Long-term exposure to high levels of the industrial degreasing chemical trichloroethylene (TCE) was associated with an increased risk for Parkinson’s disease (PD), a new study showed.

Using data from the US Environmental Protection Agency (EPA) National Air Toxics Assessment, researchers found that individuals exposed to higher TCE concentrations in outside air were 10% more likely to be diagnosed with PD than those exposed to the lowest TCE levels.

Exposure to the chemical is common, researchers said, citing a 2000 EPA report that found TCE in up to 30% of drinking water supplies across the US.

“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. This underscores the need for stronger regulations and more monitoring of industrial pollutants,” lead author Brittany Krzyzanowski, PhD, assistant professor at the Barrow Neurological Institute in Phoenix, said in a press release.

The results were published online on October 1 in the journal Neurology.

Persistent Pollutant

Primarily used as an industrial cleaner and degreaser, TCE is also found in consumer products, including lubricants, adhesives and sealants, paints and coatings, automotive care products, and cleaning products.

A persistent environmental pollutant and known carcinogen, TCE has been found in the air, water, and soil and can take years to break down through evaporation.

Although previous research implicated TCE in the pathogenesis of PD, investigators said this was the first study to identify a potential link to PD through outdoor ambient exposure.

Using a nationwide, population-based, case-control study of Medicare beneficiaries from 2014 to 2018, researchers identified 221,789 individuals (median age, 79 years; 86% White; 45% women) with PD and 1,132,765 matched control individuals (median age, 76 years; 84% White; 57% women).

Researchers used beneficiaries’ residential zip codes to estimate the level of TCE exposure 2 years before PD diagnosis. Most Medicare beneficiaries stayed in the same zip code for the 8 years preceding a diagnosis of PD (82%) or inclusion in the control group (84%).

Growing Body of Evidence

The results showed a dose-dependent 10% higher risk of developing PD (risk ratio, 1.10; P < .05) among individuals in zip codes with the highest TCE exposure (0.14-8.66 μg/m³) compared with residents in zip codes with the lowest TCE levels (0.005-0.01 μg/m³).

High TCE exposure was also associated with prodromal gait abnormalities (odds ratio [OR], 1.07; 95% CI, 1.06-1.08), falls (OR, 1.04; 95% CI, 1.03-1.05), and dementia (OR, 1.05; 95% CI, 1.04-1.06).

Ambient TCE levels were highest in the Rust Belt area of the US relative to other regions. Researchers also noted a higher rate of PD in the area near two of three TCE-emitting facilities in the country with the highest rate of ambient TCE. The risk for PD decreased as individuals moved farther away from these facilities.

“These findings add to a growing body of evidence that environmental exposures may contribute to Parkinson’s disease,” Krzyzanowski said.

‘Calling Card’ for Neurologists

While an EPA ban set to go into effect later this year would prohibit the manufacture and most uses of TCE, “the ban does little to address the thousands of sites contaminated with the persistent chemicals in the United States and globally,” wrote E. Ray Dorsey, MD, of the Atria Research and Global Health Institute in New York City, and Caroline M. Tanner, MD, PhD, of the Weill Institute for Neurosciences at the University of California, San Francisco, in an accompanying editorial.

There are more than 100 TCE and perchloroethylene-emitting sites in Rochester, New York, alone, Dorsey and Tanner added, and the use of TCE is increasing worldwide.

“Banning use, informing populations near contaminated sites, mitigating exposure, and cleaning up polluted areas can protect millions from its harmful effects and are long overdue,” they wrote.

The current study adds to a growing link between PD and chemicals in the environment such as TCE, pesticides from golf courses, and particulate matter, they added.

“Diseases have causes, and the calling card of neurologists is to find them, whether for stroke, seizures, headaches, or Parkinson’s disease,” Dorsey and Tanner wrote.

“We now need to vastly expand our efforts to find the causes of Parkinson’s and neurodegenerative diseases more broadly. Our legacy should not be millions suffering needlessly with a preventable disease, but generations spared the indignity and disability of a terrible one,” they added.

This study was supported by the US Department of Defense, the Kemper and Ethel Marley Foundation, the Barrow Neurological Foundation, and the Moreno Family. Krzyzanowski reported having no relevant financial relationships. Dorsey reported having financial relationships with the American Neurological Association, Averitas Pharma, Biogen, Biohaven Pharmaceuticals, BioSensics, Genentech/Roche, The Michael J. Fox Foundation, WebMD, and others. Tanner reported receiving consulting fees from Bial, CNS Ratings, Grey Matter, Jazz/Cavion, and Praxis; and honoraria or payment from Guidemark Health, the American Academy of Neurology, the International Association of Parkinsonism and Related Disorders, Biogen, Roche/Genentech, and others. Full disclosures are available in the original articles.