Summary: A new Canadian study links long-term exposure to common air pollutants with reduced cognitive performance and visible brain changes on MRI. The analysis, conducted within the Canadian Alliance for Healthy Hearts and Minds (CAHHM) cohort, evaluated nearly 7,000 middle-aged adults across five provinces and found associations between low-level pollution and poorer memory, comprehension, and processing speed.
Notably, the effects were apparent at pollutant concentrations generally considered low by international standards. Both fine particulate matter (PM2.5) and nitrogen dioxide (NO2) were associated with lower scores on cognitive tests, and higher traffic-related pollution corresponded with a greater likelihood of covert vascular brain injury on MRI—an association that appeared stronger in women.
Key Facts
- Low exposure, measurable effects: Unlike studies from heavily polluted regions, this research demonstrates cognitive and structural brain associations at pollutant levels commonly recorded across Canada.
- Direct neurological impact: Associations persisted after statistical adjustment for major cardiovascular risk factors (hypertension, diabetes, adiposity), suggesting pollution may affect brain tissue independently of cardiovascular disease.
- Sex differences: MRI-visible signs of covert vascular brain injury linked to traffic-related pollution were more pronounced among female participants.
- Pollutants studied: Multi-year averages of PM2.5 and NO2—typical byproducts of vehicle exhaust, industrial emissions, and wildfire smoke—were the exposure metrics.
Source: McMaster University
Overview
Researchers from McMaster University report that everyday air pollution is associated with subtle but measurable differences in cognitive function among middle-aged adults living in Canada. The study, published in the journal Stroke on May 13, 2026, used standardized cognitive tests and MRI imaging to assess brain health in relation to estimated multi-year exposures to PM2.5 and NO2.
Across 6,878 participants (mean age 57.6 years, 55.6% women), the mean five-year exposure preceding enrollment was 6.9 μg/m3 for PM2.5 and 12.9 parts per billion for NO2. Higher long-term exposure to these pollutants was associated with lower scores on the Montréal Cognitive Assessment and the Digit Symbol Substitution Test, and with increased odds of covert vascular brain injury visible on MRI.
After adjustment for age, sex, education, and cardiovascular risk factors, a 5 μg/m3 higher PM2.5 was linked to a 0.44-point lower score on the Montréal Cognitive Assessment and a 1.31-point lower score on the Digit Symbol Substitution Test. A 5 parts per billion increase in NO2 was associated with 0.12-point and 0.38-point lower scores on the respective tests, and with an adjusted odds ratio of 1.08 for covert vascular brain injury.
Lead and corresponding authors emphasize that dementia and cognitive decline develop over decades. Identifying environmental factors that exert small, cumulative effects in midlife can help target prevention strategies that might reduce risk later in life.
The study does not establish causation, but it strengthens a growing evidence base linking air quality to age-related cognitive change. The persistence of associations after accounting for established cardiovascular risk factors suggests a pathway by which pollutants may directly affect central nervous system health.
Researchers call for longitudinal follow-up to determine whether these cross-sectional associations predict accelerated cognitive decline over time and to evaluate whether improvements in air quality translate into measurable brain health benefits.
Funding: This research was conducted as part of the Canadian Alliance for Healthy Hearts and Minds (CAHHM) study and received support from the Canadian Partnership Against Cancer, the Heart and Stroke Foundation of Canada, and the Canadian Institutes of Health Research.
Key Questions Answered:
A: No. This study found that even pollution levels often considered low internationally were linked to small but detectable deficits in cognitive tests and to MRI-visible brain changes. These effects appear to accumulate quietly over many years.
A: Because cardiovascular risk factors can themselves impair brain health, adjusting for them helps isolate whether air pollution has an independent association with cognition. The study’s findings remained after these adjustments.
A: The study does not prove causation or that individual exposure will lead to dementia. However, it provides evidence that chronic exposure to common pollutants may accelerate age-related declines in memory and processing speed, underscoring prevention and exposure reduction as prudent public health measures.
Editorial Notes:
- This article was edited by an editor at Neuroscience News.
- The journal paper was reviewed in full by editorial staff.
- Additional explanatory context was added by the news team.
About this research and reporting
Author: Adam Ward
Source: McMaster University
Contact: Adam Ward – McMaster University
Image: Image credit: Neuroscience News
Original Research: Open access. Title: “Association of Air Pollution With Brain Health: A Cross-Sectional Analysis in Adults Living in Canada” by Sandi M. Azab et al. Published in Stroke. DOI: 10.1161/STROKEAHA.125.054251
Abstract
Association of Air Pollution With Brain Health: A Cross-Sectional Analysis in Adults Living in Canada
BACKGROUND:
Air pollution is an established risk factor for dementia, but its role in early cognitive dysfunction is less clear. This study examined associations between long-term exposure to NO2 and PM2.5 and measures of cognitive function, as well as MRI-detected covert vascular brain injury. The analysis also considered the influence of cardiovascular risk factors and neighborhood greenspace.
METHODS:
The CAHHM cohort recruited Canadian adults between 2014 and 2018. Five-year average exposures to NO2 and PM2.5 preceding enrollment were estimated for each participant. Cognitive outcomes included the Montréal Cognitive Assessment and the Digit Symbol Substitution Test; brain outcomes included MRI-detected covert vascular lesions. Generalized linear mixed models assessed associations in this cross-sectional analysis.
RESULTS:
Among 6,878 participants (mean age 57.6 years; 55.6% women), five-year mean exposures were 6.9 μg/m3 for PM2.5 and 12.9 ppb for NO2. After adjustment, a 5 μg/m3 higher PM2.5 was associated with a 0.44-point lower Montréal Cognitive Assessment score and a 1.31-point lower Digit Symbol Substitution Test score. A 5 ppb higher NO2 was associated with a 0.12-point lower Montréal Cognitive Assessment score, a 0.38-point lower Digit Symbol Substitution Test score, and slightly higher odds of covert vascular brain injury (adjusted OR 1.08). Results were robust to adjustment for cardiovascular risk factors and neighborhood greenspace.
CONCLUSIONS:
In this large cross-sectional sample of middle-aged adults living in Canada, higher long-term exposure to PM2.5 and NO2 was associated with lower cognitive test scores and with MRI evidence of covert vascular brain injury. These associations were independent of major cardiovascular risk factors. Longitudinal research is needed to determine whether these exposures predict faster cognitive decline over time and whether air-quality improvements would protect brain health.