Air pollution harms cognition just hours after exposure, study finds

New research shows that inhaling polluted air for just an hour can impair cognitive abilities like attention and emotional recognition within four hours, highlighting the urgent need for cleaner air policies.

Study: Acute particulate matter exposure diminishes executive cognitive functioning after four hours regardless of inhalation pathway. Image Credit: Shutterstock AI Generator / Shutterstock.com

A recent study published in the journal Nature Communications reports that acute exposure to high levels of particulate matter (PM) has negative effects on cognitive function.

How does air pollution affect cognition?

PM 2.5 micrometers (µM) in size or smaller (PM_2.5) is a type of air pollution released from various sources, including vehicles, industrial facilities, and residential fireplaces. In 2015, exposure to PM_2.5 was responsible for about 4.2 million deaths. In addition to premature mortality, PM_2.5 inhalation also increases the risk of cardiovascular disease, respiratory dysfunction, and neurodegenerative diseases, including Alzheimer’s disease and Parkinson’s disease.

Cognitive function describes a wide range of mental processes that allow individuals to complete daily tasks. Executive function, for example, involves making decisions and performing goal-oriented activities, whereas socio-emotional cognition describes the ability to convey and interpret emotions needed to participate in meaningful social interactions.

The complexity of cognitive function emphasizes the importance of considering each aspect of these processes to determine how pollutants like PM_2.5 may induce toxicity. Currently, the two primary mechanistic models used for these studies include ‘direct’ and ‘indirect’ models. These models propose that the inhalation of air pollution directly targets the olfactory and respiratory pathways to impact cognitive function or induce systemic inflammation that ultimately reaches the central nervous system.

Direct vs. indirect models

The direct model hypothesizes that nasally inhaled air pollutants like PM_2.5 may directly affect cognition by translocating along olfactory neurons to ultimately reach the olfactory bulb. Conversely, PM_2.5 may also be exchanged with soluble particle components into the lungs to reach the bloodstream, where they can reach the brain by crossing the blood-brain barrier.

Several studies support this theory, as nanoparticles resembling burnt particles have been identified within the brain tissues of animals and humans exposed to polluted environments. These observations have identified the olfactory bulb as a ‘hot spot’ for neuronal damage, thus suggesting that preventing nasal inhalation of air pollutants may mitigate its effects on cognitive function.

Conversely, PM-induced inflammation in the lungs can subsequently induce chronic and acute systemic inflammation, both of which negatively impact cognitive function. This ‘indirect’ model postulates that the effects of inhaled air pollution on cognition and neuronal activity may not be observed for several hours following exposure.  

About the study

The current study explores the impact of high PM levels on cognition four hours after exposure through different inhalation routes. To this end, the study cohort consisted of 26 adults with a mean age of 27.7 years, all of whom were exposed to either high PM_2.5 levels or clean air. PM_2.5 was generated by burning and extinguishing a candle in the exposure room prior to beginning the session.

Under each condition, study participants were asked to breathe normally with or without a nose clip to reduce air entry through their noses. Four cognitive tests assessing working memory, selective attention, emotion expression discrimination, and psychomotor vigilance were administered to the participants at baseline and four hours after a one-hour period of air exposure. During the exposure period, all study participants were unaware whether they were exposed to clean air or PM.

Study findings

Study participants who inhaled PM_2.5-polluted air exhibited reduced selective attention and emotion expression discrimination compared to those exposed to clean air. PM_2.5 exposure did not significantly impact working memory or psychomotor vigilance.

These observations corroborate previous studies reporting that acute and chronic PM_2.5 exposure adversely affects executive functioning and the social and cognitive ability to discriminate emotions from expressions. These are cognitive processes closely related to each other.  

The lack of association with the method of inhalation suggests that ambient PM_2.5 may directly or indirectly impact neurons through the lung-brain axis and that these effects may be mediated by inflammation.

Conclusions

Higher cognitive functions in healthy people are negatively affected four hours following inhaling high levels of PM_2.5; however, working memory and psychomotor vigilance may not be affected by this type of acute PM_2.5 exposure. Exposure through different olfactory pathways did not change the outcome, though this finding requires further validation.

The impact of air pollution exposure on brain function may be more significant in more vulnerable groups.”

Additional studies are also needed to further explore the effects of PM_2.5 inhalation in older individuals and others at a high risk of cognitive impairment.