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Researchers found a nationwide association between incident Parkinson's disease and air pollution, which varied in strength by region, and identified a hot spot in the Mississippi-Ohio River Valley.

The relative risk for incident Parkinson's increased by 25% (95% CI 23-26) comparing counties with the lowest to the highest quartile of PM_2.5 (fine particulate matter less than 2.5 microns in diameter) throughout the U.S., reported Brittany Krzyzanowski, PhD, of the Barrow Neurological Institute in Phoenix, in an abstract released in advance of the American Academy of Neurology annual meeting.

The strongest association between PM_2.5 and incident Parkinson's was in the Rocky Mountain region, including Lake County, Colorado. Fine particulate matter was also associated with higher rates of Parkinson's in a Mississippi-Ohio River Valley hot spot that includes Tennessee and Kentucky, but the relationship was weaker due to an apparent ceiling effect.

The Mississippi-Ohio River Valley region has some of the highest levels of particulate matter in the nation, Krzyzanowski told MedPage Today.

"Finding a relatively weaker association where we have some of the highest Parkinson's disease risks and fine particulate matter levels is consistent with the threshold effect we observed in our data," she said.

"In the Mississippi-Ohio River Valley, for example, Parkinson's disease risk increases with increasing air pollution exposure until about 15 µg/m³ of fine particulate matter, where Parkinson's disease risk seems to plateau," she added.

The findings add important evidence to the role that air pollution may play in Parkinson's disease, noted Ray Dorsey, MD, of the University of Rochester in New York, who wasn't involved with the study.

Recent research has demonstrated that fine particles and the toxicants they may be carrying , Dorsey pointed out. "Other environmental toxicants, such as certain pesticides and dry cleaning chemicals, hurt mitochondria and are also inhaled," he told MedPage Today.

"This all suggests a possible common mechanism of injury and portal of entry for environmental toxicants," he added. "The answer to what is fueling the rise of many brain diseases may be under our noses."

Recent studies have tied Parkinson's with nitrogen dioxide (NO₂) exposure, he noted. In 2022, researchers published an to help address contaminants like pesticides, metals, and industrial chemicals that are implicated in Parkinson's disease.

Krzyzanowski and co-authors conducted a geographic study of 22.5 million Medicare beneficiaries, including 83,674 people with incident Parkinson's disease in 2009. They geocoded beneficiaries to county and zip code of residence in the contiguous U.S., and performed an individual-level case-control analysis to verify county-level results.

People in the quartile with the highest fine particulate matter had an average annual exposure of 19 µg/m³; those in the lowest quartile had an average annual exposure of 5 µg/m³. Incident Parkinson's cases were 434 per 100,000 population in the highest PM_2.5 quartile and 359 per 100,000 in the lowest.

Individual-level data confirmed that Parkinson's incidence increased by 25% (95% CI 20-29) when comparing the lowest to the highest decile of PM_2.5.

"By mapping nationwide levels of Parkinson's disease and linking them to air pollution, we hope to create a greater understanding of the regional risks and inspire leaders to take steps to lower risk of disease by reducing levels of air pollution," Krzyzanowski said in a statement.

A study limitation is that fine particulate matter contains a variety of airborne pollutants, some of which may be more toxic than others, Krzyzanowski acknowledged. Air pollution is tied to other health risks including dementia, which may reduce Parkinson's diagnoses and explain the relatively weaker association found in the Mississippi-Ohio River Valley, she added.

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