A new Canadian study found that long-term exposure to common air pollutants was linked to lower scores on tests of memory, understanding, and processing speed.
Researchers observed these associations even in areas with relatively low levels of air pollution by international standards.
Higher levels of traffic-related pollution were also associated with subtle signs of brain damage on MRI scans, particularly among women.
Most people think about air pollution as a threat to lung or heart health. But new research from researchers at McMaster University suggests it may also have implications for brain health.
The study found that adults who were exposed to higher levels of common air pollutants over time tended to perform worse on tests that measured memory, comprehension, and mental processing speed.
What makes the findings especially notable is that the research was conducted in Canada, a country known for having relatively clean air. Even so, researchers found associations between pollution exposure and cognitive performance, suggesting that brain health effects may occur at pollution levels many people would consider low.
“Canada’s air is often described as clean, but our findings suggest that even low levels of air pollution are linked to worse brain health,” lead author Sandi Azab said in a news release. “These are changes that can happen quietly, years before any noticeable symptoms appear.”
The study
The study analyzed data from nearly 7,000 middle-aged adults living across five Canadian provinces as part of the Canadian Alliance for Healthy Hearts and Minds project.
Researchers compared participants’ long-term exposure to air pollution with their performance on cognitive assessments, and for some participants, brain MRI scans.
The team focused on two common pollutants: fine particulate matter, known as PM2.5, and nitrogen dioxide. PM2.5 can come from sources such as wildfire smoke, industrial activity, and vehicle emissions, while nitrogen dioxide is commonly associated with traffic pollution.
The results
People living in areas with higher levels of these pollutants generally scored lower on cognitive tests.
Researchers also found that greater exposure to traffic-related pollution was linked to subtle but visible changes on brain MRI scans. These associations remained even after accounting for factors such as high blood pressure, diabetes, and body fat levels. The MRI findings appeared to be stronger among women.
The researchers emphasize that the study does not prove air pollution causes dementia or other cognitive disorders. Instead, it identifies a link between long-term pollution exposure and measures of brain health in midlife.
Still, the findings add to a growing body of research suggesting that environmental factors may play a role in how the brain ages. According to the study authors, changes associated with pollution exposure may occur gradually and long before noticeable symptoms develop.
The researchers say more long-term studies are needed to better understand whether reducing pollution exposure could help protect cognitive health over time. For now, the study highlights that air quality may be an important piece of the broader brain-health picture — even in places where the air is generally considered clean.
“Dementia doesn’t happen overnight,” researcher Russell de Souza said in the news release. “It develops over decades. Identifying factors that may damage the brain early, and that are potentially preventable, is critical for protecting brain health later in life.”