Air pollution linked to neurological disorders

BOSTON, Mass.—Air pollution has been significantly associated with an increased risk of hospital admissions for neurological disorders — including Parkinson’s, Alzheimer’s and other dementias — in a long-term study of more than 63 million older adults in the U.S. The research was led by researchers at Harvard T.H. Chan School of Public Health.

A study, conducted with colleagues at Emory University’s Rollins School of Public Health and Columbia University’s Mailman School of Public Health, is reportedly the first nationwide analysis of the link between fine particulate (PM2.5) pollution and neurodegenerative diseases in the U.S. The researchers crunched an unparalleled amount of data compared to previous studies of air pollution and neurological disorders.

The article, entitled “Long-term effects of PM2.5 on neurological disorders in the American Medicare population: a longitudinal cohort study,” has been published online in The Lancet Planetary Health.

“The 2020 report of the Lancet Commission on dementia prevention, intervention, and care has added air pollution as one of the modifiable risk factors for these outcomes,” said Xiao Wu, doctoral student in biostatistics at Harvard Chan School and co-lead author of the study. “Our study builds on the small but emerging evidence base indicating that long-term PM2.5 exposures are linked to an increased risk of neurological health deterioration, even at PM2.5 concentrations well below the current national standards.”

Researchers looked at 17 years (2000-2016) of hospital admissions data from 63,038,019 U.S. Medicare recipients, and linked it with estimated PM2.5 concentrations by zip code. Taking into account potential confounding factors like socioeconomic status, they found that for each 5 microgram per cubic meter of air (μg/m3) increase in annual PM2.5 concentrations, there was a 13% increased risk for first-time hospital admissions for Parkinson’s, Alzheimer’s and related dementias.

This risk remained elevated even below supposedly safe levels of PM2.5 exposure — which, according to current U.S. Environmental Protection Agency standards, is an annual average of 12 μg/m3 or less. The study also found that women, white people and urban populations were particularly susceptible.

“Among the effect modifiers, we found PM2.5 effect estimates that were significantly larger in magnitude among individuals in more urban areas versus those in less urban areas (as expressed in quartiles of population density),” states the article. “We also observed higher HRs among those who identified as white than those who identified as Black or Asian, Hispanic, American Indian or Alaskan Native, and unknown, and for women compared with men (figure 3).

“Our findings regarding associations between PM2.5 and Alzheimer’s disease and related dementias are consistent with previous research, both in terms of direction and magnitude; of these, one was done in Ontario’s Canadian population,⁸ and the other two were done in regional subpopulations of U.S. Medicare enrollees.^{7, 9} Mixed results, including both positive and null findings, however, were reported for the association between PM2.5 and Parkinson’s disease in the literature.^{9, 22, 23}

“It is worth noting that a comprehensive city-level study in 50 northeastern US cities among Medicare enrollees found higher estimates in magnitude for Parkinson’s disease and Alzheimer’s disease and related dementias than the ones estimated in this study,⁹ which matches our finding of significantly higher PM2.5 effects among urban dwellers,” the article notes. “Other studies also found similar results in the urban populations they investigated.^{7, 8} The observed associations for the other grouping within race are not clear and more work is needed to understand these results. We note, however, that the percentage of the population aged older than 65 years in the USA who are not white or Black is 6-8%.”

“Our U.S.-wide study shows that the current standards are not protecting the aging American population enough, highlighting the need for stricter standards and policies that help further reduce PM2.5 concentrations and improve air quality overall,” pointed out Antonella Zanobetti, principal research scientist in Harvard Chan School’s Department of Environmental Health and co-senior author of the study.

The highest risk for first-time Parkinson’s hospital admissions was among older adults in the northeastern U.S. For first-time Alzheimer’s disease and related dementias, older adults in the Midwest faced the highest risk.

“Air pollution might play a key role in neuroinflammation and further exacerbate or initiate dysfunctional protein handling, in the context of amyloid plaques, tau hyperphosphorylation, and neurofibrillary tangles.²⁷ Several air pollutants, including PM2.5 and ultrafine (<0·1 μm) particulate matter, have been shown to easily cross the blood–brain barrier, providing an important route for air pollutants to interact with the CNS [central nervous system],” the article continues. “Indeed, increases in air pollution can elicit increases in the inflammatory response in the prefrontal lobes, with concomitant increases in oxidative damage and amyloid β deposition.

“Given the pathogenesis of Alzheimer’s disease and other neurogenerative diseases that are defined by neuroinflammation, oxidative damage, and protein misfolding, exposure to air pollution might serve as an important risk factor in the development and progression of Alzheimer’s disease and Parkinson’s disease pathology and concomitant neurobehavioral deficits.”

This study was supported by the Health Effects Institute, the National Institute of Environmental Health Sciences, the National Institute on Aging and the HERCULES Center. Research described in this article was completed under contract to the Health Effects Institute, an organization jointly funded by the U.S. Environmental Protection Agency and some motor vehicle and engine manufacturers.