Does air pollution reduce cognitive function over time?

In a recent Science of the Total Environment study, researchers in the United Kingdom examine available studies for significant correlations between declining cognitive function in childhood and adult life and air pollution parameters. The study findings provide evidence of the inextricable interweaving of networks linking human environmental and individual health to productivity and socioeconomic background.

Study: Air pollution and human cognition: A systematic review and meta-analysis. Image Credit: Lemberg Vector studio / Shutterstock.com

Introduction

Air pollution, directly and indirectly, harms health due to climate change, higher temperatures favoring the emergence of new diseases, and spreading existing disease vectors beyond their accustomed habitats. Air pollution is also a threat to the feasibility and sustainability of healthcare systems as they exist today.

Particulate matter, ozone, and nitrogen oxides (NOx) are some of the most prevalent air pollutants, according to the World Health Organization (WHO) and European Environmental Agency (EEA).

Cognition refers to mental processes involved in learning and using knowledge or information. This includes acquiring, processing, transforming, and storing such data with timely retrieval. Good cognitive skills are key to maintaining good physical and mental health, achieving academic success, rising in society, and earning more. 

Air pollutants may not reach the brain directly but produce inflammation and oxidative stress that have neurological effects. Inflammation may be neuronal or systemic and may also involve dysregulated immunity that can lead to neuronal degeneration.

About the study

Earlier research has shown a link between cognition at the population level and the degree of air pollution and cumulative exposure. The current review supports these previous findings while also focusing on cognition as experienced by the people at large rather than in terms of specific clinical diagnoses such as autism or dementia.

The researchers included 86 studies in their qualitative analysis, with 14 in the meta-analysis. Except for Africa, all other continents were included.

Most studies in the meta-analysis explored air quality at home or school, thus measuring potential exposure to air pollution in the form of particulate matter less than or equal to 2.5 micrometers in size (PM_2.5). For children and adolescents, the risk of exposure-linked general cognitive deterioration was not supported by research; however, the strength of the evidence is too weak to make a definitive conclusion.

In other words, the studies came to varying conclusions, might have tested different sets of cognitive skills, and, as a result, may have used too different methods to be clustered together in a single meta-analysis. Standardized cognitive tests might help avoid such deficits in future studies.

What did the study show?

Some studies indicated lower intelligence in children between the ages of eight and 11 exposed to higher levels of black carbon (BC) but not coarse PM, PM of 10 micrometers or less (PM₁₀), or ozone in younger children up to eight years of age. In addition, several studies showed a decline in executive function, especially working memory and attention span.

PM_2.5, PM_10, and NOx exposure were linked to poor executive function in several studies that did not depend on a single cohort, unlike the above.

Available research does not support an association between memory and learning or between reaction time and the speed at which a child processes data or exposure to various air pollutants like NOx, PM_2.5, and ultrafine particles (UFP).

With young adults, few studies have explored cognitive outcomes with exposure to air pollution.

In those above the age of 40, some associations with general cognitive decline and PM_2.5 or NOx exposure were identified. In addition, PM_2.5 exposure was also associated with reduced verbal fluency and executive function.

Previous meta-analyses showed significant adverse effects were due to increasing exposure to air pollution in low-exposure areas but not high-exposure areas. This could be due to the overall high level of exposure-related harmful effects in high-exposure areas; therefore, the range of exposures used in these areas might fail to detect the change in harm level.

Prior studies that covered long periods showed significant negative associations between cognition and exposure levels. However, cognition studies were of relatively good quality only in older adults.

Most studies focused on children or older adults above 40 who are considered at higher risk due to rapid changes in their cognitive processes. Intelligence and reasoning skills were not well studied; however, verbal fluency in older adults showed a reduced association with an increase in PM_2.5.

Despite the limited number of studies on young adults, this group appears to be more affected by exposure to air pollution than children or older adults. Further research is thus essential in this group, as the brain rapidly develops up to the age of 25 years and continues after that at a slower pace until the end of life.

The extant studies also did not account for the confounding effects of noise pollution, which is often co-existent with air pollution. Moreover, the effects of exposure to air pollution at one period of life may be heavily influenced by previous exposure and its developmental impact.

Cognitive effects due to such exposures may vary depending on the developmental phase and period of life. At present, cumulative slow mechanisms such as attrition of neurons by slow injury or chronic inflammation affecting the whole body may be implicated. However, more acute effects have been shown to possibly affect the brain.

Immediate and acute exposure, therefore, could disrupt contemporaneous cognitive processes and have a lasting cognitive impact through disruption to longitudinal cognitive processes.”

Such differences in the latent period before injury become apparent following an acute injury or with different pollutants. This phenomenon was evident in one study where short-term effects on general cognitive function were more significantly associated with PM_2.5 than with NOx. However, the converse effects were seen with long-term consequences for these two pollutant types.

Notably, the high variation in significance and direction of associations could be due to the combination of effects from performing different tasks. With the single task of verbal fluency, where the same task was applied across various studies, heterogeneity of the effect on meta-analysis was low.

Task similarity alone does not explain heterogeneous effects since heterogeneity was low for the meta-analysis of executive function using different tasks but high for other single-task meta-analyses. Instead, exposure levels, latency period, and bias could play a role.

Nevertheless, most associations did find support in the outcomes reached by the meta-analysis, thus indicating an association between air pollution and some cognitive processes.

Future directions

This review identified much evidence that was supportive of associations between environmental air pollution and cognition in humans, but not for all pollutants and all cognitive outcomes.”

However, the evidence could not be classified with a high degree of certainty.

The researchers also make several recommendations. First, using standardized tools in global research would improve the meta-analysis by ensuring better comparability.

Secondly, much more research must be conducted to examine how air pollution affects cognition during the vulnerable periods of adolescence and young adulthood when the brain undergoes dramatic changes. Such analysis should also be extended to cover a broader spectrum of cognitive functions.

Similarly, a range of air pollutants, especially those which often occur together or affect the response to another, should be studied. Unfortunately, the current study only assessed a select list of pollutants.

The importance of adjusting for pre-existing risk factors such as birth difficulties, other forms of pollution, and risk of injury during childhood is also highlighted. These need further exploration to better understand their relationships and modifying effects on the results of pollution exposures.

Mechanistic studies are also indicated to strengthen the potential causality of an association.

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.