A study published in JAMA Neurology on 15 May 2023 suggests that exposure to trichloroethylene (TCE), a persistent liquid chemical present in the air, water, and soil, may increase the risk of Parkinson’s disease by 70%. This study is believed to be the first large-scale research to establish a link between TCE and Parkinson’s.
TCE has been used for industrial and commercial purposes for nearly a century. It was previously used as a surgical anaesthetic until it was banned in 1977, and more recently as a degreasing solvent. It is now primarily used to degrease industrial metal parts, a process that involves heating TCE in degreasing tanks to create vapour that dissolves grease, releasing the chemical into the environment. TCE can persist in soil or groundwater for extended periods once it enters these mediums.
The study, led by researchers from the University of California, San Francisco (UCSF) and the San Francisco VA Medical Center, compared the incidence of Parkinson’s disease among approximately 160,000 Navy and Marine veterans. The study group consisted of over half who served at Camp Lejeune in North Carolina, where TCE was used to degrease military equipment and water contamination occurred, while the remaining participants were from Camp Pendleton in California, where water contamination did not occur.
The service members spent a minimum of three months in these camps between 1975 and 1985, a period when TCE in the water at Camp Lejeune exceeded the maximum safety levels by 70 times. The researchers had access to follow-up health data on the service members between 1997 and 2021, during which Parkinson’s disease might have been expected to develop.
The findings revealed that 430 veterans had been diagnosed with Parkinson’s disease, with the Lejeune veterans having a 70% higher risk compared to the Pendleton veterans. On average, service members from both camps were stationed there for approximately two years from 1975 to 1985. The onset of residence occurred at an average age of 20, while the average age of Parkinson’s diagnosis was 54 at Lejeune and 53 at Pendleton, indicating that the disease took decades to manifest after TCE exposure.
Samuel M. Goldman, MD, MPH, from the UCSF Division of Occupational, Environmental and Climate Medicine and the San Francisco VA Medical Center, noted that TCE exposure is not limited to the military population. He stated that between 9% and 34% of US water supplies contain measurable amounts of the chemical, highlighting the potential risk to the civilian population.
Goldman further explained that TCE remains a widely used chemical in the United States and globally, with its production increasing in recent years and its availability online. However, determining individual exposure to TCE is challenging unless there has been direct contact or work with the chemical. TCE gets metabolized and eliminated from the body quickly, making blood and urine tests reflective of recent exposure only.
The researchers also observed a higher prevalence of prodromal Parkinson’s symptoms, which are indicative but do not fulfil diagnostic criteria for the disease, among the Lejeune veterans. These symptoms include loss of sense of smell, rapid eye movement sleep behaviour disorder (RBD), anxiety, depression, and constipation. Caroline M. Tanner, MD, PhD, from the UCSF Department of Neurology, the Weill Institute for Neurosciences, and the San Francisco VA Medical Center, emphasised that while these symptoms can be early signs of Parkinson’s, only a small fraction of individuals displaying them will develop the disease.
Tanner added that a risk score based on these symptoms can estimate the likelihood of developing Parkinson’s in the future. The Lejeune veterans exhibited higher risk scores than the Pendleton veterans, indicating a higher likelihood of developing Parkinson’s in the future.
The study’s co-authors and their disclosures can be found in the published paper.
The research received support from the US Department of Veterans Affairs through a clinical science research and development merit award (I01 CX002040-01). The Veterans Administration (VA)/Centers for Medicare & Medicaid Services data support was provided by the US Department of Veterans Affairs, VA Health Services Research and Development Service, and project numbers SDR 02-237 and 98-004 from the VA Information Resource Center.