Revision #1

Roughly one million Americans live with Parkinson's disease, and nearly 90,000 new cases are diagnosed each year — a figure 50% higher than previous estimates [1]. For decades, researchers attributed most cases to genetics and aging. But a growing body of evidence now implicates a colorless, volatile industrial solvent that has saturated American soil and groundwater for the better part of a century: trichloroethylene, or TCE.

First synthesized in the 1860s and adopted for industrial degreasing in the 1920s, TCE became one of the most widely used chemicals in American manufacturing [2]. U.S. production peaked in the 1970s at more than 600 million pounds annually [3]. It was used to clean metal parts in factories, degrease electronics in Silicon Valley, strip paint, and dry-clean clothes. Approximately 10 million Americans worked directly with TCE or similar chlorinated solvents during this period [3].

The chemical's legacy is now measured in contamination. TCE has been detected at more than 850 of the EPA's Superfund sites [4]. According to the Environmental Working Group, drinking water supplies serving more than 19 million Americans are contaminated with TCE [5]. Because TCE is denser than water and degrades slowly, it sinks through soil into aquifer systems, where it can persist for decades. It also evaporates readily, seeping into homes, schools, and workplaces through a process called vapor intrusion [4].

The Evidence Linking TCE to Parkinson's

The connection between TCE exposure and Parkinson's disease has been built through animal models, occupational studies, and large population analyses over the past 15 years.

The foundational human evidence came from a 2012 twin study published in Annals of Neurology by Drs. Samuel Goldman and Caroline Tanner of the Parkinson's Institute. Studying 99 twin pairs from a World War II veteran cohort — in which one twin had Parkinson's and one did not — they found that occupational exposure to TCE was associated with a sixfold increase in Parkinson's risk (odds ratio 6.1, 95% CI 1.2–33, p = 0.034) [6]. The study also identified a dose-response relationship, with cumulative exposure intensity scored across jobs and hobbies at thresholds of 2.5, 25, and 100 parts per million [6].

Source: Multiple peer-reviewed studies

Data as of Oct 1, 2025CSV

In 2023, a study published in JAMA Neurology examined veterans stationed at Marine Corps Base Camp Lejeune between 1975 and 1985, where water supplies were contaminated with TCE at levels exceeding 70 times the permissible amount. Among 84,824 Camp Lejeune veterans and 73,298 veterans stationed at Camp Pendleton (the control group), Camp Lejeune veterans had a 70% higher risk of developing Parkinson's disease (odds ratio 1.70, 95% CI 1.39–2.07, p < 0.001) [7]. The prevalence was 0.33% at Camp Lejeune versus 0.21% at Camp Pendleton [7]. Camp Lejeune veterans also showed elevated rates of Parkinson's prodromal symptoms, including tremor, anxiety, and erectile dysfunction [7].

A nationwide study published in Neurology in October 2025, using Medicare data on seniors over age 67 diagnosed between 2016 and 2018, found that individuals living in census tracts with the highest outdoor TCE levels had a 10% greater risk of Parkinson's compared with those in the lowest exposure areas [8]. The study also found a roughly 3% increase in Parkinson's risk for every one-unit increase in the log scale of TCE exposure [8]. Near one Oregon factory that used TCE, the risk increased approximately fourfold for people living one to five miles downwind [9].

The Biological Mechanism

TCE enters the body through inhalation, skin contact, or ingestion of contaminated water. Once inside, it readily crosses the blood-brain barrier [3].

The chemical undergoes biotransformation primarily through two pathways: cytochrome P450-mediated oxidation and glutathione conjugation [10]. These processes generate reactive metabolites — most notably 1-trichloromethyl-1,2,3,4-tetrahydro-β-carboline, known as TaClo — that are capable of disrupting cellular function [10].

TaClo and related metabolites inhibit Complex I of the mitochondrial electron transport chain, the same molecular target affected by other known Parkinson's-associated toxins like rotenone and MPTP [10][11]. Complex I is essential for ATP synthesis — the cell's primary energy currency. Its inhibition triggers oxidative stress, the release of cytochrome C from mitochondria, and ultimately apoptosis (programmed cell death) [11].

Dopaminergic neurons in the substantia nigra — the brain region whose degeneration defines Parkinson's — are particularly vulnerable to this damage because of their high metabolic demands and baseline oxidative load [11]. In animal models, systemic administration of TCE through oral gavage or inhalation produces selective loss of these dopamine-producing neurons, replicating the pathology observed in human Parkinson's patients [10][11].

A Century of Regulatory Delay

The gap between scientific evidence and regulatory action on TCE spans decades.

Occupational health concerns about TCE appeared in medical literature as early as the 1970s, when the chemical's neurotoxic properties were first documented [12]. OSHA established a permissible exposure limit (PEL) of 100 parts per million in 1971, shortly after the agency's creation [12]. That standard remains in effect today. OSHA itself has acknowledged that many of its PELs from this era, including TCE's, are "outdated and inadequate for ensuring protection of worker health" [12].

The EPA set a maximum contaminant level (MCL) for TCE in drinking water at 5 parts per billion [13]. Both the International Agency for Research on Cancer and the U.S. Department of Health and Human Services have classified TCE as a known human carcinogen, citing links to kidney cancer, liver cancer, and non-Hodgkin's lymphoma [4].

Yet comprehensive federal action on TCE's non-cancer risks — including neurotoxicity — did not begin until 2023. On October 31 of that year, the EPA proposed banning all uses of TCE after determining it presents "an unreasonable risk of injury to human health" [14]. On December 17, 2024, the EPA finalized the rule under the Toxic Substances Control Act, prohibiting most manufacturing, processing, distribution, and industrial uses of TCE, with most prohibitions taking effect within one year [15].

The American Chemistry Council, which represents chemical manufacturers, opposed the ban as "overly restrictive," arguing the EPA overestimated TCE exposure [16]. Some lawmakers and industry groups have since lobbied to reinstate TCE use [17]. The EPA has already extended compliance deadlines multiple times, with the SBA's Office of Advocacy requesting further delays for small businesses [18]. As of late 2025, the EPA further extended deadlines for certain exemption-related provisions in the final rule pending judicial review [18].

Source: OpenAlex

Data as of Jan 1, 2026CSV

Camp Lejeune: A Case Study in Federal Liability

Camp Lejeune represents the most extensively documented case of TCE exposure affecting a defined population. Drinking water at the base was contaminated with TCE, perchloroethylene (PCE), and other volatile organic compounds from 1953 until 1987, when contaminated wells were finally shut down [7].

On August 10, 2022, President Biden signed the PACT Act, which included the Camp Lejeune Justice Act under Section 804 [19]. The law allows military personnel, civilians, and family members who lived or worked at Camp Lejeune during the contamination period to seek compensation from the federal government [19]. Parkinson's disease is now a presumptive condition for VA disability benefits for veterans who were stationed at Camp Lejeune for at least 30 days between August 1953 and December 1987 [19].

In 2023, the VA expanded its Camp Lejeune Family Member Program to include Parkinson's disease, allowing family members to receive reimbursement for out-of-pocket healthcare costs [19].

Beyond military contamination, tort litigation against private manufacturers and industrial users of TCE has expanded. Lawsuits have been filed against companies including Dow Chemical and Zep Inc. by workers alleging that occupational TCE exposure caused kidney disease and other illnesses [20]. Municipalities have also sued over groundwater contamination. The aggregate scope of pending litigation is difficult to quantify, as cases are distributed across federal and state courts with varying procedural stages.

Methodological Critiques and Limitations

The TCE-Parkinson's link has skeptics, and their objections address real methodological constraints in the existing research.

A central criticism is exposure measurement. Most epidemiological studies rely on indirect or ecological proxies for TCE exposure — residential proximity to contaminated sites, census tract air modeling, or self-reported occupational history — rather than biological dosimetry (direct measurement of TCE or its metabolites in tissue or blood) [21]. Because previous exposures cannot currently be measured retroactively, there is an inherent imprecision in all retrospective exposure assessment.

Co-exposure confounding is another concern. TCE contamination frequently co-occurs with other chemicals at Superfund sites, and Parkinson's risk is also influenced by pesticide exposure, manganese, and genetic factors such as LRRK2 variants [21]. Studies vary in how thoroughly they control for these co-exposures.

The Camp Lejeune study illustrates both the strengths and limitations. The study's power came from a well-defined cohort with documented contamination. But assignment to Camp Lejeune did not guarantee individual exposure — some personnel may have consumed bottled water or been stationed in less-affected buildings [21]. When researchers restricted their analysis to cases ascertained before 2017 — before a new VA policy may have increased diagnosis-seeking among Camp Lejeune veterans — the elevated risk was still present but lower, at 28% rather than 70% [21].

The 2025 Medicare study acknowledged that its TCE exposure estimates were based on outdoor air modeling from 2002 and may not reflect individual lifetime or indoor exposures [8]. It also studied only adults over 67, limiting generalizability.

Researchers who have published these studies are transparent about these limitations. Dr. Ray Dorsey and colleagues at the University of Rochester, who authored a 2023 hypothesis paper in the Journal of Parkinson's Disease, explicitly presented their TCE-Parkinson's thesis as a hypothesis requiring further study, while pointing to the consistency of evidence across animal models, occupational studies, and population analyses [3][21].

The Substitution Problem: Is PERC Any Safer?

As TCE faces regulatory restriction, many industrial users and dry cleaners have shifted to perchloroethylene (PERC, also called tetrachloroethylene or PCE). In Europe, where REACH authorization requirements took effect in 2016, most firms that stopped using TCE substituted PERC [22].

This substitution raises its own concerns. PERC exposure has been linked to cognitive damage, kidney and liver disease, immunotoxicity, and reproductive harm [23]. While PERC is classified as a "suspected" rather than "proven" carcinogen in the EU — a less severe designation than TCE's — the EPA in December 2024 also issued rules restricting PERC, determining it poses unreasonable health risks in dry cleaning and other applications [24].

Whether PERC carries comparable Parkinson's risk is less well-studied. The Camp Lejeune water supply was contaminated with both TCE and PCE, making it difficult to isolate their individual contributions in that cohort [7]. Independent toxicologists have noted that the shift from TCE to PERC may represent lateral movement to a chemical with a shorter but potentially similar risk profile, rather than a meaningful reduction in neurotoxic hazard [23].

Safer alternatives do exist. Approximately one-third of dry cleaners in California have converted from PERC to hydrocarbon-based solvents, and water-based wet cleaning methods can satisfactorily clean 30% to 70% of garments previously dry-cleaned with PERC [23]. Silicone-based solvents (marketed as GreenEarth), glycol ethers, and carbon dioxide-based systems are also commercially available [23].

International Regulatory Comparisons

The U.S. and Europe have taken different paths to TCE regulation, with different results.

The EU placed TCE on the REACH authorization list, requiring specific authorization for continued industrial use beginning in April 2016 [22]. The effect was substantial: TCE available on the EU market dropped from approximately 50,000 tonnes in 2010 to roughly 12,000 tonnes by 2019, and estimated current use stands at approximately 1,200 tonnes — a reduction of more than 95% [22].

In the United States, the EPA's MCL for TCE in drinking water is 5 parts per billion [13]. The WHO guideline value is 30 micrograms per liter (approximately 30 ppb), a less stringent threshold [25]. Minnesota has adopted its own health-based standard of 0.4 ppb, based on TCE's immunotoxic effects at low doses and designed to protect all age groups and developmental stages [5]. Only Minnesota and New York had state-level bans before the 2024 federal rule [3].

The gap between the EU's effective 95% reduction in TCE use (achieved by 2019) and the U.S. federal ban (finalized in 2024, with ongoing implementation delays) illustrates how regulatory frameworks produce different timelines for controlling the same chemical. The REACH system's authorization requirement created economic incentives for substitution years before a formal ban. The U.S. approach under TSCA, by contrast, required a full risk evaluation and rulemaking process that took decades.

The Scale of the Problem

Parkinson's disease is the fastest-growing neurological disorder in the world [3]. In the United States, incidence has increased at an estimated annual percentage change of 2.87%, a trend that reflects both an aging population and, according to some researchers, accumulated environmental exposures [1]. Higher incidence has been observed in certain areas of the Midwest and South, Southern California, Southeastern Texas, Central Pennsylvania, and Florida [1].

The attributable fraction of Parkinson's cases specifically caused by TCE remains uncertain. With an estimated 10 million workers historically exposed to TCE, and 19 million or more Americans drinking contaminated water, the population at risk is substantial [3][5]. But the long latency between exposure and disease onset — often decades — and the multiple risk factors involved make precise attribution difficult. Researchers have characterized the relationship as one where TCE "may be an invisible cause" rather than claiming a definitive causal share [21].

What is clearer is that the science linking TCE to Parkinson's has accelerated dramatically. Since 2011, 490 peer-reviewed papers have been published on the TCE-Parkinson's connection, with annual output more than tripling from 21 papers in 2011 to a peak of 78 in 2024 [26]. The question facing regulators, courts, and communities is whether the response will match the pace of the evidence.