Study shows traffic pollution impairs human minds

A new study by researchers at the University of British Columbia and the University of Victoria has shown that common levels of traffic pollution can impair human brain function in only a few hours.

Physiological evidence of traffic pollution’s impact on our bodies — or more specifically, our minds — has been discovered by scientists in Canada.

A study, published in the journal Environmental Science, found that exposure to just two hours of diesel exhaust fumes led to a decrease in the brain’s functional connectivity – a measure of how well different regions of the brain interact with each other.

While the exact ramifications are as yet unknown, previous observational studies have linked weakened functional connectivity with worsened working memory, behavioural performance and productivity at work, the study authors wrote.

The study was the world’s first to expose humans to air pollutants in a lab setting, according to the authors from the UBC and University of Victoria. In total, 25 adults between 19 and 49 were exposed to two hours of both filtered air and air contaminated with diesel exhaust.

“In our study, Joe is compared to Joe, so the only thing that is different is the exposure to diesel,” said senior study author Chris Carlsten, professor and head of respiratory medicine and Canada Research Chair in occupational and environmental lung disease at UBC.

“A crossover design where each individual serves as their own control has vastly more statistical power than if you looked at a hundred people,” Carlsten said.

Participants received a functional MRI scan before and after each treatment, enabling researchers to monitor each subject’s brain activity. They were especially interested in a brain region called the default mode network (DMN), as it’s particularly sensitive to stresses like toxicity, aging and disease.

After analyzing the MRIs, the researchers recorded a significant decrease in functional connectivity inside the default mode network after breathing diesel exhaust compared to filtered air.

“We know that altered functional connectivity in the DMN has been associated with reduced cognitive performance and symptoms of depression, so it’s concerning to see traffic pollution interrupting these same networks,” Jodie Gawryluk, psychology professor at the University of Victoria and the study’s first author, said in a statement.

“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.”

Additionally, Carlsten said his team is planning to publish another set of data from the same study that focused on cognition and reaction times.

“In addition to the MRI, we had participants take a standardized computer test called CANTAB, where a computer basically says, ‘Press the button if there’s a yellow square,’ and they have to do that as quickly as possible.

“Our preliminary results is that reaction time is slower under diesel conditions.”

Notably, the changes in the brain were temporary and the healthy participants’ brain function returned to their normal baseline after the exposure. However, Carlsten speculated that the effects could be long-lasting when exposure is continuous.

“It would also be important for another study to look at effects from fire smoke as well, which is arguably a bigger problem in some places … Wood smoke is quite different to diesel exhaust,” he said.

Although the experiment was the first of its kind, it’s preceded by thousands of observational studies on the health impacts of traffic pollution, said Dr. Samantha Green, a Toronto-based family physician and board member of the Canadian Association of Physicians for the Environment.

“The findings are not surprising to me, given the tremendous amount of observational research that already exists,” she said. “But it’s certainly a fascinating methodology and super-interesting that they were able to actually find, with a randomized controlled trial, and observe” traffic pollution’s impact.

According to Green, traffic pollution has been linked to a broad spectrum of health disorders including issues with pregnancy, heart disease, various cancers and even an increased rate of dementia and Alzheimer’s disease.

“We reviewed 1,858 articles related to the health harms of traffic-related air pollution,” Green said, referencing a 2022 report she helped review. “Those studies linked traffic related air pollution to, of course, respiratory related outcomes, but also many, many, many other harms in other parts of the body.”

Green believes the study represents a foundational step in taking air pollution research further, despite its relatively small sample size of 25 people: “Given that this is the first study of its kind, I actually I think it’s a reasonable sample size,” she said — “especially given what they were doing with the MRIs and also given the amount of observational evidence we already have.”

She also said a larger cohort could raise ethical questions around exposing more people than necessary to substances we know are harmful.

Carlsten noted the ethical issues associated with conducting a similar study on children, but he added that such data would be helpful to understand any long-term effects of pollution on development since “patterns and trajectories are set early.”

Michael Brauer, a professor at UBC’s school of population and public health, and unaffiliated with the study, agreed with Green about the study’s merits: “I’m not concerned with the numbers per se, because you’re basically comparing every person to themselves. So you have 25 different comparisons.”

That being said, the 25 people chosen for the study might not represent the broader population, Brauer continued. Study participants tend to be “better off, better educated, overall healthier people than the general population,” he said.

Given all we know and are still learning about traffic pollution, Brauer says it’s high time for systemic change: “That means more regulations on the sources of pollution.”

Brauer recommends individuals avoid areas with heavy traffic when walking or exercising if possible, install better air ventilation at home or even change commuting patterns.

“I think most people are aware of things like (not) smoking and getting enough sleep and exercise” when it comes to having a healthy brain, Brauer said. “Air pollution is just now another one of those things that people should be thinking about.”

This article was reported by the Star.