Increases in chronic non-communicable diseases associated with changes in global economies and population ageing can be attributed at least partly to the exposure of urban populations to airborne particulate matter and other pervasive pollutants, poverty, dietary practices, and decreased levels of physical activity. Understanding the importance of particulate matter—a complex mixture of solid and liquid particles suspended in air—is at the crux of world epidemiological associations with short-term and long-term cardiovascular morbidity and mortality. In The Lancet, Hong Chen and colleagues concluded that living close to heavy traffic was associated with a higher incidence of dementia. Using data from two population cohorts to explore the development of multiple sclerosis in the younger cohort (around 4·4 million adults aged 20–50 years) and dementia and Parkinson's disease in the older cohort (around 2·2 million, aged 55–85 years), the authors included almost the entire adult population in the most populous province in Canada (Ontario) with a lagged exposure up to 10 years. Chen and colleagues statistically assessed the associations between traffic road proximity and incident dementia, Parkinson's disease, and multiple sclerosis, adjusting for key variables such as diabetes, brain injury, and neighbourhood income. The significant association of newly diagnosed cases of dementia in the study period between 2001 and 2012 with the proximity to traffic road less than 50 m–300 m versus more than 300 m, and the robust observation of dementia involving predominantly urban versus rural residents, opens up a crucial global health concern for millions of people.