The specific component of fine particle air pollution, or PM2.5, ammonium nitrate, is also implicated in Alzheimer’s and dementia risk in adults, suggesting that PM2.5 may cause neurocognitive harm across the lifespan. Ammonium nitrate forms when ammonia gas and nitric acid, produced by agricultural activities and fossil fuel combustion, respectively, react in the atmosphere.

The findings appear in Environmental Health Perspectives.

“Our study highlights the need for more detailed research on particulate matter sources and chemical components,” said senior author Megan Herting, an associate professor of population and public health sciences at the Keck School of Medicine of USC. “It suggests that understanding these nuances is crucial for informing air quality regulations and understanding long-term neurocognitive effects.”

For the last several years, Herting has been working with data from the largest brain study across America, known as the Adolescent Brain Cognitive Development Study, or ABCD, to understand how PM2.5 may affect the brain.

PM2.5, a key indicator of air quality, is a mixture of dust, soot, organic compounds and metals that come in a range of particle sizes less than 2.5 micrometers in diameter. PM2.5 can travel deep into the lungs, where these particles can pass into the bloodstream, and bypass the blood-brain barrier, causing serious health problems.

Fossil fuel combustion is one of the largest sources of PM2.5, especially in urban areas, but sources like wildfires, agriculture, marine aerosols and chemical reactions are also important.

In 2020, Herting and her colleagues published a paper in which they looked at PM2.5 as a whole, and its potential impact on cognition in children, and did not find a relationship.

For this study, they used special statistical techniques to look at 15 chemical components in PM2.5 and their sources. That’s when ammonium nitrate — which is usually a result of agricultural and farming operations — in the air appeared as a prime suspect.

“No matter how we examined it, on its own or with other pollutants, the most robust finding was that ammonium nitrate particles were linked to poorer learning and memory,” Herting said. “That suggests that overall PM2.5 is one thing, but for cognition, it’s a mixture effect of what you’re exposed to.”

For their next project, the researchers hope to look at how these mixtures and sources may map on to individual differences in brain phenotypes during child and adolescent development.

In addition to Herting, other study authors include Rima Habre, Kirthana Sukumaran, Katherine Bottenhorn, Jim Gauderman, Carlos Cardenas-Iniguez, Rob McConnell and Hedyeh Ahmadi, all of the Keck School of Medicine; Daniel A. Hackman of the USC Suzanne Dworak-Peck School of Social Work; Kiros Berhane of the Columbia University Mailman School of Public Health; Shermaine Abad of University of California, San Diego; and Joel Schwartz of the Harvard T.H. Chan School of Public Health.

The research was supported by grants from the National Institutes of Health [NIEHS R01ES032295, R01ES031074, P30ES007048] and the Environmental Protection Agency [RD 83587201, RD 83544101].