The Environmental Protection Agency estimates that the United States generated 67 million tons of air pollution in 2021. Ever since the Industrial Revolution in the 1800s, the amount of air pollutants (nitrogen oxides, carbon dioxide, methane, and sulfur dioxide, to name a few) in the Earth’s atmosphere skyrocketed. With the increasing toxicity present in the air we breathe comes consequences for the way we think and behave. Past studies on the effects of air contaminants have studied respiratory impacts, cardiovascular risks, and developmental impacts. But, much is unknown about the cognitive effects of traffic-induced air pollution, as the neurological effects of air pollution are not as widely studied.
Earlier this year, researchers at the University of Victoria and the University of British Columbia released data from a study describing the negative impact of traffic pollution on the brain’s default mode network (DMN). They describe the DMN as “a set of interconnected brain regions that play an important role in memory and internal thought.” The DMN impacts our emotional perception of others and feelings of empathy, and it tends to activate in times when our attention is diverted from our surroundings and in instances of self-awareness (for example, daydreaming and self-analysis).
Through a brain scan analysis, the scientists concluded that exposure to 300 micrograms per cubic meter of diesel exhaust over a two-hour period causes a temporary reduction of DMN activity when compared to the effects of filtered air. They attributed this short-term effect to the fact that exposure only lasted for two hours, rather than the more continuous exposure that occurs in your typical everyday traffic. This decrease in DMN activity has complications. Since the DMN is linked to our sociocognitive abilities, scientists are aware of the potential for lower DMN activity caused by air pollution to result in sociocognitive deficiencies and symptoms of depression or other mental health conditions.
Unlike the majority of other research done by scientists on the impacts of air pollution, this study used 25 adult human subjects between the ages of 19 and 49. Each person was introduced to two environments — one with diesel exhaust and one with filtered air — with a two-week interval between each. The order of exposures was randomized. During the exposure periods, each individual was instructed to ride a stationary bike for 15 minutes in order to gain a base-level image of brain activity. Before and after the exposures, brain activity was measured using functional magnetic resonance imaging (fMRI), which records changes in blood flow and oxygen levels.
The general results from the fMRIs indicated that there was the greatest DMN activity post-exposure to the filtered air in regions of the brain controlling language processing and visual perception. The researchers ultimately concluded that brain activity in the DMN is diminished post-exposure to diesel exhaust. They also mention that, in the diesel exhaust environment, the individuals did not gain any of the cognitive boosts from light exercise on the stationary bike. This study is very important in the world of environmental science and neuroscience as it “provides the first evidence in humans, from a controlled experiment, of altered brain network connectivity acutely induced by air pollution,” according to the authors of the recent paper.
This research echoes similar findings to a 2015 review by Nicholas Woodward and colleagues exploring the neurological effects of traffic-related air pollution on rodents, prenatal fetuses, and neonatal children. They explain how past research has found that exposure to traffic-caused air pollution can cause inflammation and cognitive changes, especially in male rodents. The authors also discuss how excessive air pollution exposure during the developmental stage of rodents can cause symptoms of depression, lower reaction times when introduced to stimuli, and attention deficits. Similar to the authors of the recent study, Woodward also acknowledges the gap in cognitive research in relation to pollution and advocates for future studies.
Since the University of Victoria and the University of British Columbia study only examined the effects after a two-hour period, Dr. Chris Carlsten, one of the authors of the paper, speculates that continuous exposure to air contaminants in traffic may lead to long-lasting impacts on the brain. Scientists as a whole admit that further study is needed on this topic to determine the actual neurological impacts for those who experience long stretches of traffic-induced air pollution on a regular basis.
Image courtesy of Wikimedia Commons
