methods — We assembled two population-based cohorts including all adults aged 20-50 (~4*4 million; MS cohort) and all adults aged 55-85 (~2*2 million; dementia/PD cohort) who resided in Ontario, Canada on 1 April 2001. We included those who were free of these neurological diseases, Ontario residents for ≥5 years, and Canadian-born. We ascertained individuals' proximity to major roadways based on their residential postal-code address in 1996, five years before cohort inception. Incident diagnoses of dementia, PD, and MS were ascertained from validated provincial chronic disease databases. We assessed the associations between traffic proximity and incident dementia, PD, and MS using Cox proportional hazards models, adjusting for individual and contextual factors such as diabetes, brain injury, and neighbourhood income. We conducted various sensitivity analyses, such as adjusting for access to neurologists and exposure to selected air pollutants, and restricting to never movers and urban dwellers.

Findings — Between 2001 and 2012, we identified 243,611 incident cases of dementia, 31,577 cases of PD, and 9,247 cases of MS. The adjusted hazard ratios (HR) of incident dementia were 1*07 (95%CI: 1*06-1*08), 1*04 (95%CI: 1*02-1*05), 1*02 (95%CI: 1*01-1*03), and 1*00 (95%CI: 0*99-1*01) for persons living <50, 50-100, 101-200, and 201-300 versus >300m, respectively (Ptrend=0.0349). The associations were robust to sensitivity analyses and appeared stronger among urban residents, especially those who lived in major cities (HR=1*12, 95%CI: 1*10-1*14 for living <50 m), and who never moved (HR=1*12, 95%CI: 1*10-1*14 for living <50 m). No association was found with PD or MS.

Interpretation — In this large population-based cohort, living close to heavy traffic was associated with a higher incidence of dementia, but not with PD or MS.