Persistent Pollutant Outdoor Air Exposure to Chemical May Raise Risk of Parkinson’s Disease

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Long-term exposure to the industrial solvent trichloroethylene (TCE) outdoors may be linked to an increased risk of Parkinson’s disease, according to a large nationwide study published in the October 1, 2025, issue of Neurology, the medical journal of the American Academy of Neurology.

Trichloroethylene is a chemical used in metal degreasing, dry cleaning and other industrial applications. Although TCE has been banned for certain uses, it remains in use today as an industrial solvent and is a persistent environmental pollutant in air, water and soil across the United States. The study does not prove that TCE exposure causes Parkinson’s disease, it only shows an association.

“In this nationwide study of older adults, long-term exposure to trichloroethylene in outdoor air was associated with a small but measurable increase in Parkinson’s risk,” said study author Brittany Krzyzanowski, PhD, of Barrow Neurological Institute in Phoenix. “These findings add to a growing body of evidence that environmental exposures may contribute to Parkinson’s disease.”

Researchers used Medicare data to identify people over age 67 newly diagnosed with Parkinson’s between 2016 and 2018. Each person was compared with five people who did not have the disease. After removing people without home ZIP+4 information, the study included 221,789 people with Parkinson’s and over 1.1 million people without the disease.

They mapped exposure to outdoor TCE concentrations using U.S. Environmental Protection Agency data and participants’ residential neighborhood based on their ZIP +4 location. Air levels of TCE were estimated by U.S. Census tract, a small area within a county. Each participant’s exposure was based on their neighborhood two years prior to diagnosis.

Researchers divided participants into 10 groups based on their estimated TCE exposure. Those in the lowest exposure group experienced levels between 0.005 and 0.01 micrograms per cubic meter (μg/m³), while those in the highest group had exposures ranging from 0.14 to 8.66 μg/m³.

After adjusting for other factors that could affect the risk of Parkinson’s, including age, smoking history and exposure to fine particulate air pollution, researchers found people exposed to the highest outdoor TCE levels had a 10 % increased risk of Parkinson’s disease compared to people exposed to the lowest levels.

The researchers also identified several geographic “hot spots” where outdoor TCE levels were highest, particularly in the Rust Belt region of the U.S. and smaller pockets across the country. They then analyzed Parkinson’s risk in the 10 miles surrounding the three top TCE-emitting facilities in the U.S. from 2002. For two of the areas, risk was higher closer to the facilities, and at one of those sites, there was a clear increasing incremental risk the closer people lived to the facility.

“While the increased risk was modest, the sheer number of people exposed to TCE in the environment means the potential public health impact could be substantial,” said Krzyzanowski. “This underscores the need for stronger regulations and more monitoring of industrial pollutants.”

A limitation of the study is that it focused only on Medicare-aged individuals, so findings may not apply to younger people or those with early-onset Parkinson’s disease. In addition, TCE exposure estimates were based on outdoor air levels in 2002 and may not reflect individual lifetime or indoor exposures.

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