"Safe" levels of air pollution can negatively impact brain development and cause cognitive issues

Since air pollution is known to contribute to a variety of diseases and hinder brain development, international regulators such as the Environmental Protection Agency The EPA has set strict limits on emissions.

However, according to a new longitudinal study led by the University of Southern California (USC), even pollution levels long thought to be safe could increase the risk of health problems. This includes cognitive and developmental issues with the brain.

The researchers used brain scan data from over 9,000 participants in the Adolescent Brain Cognitive Development (ABCD) study. This was the largest nationwide study of youth brain health in the United States.

The experts found that children exposed to air pollutants showed significant changes in connectivity between various brain areas. They found more connections than normal in certain brain regions, and fewer in others.

“A deviation in any direction from a normal trajectory of brain development – whether brain networks are too connected or not connected enough – could be harmful down the line,” said study lead author Devyn L. Cotter, a doctoral student in Neuroscience at USC.

Heightened risk of hindering brain development

Communications between different regions of the brain are crucial for navigating each moment of our day. This ranges from how we collect information about our surroundings to how we think and feel.

Many of these connections are forged early, between the ages of nine and 12. They play a significant role in children’s cognitive and emotional brain development.

“Air quality across America, even though ‘safe’ by EPA standards, is contributing to changes in brain networks during this critical time, which may reflect an early biomarker for increased risk for cognitive and emotional problems later in life,” explained senior author Megan Herting, an associate professor of Population and Public Health Sciences at USC.

How the research on brain development was conducted 

To explore the relation between air pollution and brain development, Herting and her team analyzed functional MRI scans from 9,497 participants aged 9-10 from the ABCD study. They then selected a subset of this cohort for collecting additional scans two years later to assess how brain connectivity changed over time.

Their main focus was on the salience, frontoparietal, and default-mode brain networks, along with the amygdala and the hippocampus. These are critical regions of the brain involved in emotion, learning, memory, and other complex cognitive functions.

Afterwards, the scientists used EPA and other data to map air pollution at each participant’s residence. These included levels of fine particulate matter (PM_2.5), nitrogen dioxide (NO₂), and ground-level ozone (O₃). They used these measurements to investigate how these levels relate to changes in brain connectivity over time. 

What the researchers discovered about brain development

The analysis revealed that greater exposure to PM_2.5 led to relative increases in functional connectivity between different brain areas. Also, increased exposure NO₂ was linked to relative decreases in connectivity.

Finally, exposure to O₃ was associated with greater connections within the brain’s cortex. However, fewer connections were associated between the cortex and other areas, such as the hippocampus and amygdala.

These findings highlight the urgent need for regulators to also take into account brain health, in addition to respiratory and cardiometabolic health, when setting or adjusting recommendations for safe air quality levels. For instance, although EPA proposed strengthening standards for PM_2.5 this year, guidelines for NO₂ levels have not changed since they were first set over half a century ago.

In future research, the scientists plan to examine more closely how the chemical structures of each pollutant impacts the brain in order to help further refine regulations, while continuing to use data from the ABCD study to analyze brain health over time.

“On average, air pollution levels are fairly low in the U.S., but we’re still seeing significant effects on the brain. That’s something policymakers should take into account when they’re thinking about whether to tighten the current standards,” Cotter concluded.

The study is published in the journal Environment International.

More about the EPA and air quality standards

The Environmental Protection Agency (EPA) in the United States sets and enforces standards for air quality under the authority of the Clean Air Act (CAA).

The CAA was originally passed in 1970 and significantly amended in 1977 and 1990. The main purpose of these standards is to protect public health and the environment from harmful effects of air pollution.

The Clean Air Act established two types of national air quality standards:

National Ambient Air Quality Standards (NAAQS)

NAAQS are set for six principal pollutants, also known as “criteria” pollutants. These include carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter (both PM10 for particles less than or equal to 10 micrometers in diameter, and PM2.5 for particles less than or equal to 2.5 micrometers in diameter), and sulfur dioxide.

NAAQS are set at levels that EPA has determined will protect public health. They include an adequate margin of safety from a variety of adverse effects.

There are two types of NAAQS:

Primary

Primary standards provide public health protection, including protecting the health of “sensitive” populations. These include asthmatics, children, and the elderly.

Secondary

Secondary standards provide public welfare protection. These include protection against decreased visibility and damage to animals, crops, vegetation, and buildings.

New Source Performance Standards (NSPS)

The NSPS are industry-wide standards for new and modified sources within specific industrial sectors. The standards are designed to implement feasible measures to limit emissions from these sources. They cover a range of pollutants and industries and are based on the performance of technologies.

In addition to these, the Clean Air Act also regulates Hazardous Air Pollutants (HAPs). These are pollutants that are known or suspected to cause cancer or other serious health effects. There are currently 187 pollutants and compound groups listed as HAPs under the Clean Air Act.

The EPA regularly reviews these standards and updates them if necessary based on the latest scientific research. The process for setting and reviewing these standards is rigorous and transparent. It involves a comprehensive review of the scientific literature, risk and exposure assessments, and opportunities for public input and comment.

The Clean Air Act also establishes mechanisms for enforcing these standards. If an area is not meeting the standards for a particular pollutant (referred to as a “nonattainment area”), it must develop a plan to reduce emissions and improve air quality.

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By Andrei Ionescu, Earth.com Staff Writer

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