First-of-its-kind study shows how traffic pollution impairs brain function

Researchers in Canada have, for the first time ever, demonstrated how acute exposure to traffic pollution can immediately impair human brain function, offering unique evidence of the connection between air quality and cognition. Healthy adults were exposed to diesel fumes before having their brain activity imaged in a fMRI machine.

Air pollution in urban environments has long been associated with poor cardiovascular, respiratory and brain health. But connecting the dots between air quality and human health has been challenging for researchers. It’s difficult to accurately quantify a person’s exposure to air pollution beyond associating rates of certain diseases in geographical areas of high pollution.

Plenty of cell and animal studies can demonstrate how air pollution affects organisms. But as we know, there can often be a huge chasm between the effects of toxins on a mouse in a lab and chronic exposure to a human in the real world.

So perhaps the final missing piece in the puzzle for researchers has been direct human exposure studies. Of course, it’s not exactly ethical to expose volunteers to high levels of toxic fumes just to watch what happens, so these kinds of experiments, unsurprisingly, have been lacking.

This new research used a model of human exposure to diesel exhaust fumes developed over a decade ago. The technique delivers controlled and diluted concentrations of diesel exhaust particulate matter to human subjects at levels deemed to be representative of real-world exposure but also proven to be safe. In a lab setting, 25 healthy adults were exposed to either diesel exhaust, or filtered air for two hours and had their brain activity measured using fMRI before and after each exposure.

The main focus of the study was on the impact of this kind of traffic-associated air pollution on what is known as the default mode network (DMN). This is a set of inter-connected cortical brain regions that play a crucial role in cognition, memory and emotion.

The findings revealed brief exposure to diesel exhaust caused a decrease in DMN activity, essentially yielding a drop in functional connectivity between different brain regions, compared to what was seen when subjects were exposed to filtered air. Jodie Gawryluk, first author on the study, said these kinds of DMN alterations have been linked to depression and cognitive decline.

“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,” said Gawryluk. “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.”

Alone, these new findings are not particularly meaningful. No evaluations were performed in the study to suggest the observed DMN changes impacted cognition. But alongside a growing body of epidemiological and preclinical studies linking air pollution with a number of neurodegenerative diseases, these findings may be much more significant. They effectively demonstrate the acute effects of air pollution on the human brain in a way never before shown.

According to senior author on the study Chris Carlsten, it is unclear what long-term effects this kind of pollution exposure will have on a human brain. On the positive side of things the researchers did seen DMN brain activity return to normal relatively soon after the diesel fume exposure. So Carlsten is only able to hypothesize what the impact of more chronic, continuous exposure could be.

“People may want to think twice the next time they’re stuck in traffic with the windows rolled down,” said Carlsten. “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.”

The study was published in Environmental Health.

Source: University of British Columbia

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