Parkinson’s risk reflected in regional differences in air pollution: US study

Researchers hope study will lead to stricter pollution policies to mitigate risk

Steve Bryson, PhD avatar

by Steve Bryson, PhD |

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Exposure to an air pollutant called fine particular matter (PM2.5) is significantly associated with an increased risk of developing Parkinson’s disease, according to a U.S. population study.

“Using state-of-the-art geospatial analytical techniques, we were, for the first time, able to confirm a strong nationwide association between incident Parkinson’s disease and fine particulate matter in the U.S.,” Brittany Krzyzanowski, PhD, the study’s senior author at the Barrow Neurological Institute in Arizona, said in a press release.

Parkinson’s hot spots, or regions with a high rate of the neurodegenerative disease, were associated generally with higher air levels of PM2.5, whose sources include motor vehicles, fires, and power plants.

Exposure to median levels of the pollutant is associated with a 56% increased risk of developing Parkinson’s relative to the lowest level exposure, researchers found.

Those living in the Mississippi-Ohio River Valley in particular had a 19% greater Parkinson’s risk than the rest of the nation, while those in the western half of the country had a reduced risk.

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“Regional differences in Parkinson’s disease might reflect regional differences in the composition of the particulate matter,” Krzyzanowski said, adding that “some areas may have particulate matter containing more toxic components compared to other areas.”

The study, “Fine Particulate Matter and Parkinson Disease Risk Among Medicare Beneficiaries,” was published in the journal Neurology.

Numerous studies suggest environmental exposure to air pollution plays a role in the development of Parkinson’s, a neurodegenerative condition known for its movement-related symptoms.

“Previous studies have shown fine particulate matter to cause inflammation in the brain, a known mechanism by which Parkinson’s disease could develop,” Krzyzanowski said.

PM2.5 are particles less than 2.5 micrometers in diameter, about 30 times smaller than a human hair, that comprise soot, aerosols, mold spores, and the like.

Their tiny size allows them to enter the body through breathing and cross the highly selective membrane that prevents microbes and large molecules in circulation from entering the brain.

A previous European study of more than 200,000 adults found an increased risk of dying from Parkinson’s due to long-term exposure to air pollutants such as fine particulate matter.

To investigate the potential links between PM2.5 exposure and the risk of Parkinson’s across the U.S., Krzyzanowski’s team used 2009 data from Medicare to calculate the number of newly diagnosed Parkinson’s patients for several U.S. regions.

Medicare is a government-funded health insurance program that includes people 65 years and older, and those with certain types of disabilities.

Then, average air pollution exposure levels from 1998 to 2000, before Parkinson’s onset, were established based on counties and ZIP codes sourced from annual PM2.5 concentrations. Nationwide PM2.5 exposure was divided into 10 levels, ranging from 2.16 to 23.30 micrograms per cubic meter (mcg/m3).

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89,790 million people on Medicare in 2009 newly diagnosed with Parkinson’s

Results showed that out of nearly 22 million people enrolled in Medicare in 2009, about 89,790 had been newly diagnosed with Parkinson’s. After adjusting for potential influencing factors, such as age, sex, race, smoking history, and medical care use, the team found higher PM2.5 levels were significantly associated with a higher risk of Parkinson’s.

There was an 11% to 28% increased risk in the top nine levels of PM2.5 exposure compared with the lowest level. The risk increased significantly and gradually to 24% in the five lower levels up to 13 mcg/m3, corresponding to a 4.2% greater Parkinson’s risk for each additional mcg/m3 of PM2.5.

Beyond this level, the risk association rose more slowly and non-significantly, with a maximum increased risk, by 28%, in the eighth level.

People exposed to the median level of PM2.5 (12.93 mcg/m3) had a 56% significantly greater risk of Parkinson’s compared with those with the lowest level (2.16 mcg/m3), and those exposed to the highest levels had a 60% higher risk.

Similar risks were obtained when analyses were adjusted for medical conditions associated with PM2.5 that might facilitate Parkinson’s diagnosis, socioeconomic factors, access to neurologists, and pesticide use. This pattern was also the same across ages and for men and women, with slightly stronger associations among women.

“Despite years of research trying to identify the environmental risk factors of Parkinson’s disease, most efforts have focused on exposure to pesticides,” Krzyzanowski said. “This study suggests that we should also be looking at air pollution as a contributor in the development of Parkinson’s disease.”

When looking at different geographic regions across the U.S., the team found a general spatial association, with counties with above-average Parkinson’s risk also showing above-average PM2.5 levels and those with below-average risk having below-average exposure levels.

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Parkinson’s risk 19% greater in Mississippi-Ohio River Valley hot spot

The risk of Parkinson’s was 19% greater in the Mississippi-Ohio River Valley hot spot, compared with the rest of the nation. This region has a high road network density, Krzyzanowski said, meaning “the pollution in these areas may contain more combustion particles from traffic and heavy metals from manufacturing which have been linked to cell death in the part of the brain involved in Parkinson’s disease.”

However, the strongest links between PM2.5 and Parkinson’s risk were detected in the western region — a cold spot — with 51 counties in the mountainous regions of Colorado and Wyoming showing a 15%-16% increased risk with each increasing level of PM2.5 exposure.

A weaker association was detected within the hot spot region, which included 118 counties north of the Mississippi-Ohio rivers in Indiana, Ohio, Illinois, and Missouri. Here, Parkinson’s risk increased by 3%-4% with each additional level of PM2.5 exposure.

There were also regions where low PM2.5 levels were linked to a higher Parkinson’s risk, including the North Dakota-Minnesota border, parts of the Mid-Atlantic, South-Atlantic, and part of Washington State, Idaho, and Montana.

“We identified strong regional associations between PM2.5 and [Parkinson’s disease] in the U.S,” the researchers wrote. “A deeper investigation into the subfractions of PM2.5 in those regions may provide insight into [Parkinson’s disease] risk factors.”

“Population-based geographic studies like this have the potential to reveal important insight into the role of environmental toxins in the development and progression of Parkinson’s,” Krzyzanowski said.

The study was supported by The Michael J. Fox Foundation for Parkinson’s Research, the U.S. Department of Defense, and the National Institute of Environmental Health Sciences.