Outdoor air pollution from power plants, fires and cars continues to degrade human, animal and environmental health around the globe. New research shows that even pollution levels that are below government air-quality standards are associated with differences in children’s brains.

A University of California, Davis, research team systematically analyzed 40 empirical studies, the majority of which had found that outdoor air pollution is associated with differences in children’s brains. These differences include volumes of white matter, which is associated with cognitive function, connections throughout the brain and even early markers for Alzheimer’s.

The study, “Clearing the Air:  A systematic review of studies on air pollution and childhood brain outcomes to mobilize policy change,” was published this month in Developmental Cognitive Neuroscience.

“We're seeing differences in brain outcomes between children with higher levels of pollution exposure versus lower levels of pollution exposure,” said Camelia Hostinar, an associate professor of psychology and the study’s corresponding author.

Children and teens are especially vulnerable to air pollution because their brains and bodies are still developing. They tend to spend more time outdoors, and their bodies absorb more contaminants relative to their bodyweight than adults, researchers said.

Outdoor air pollution and brain development

This study surveyed 40 published, peer-reviewed studies that all included measures of outdoor air pollution and brain outcomes for children at various ages, from newborns up to 18-year-old adults. The majority of studies came from the United States, Mexico and Europe, with one each from Asia and Australia.

The studies ranged in how they measured brain differences. Some used advanced scanning methods like magnetic resonance imaging, or MRI. Others tested changes in chemical compounds that aid in brain function and health. Some studies looked for tumors in the brain or central nervous system.

Studies from Mexico City that compared children from high- and low-pollution areas found significant differences in brain structure.

Each study included measures of air pollution linked to the child’s address or neighborhood, which showed that the children’s brain differences were observed in places with high levels of air pollution as well as places that met local air pollution standards.

“A lot of these studies include children in places with air pollutant levels that are well below limits set by U.S. or European regulations,” said Anna Parenteau, a Ph.D. student in psychology at UC Davis and the study’s co-first author.

Outdoor air pollution

Sources of outdoor air pollution include coal-fired plants, wildfires and many other sources near where people live. This systematic review is unique because most others have focused on how air pollution affects adults or animals, researchers said.

“We can't necessarily apply findings from adults and assume that it's going to be the same for children,” said Johnna Swartz, an associate professor of human ecology and co-author on the study. “We also have to look more at different developmental windows because that might be really important in terms of how air pollution might impact these brain outcomes.”

To establish a causal link between outdoor air pollution and differences in the brain, the research team looked to experimental research on animals. That research showed that pollution does lead to many of the same outcomes identified by the studies in this review, including markers for Alzheimer’s disease.

“A lot of researchers working on brain development, whether it's autism, Alzheimer's or something else, really discounted for a long time the environmental factors,” said Anthony Wexler, a professor at UC Davis and director of its Air Quality Research Center. “They argued that it’s genetic or some other factor other than exposure to air pollution. That's changed a lot recently because of all this research literature.”

Reducing harm

This systematic review proposed steps for both parents and policymakers to protect their children from outdoor air pollution by, for example, adding air filters to homes and schools near freeways.

“We listed air purifiers as one of the policy recommendations, and that is something that could be subsidized or provided in schools and other places where children spend a lot of time,” said Hostinar. “These can be quite effective.”

Researchers can also incorporate measures of air pollution into studies related to brain health or other health outcomes.

“Anybody collecting data from human participants on brain outcomes or cardiovascular outcomes or anything else could easily add questions to assess air pollution exposure, such as obtaining their addresses,” said Sally Hang, a Ph.D. student in psychology and the study’s first co-author.