Specifically, two hours of exposure to diesel exhaust is enough to cause a measurable decrease in the brain’s functional connectivity. The scientists discovered this through an experiment in which 25 healthy adults were exposed to diesel exhaust and filtered air at different times in a laboratory.

All the participants had their brain activity easured before and after each exposure with functional magnetic resonance imaging (fMRI). The researchers then analyzed changes in the brain’s default mode network (DMN), a set of inter-connected brain regions that have impacts on memory and internal thought. The scans showed that functional connectivity in widespread regions of the DMN decreased in the brains of participants after they were exposed to diesel exhaust.

“For many decades, scientists thought the brain may be protected from the harmful effects of air pollution,” explains Chris Carlsten, a professor of respiratory medicine at UBC who was a senior author of a study. “This study, which is the first of its kind in the world, provides fresh evidence supporting a connection between air pollution and cognition.”

These findings are concerning because “altered functional connectivity in the DMN has been associated with reduced cognitive performance and symptoms of depression,” stresses Jodie Gawryluk, a professor of psychology at the University of Victoria.

“While more research is needed to fully understand the functional impacts of these changes, it’s possible that they may impair people’s thinking or ability to work,” Gawryluk adds.

On the upside, the changes were temporary and connectivity in the brains of participants soon returned to normal. However, prolonged exposure to diesel exhaust could trigger long-lasting harm, the scientists speculate. That is why it is important to try and avoid exposure to traffic pollution as best as possible.

“People may want to think twice the next time they’re stuck in traffic with the windows rolled down,” Carlsten says. “It’s important to ensure that your car’s air filter is in good working order, and if you’re walking or biking down a busy street, consider diverting to a less busy route.”

Nor is it only diesel exhaust that is a concern. So are other airborne pollutants, the experts point out.

“Air pollution is now recognized as the largest environmental threat to human health and we are increasingly seeing the impacts across all major organ systems,” Carlsten says.

“I expect we would see similar impacts on the brain from exposure to other air pollutants, like forest fire smoke. With the increasing incidence of neurocognitive disorders, it’s an important consideration for public health officials and policymakers,” the scientist adds.