Exposure to particulate air pollution, even at low levels, can reduce children’s cognitive abilities

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Children are especially vulnerable to the impact of ambient air pollution given their developing bodies and underdeveloped immunological and respiratory systems. (Photo credit: iStock by Getty Images)

Increased exposure to outdoor air pollution in early life can make it harder for children to learn and process information, especially in terms of critical thinking and non-verbal tasks, a new study reports.

The findings, published Nov. 18 [2024] in Environmental Health, are based on a review and analysis of data from six epidemiological studies representing 4,860 children from diverse populations in North America, Europe, and Asia. They underscore growing evidence that air pollution severely impacts childhood health even before birth, with new research published this month showing it can harm embryo quality during in vitro fertilization (IVF). 

This marks the first time, however, that researchers have quantified the correlation between early life exposures to fine airborne particulate matter (PM2.5), a common air pollutant, and declines in verbal and nonverbal mental abilities, as measured by Intelligence Quotient (IQ) scores. 

IQ tests assess a range of mental skills, including memory, reasoning, working with numbers, processing language, and thinking about objects in three dimensions.

“Without exception, each study reported a negative association between PM2.5 exposure and children’s cognitive function,” say the researchers, including Dr. Philip Landrigan, director of Boston College’s newly launched Global Observatory on Pollution and Health and former cochair of The Lancet Commission on pollution and health.

The results are important because a child’s IQ is a strong predictor of their neurodevelopment, future academic achievement, employment opportunities, financial earnings, and economic mobility in adulthood, says study co-author Ella Whitman.

“At a population level, reduced cognitive function in children can increase the educational support needed in schools, reduce a society’s gross domestic product (GDP), and future leadership potential,” she says. “Our results demonstrate the urgent need to reduce exposure to PM2.5 in the early stages of life to promote longitudinal outcomes in children’s health, and human capital.”

Children at greater risk of sickness than adults

Children are especially vulnerable to the impact of ambient air pollution given their developing bodies and underdeveloped immunological and respiratory systems. Children also breathe at a faster rate, inhaling more air than adults, and are more physically active outdoors. 

In 2021, exposure to air pollution was linked to more than 700,000 deaths of children globally under five years old, making it the second-leading risk factor for death for this age group, after malnutrition, according to the Institute for Health Metrics and Evaluation (IHME). 

An estimated 201,000 of those deaths were linked specifically to PM2.5 exposure. PM2.5 is a combustion (burning) byproduct of emissions from vehicles, power plants, and industries or sources such as construction sites and fires. 

With a diameter of less than 2.5 micrometers, about 30 times smaller than the width of a human hair, its particles can penetrate directly into the brain via the olfactory bulb (brain structure involved in our sense of smell). They can also lodge deeply in the lungs when we inhale and enter the bloodstream, causing respiratory (breathing) and cardiovascular (heart and blood vessel) issues, among other problems.

PM2.5 has been identified as a risk factor in adults for neurodegenerative conditions, including cognitive impairment. In children, studies have shown that high exposure to air pollution may:

Another recent study, involving more than 8,500 children ages 9 to 10 and published Oct. 30 [2024] in Environmental Health Perspectives, also detected an association between cumulative exposure to PM2.5 and worse cognitive performance.  

Children’s nonverbal intelligence most harmed by air pollution

In the latest report, children across the six studies were exposed to an average PM2.5 concentration of 30.4 micrograms per cubic meter (µg/m³). For context, the World Health Organization (WHO) recommends an annual average of 5 µg/m³, while the U.S. Environmental Protection Agency (EPA) sets the primary standard at 9 µg/m³. 

About one-third of the children included in the study were exposed to concentrations below the EPA’s primary standard, Whitman says. 

Exposure times varied, ranging from the prenatal period to mid-childhood. Children were approximately 9 years old at the time of IQ testing. IQ is typically scored on a scale greater than 100, but children have a mean (average) score of 100.

For every 1 microgram per cubic meter increase in PM2.5, the researchers found “statistically significant” decreases of 0.39 points in performance IQ (PIQ), an important aspect of cognition. That compares to a loss of 0.24 points in verbal IQ (VIQ) and 0.27 points in Full Scale IQ (FSIQ, a composite measure of PIQ, VIQ, and other indices of cognitive function).

While the study showed a fractional decrease of PIQ, its implications are important, Whitman says. PIQ is a measure of critical thinking and “fluid intelligence,” including tasks like nonverbal problem-solving, pattern recognition, and visual-spatial reasoning.

“[The] significant association demonstrates that air pollution can affect a child’s cognitive function even at extremely low levels that are considered ‘safe’ by current air quality standards,” she says. 

Societal consequences of children’s cognitive decline 

Additionally, the impact of IQ loss attributable to PM2.5 exposure becomes more profound when considered at the population level, Whitman says. 

PM2.5 concentrations are often disproportionately higher in low-income and marginalized communities, meaning children in these areas are more likely to be affected by the cognitive impacts of pollution—and less likely to have the resources to mitigate adverse outcomes. 

A population-wide reduction in mean mental abilities by as little as five IQ points results in a more than 50% decrease in the number of children with superior intelligence (IQ above 130). It also leads to a corresponding increase in the number of children with IQ scores below 70, the researchers say.

“With approximately 2.2 billion children worldwide, even modest increases in pollution can result in a significant excess number of children experiencing cognitive declines, potentially affecting their academic performance and future opportunities,” Whitman says.

The potential for total losses in cognitive function due to PM2.5, given its prevalence, could be as large or more prominent than those caused by exposures to other widespread neurotoxins, such as lead and methylmercury, researchers say. The chemicals in flame retardants (polybrominated diphenyl ethers, or PBDEs) and pesticides also are associated with children’s cognitive loss.

Significant loss of cognitive capacity in a population represents a “massive erosion” of human capital, the researchers say.  

“At the same time, the significant increase in the number of children with reduced cognitive capacities imposes substantial economic and social burdens on societies by reducing the lifelong productivity of future generations and increasing the need for remedial education and custodial care,” they say.

Other potential factors may be at play in reduced IQ

Limitations of the study include the scarcity of available data on early-life exposure to air pollution, note the researchers, saying their findings should be “interpreted cautiously” and reexamined as more data emerges.

Among other factors, adverse environmental exposures such as endocrine disruptors may have impacted children’s IQ scores. The study also could not account for differences in the chemical constituents of PM2.5 pollution that may occur in different places around the world. For example, biomass burning in wildfires produces PM2.5 with higher concentrations of chemicals than vehicle emissions, the study shows.

To better understand the health risks, the researchers recommend that all IQ loss—not only severe IQ loss—caused by harmful environmental exposures such as PM2.5 should be included in the Global Burden of Disease (GBD). Future studies should also seek to track children’s IQ over time and PM2.5 exposure at the individual level, with biomarkers and wearable devices, to better quantify its neurotoxic impact. 

“Understanding the magnitude of this effect is critical for establishing public health policies that will protect children’s health, preserve human capital, and support societal progress,” they say. 

Discover how the air in your city impacts health at BreatheLife.

Reference

Alter, N.C., Whitman, E.M., Bellinger, D.C. et al. Quantifying the association between PM2.5 air pollution and IQ loss in children: a systematic review and meta-analysis. Environ Health 23, 101 (2024). https://doi.org/10.1186/s12940-024-01122-x