Long-term Exposure to Air Pollution May Increase Risk of Alzheimer’s, Other Neurological Disorders

A version of this media release was first published on the website of Emory University's Rollins School of Public Health.

A recent nationwide cohort study published in Nature Communications has found that long-term exposure to air pollution may increase the risks for developing dementia and Alzheimer’s disease.

The study was led by researchers at Emory University’s Rollins School of Public Health, in collaboration with assistant professor Pengfei Liu and professor Rodney Weber of Georgia Tech's School of Earth and Atmospheric Sciences. Harvard University's T.H. Chan School of Public Health was also involved in the study, which is the first nationwide analysis of the links between key criteria air pollutants — including fine particulate (PM2.5), nitrogen dioxide (NO₂), and ozone (O₃) — and neurodegeneration incidence in the United States Medicare population.

"The results point to traffic emissions as a main culprit," says Weber. "Whether the PM2.5 specimens that cause the neurodegeneration are from vehicle tail pipes or from tire and brake wear is important to determine. It not only affects mitigation strategies, but the conversion to electric vehicles in the future may or may not help to mitigate this hazard."

Of the pollutants analyzed, exposures to PM2.5 and NO₂ showed the greatest risk for incidence of dementia and Alzheimer’s disease (AD), with effects being the strongest for PM2.5. Putting these findings into context, the national average PM2.5 is around 7 micrograms per cubic meter of air (µg/m³). In looking at larger cities, like Houston and Los Angeles, annual levels in 2020 were above 10 µg/m³.

The study’s findings indicate that air pollution differences like these, of 3 µg/m³, would lead to a predicted 7% increase in AD between more polluted and less polluted cities. Despite variations in the level of PM2.5 from city to city, the authors note that no safe levels actually exist when it comes to the risk of PM2.5 on neurodegeneration incidence.

“We observed a very strong signal between PM2.5 exposure and increased risks of dementia and Alzheimer’s disease,” says Liuhua Shi, assistant professor at Rollins and co-lead author on the paper. “To better inform policy for targeted source-specific regulations, it is important to further investigate the relative contributions of various PM2.5 components to these conditions, which we are planning to do next.”

Additional findings showed that NO₂ was also associated with increases in incidences of dementia and Alzheimer’s disease. Exposure to O₃ did not show an increase in incidence.

The Environmental Protection Agency’s (EPA) current air quality standards for fine particle pollution is 12 µg/m3. However, according to these findings, that may not be nearly low enough.

“We still found strong effects on the risk of developing Alzheimer’s disease down to 4 µg/m³, and the effect was even stronger between 4 and 8 than it was above 8 µg/m³,” says Joel Schwartz, professor of environmental epidemiology at the Harvard Chan School, and a co-author on the paper.

The researchers utilized Medicare data from 2000-2018 for the study. “It provided a very rich database, with 2 million cases of dementia and 800,000 cases of AD in our population of 12 million Medicare patients,” says Kyle Steenland, professor at Rollins and a co-lead author on the paper.

“This study suggests that air pollution might serve as a risk factor for dementia and Alzheimer’s disease,” says Liu, a co-author on the paper. “However, this risk could be potentially mitigated if we further reduce the emissions of hazardous air pollutants.”

This project was supported by the National Institute on Aging (NIA/NIH R01 AG074357), the HERCULES Center P30 ES019776, and the Goizueta Alzheimer's Disease Research Center of Emory University (P50 AG025688). https://doi.org/10.1038/s41467-021-27049-2