A frequently reported contaminant found in groundwater may be linked with a higher risk for Parkinson’s disease.
Exposure to trichloroethylene (TCE)—a common and hazardous contaminant found in the air, soil, and groundwater—may increase the risk for Parkinson’s disease, according to research published in the November 14 online Annals of Neurology.
Ever exposure to TCE was associated with a significantly increased risk of Parkinson’s disease, while exposure to other solvents such as perchloroethylene (PERC) and carbon tetrachloride (CCl4) tended toward significance, reported Samuel M. Goldman, MD, and colleagues.
“Although the present work focused on occupational exposures, solvents are ubiquitous in the environment, and this is particularly true for those implicated in this study—TCE, PERC, and CCl4,” stated Dr. Goldman, who is an Associate Professor at the Parkinson’s Institute and Clinical Center in Sunnyvale, California. “Our findings require replication in other populations with well-characterized exposures, but the potential public health implications are considerable.”
A Common Contaminant
TCE has been used extensively for decades as a dry cleaning and degreasing agent and as an additive in common household products such as adhesives, paints, carpet cleaners, spot removers, and typewriter correction fluid. It continues to be used as a degreasing agent in metal parts fabrication.
“Approximately 50 millions of pounds of TCE are still released annually into the environment in the United States,” noted the investigators. “It is detected in air, soil, food, and human breast milk and is the most frequently reported organic contaminant in groundwater, found in up to 30% of US drinking water supplies.”
Uses of PERC are similar to those of TCE, while CCl4 has been applied primarily in the production of chlorofluorocarbons for use as refrigerants. PERC can remain in the air and groundwater for several months or longer, and CCl4 is degraded slowly, allowing it to gradually accumulate in the environment.
Solvent Exposure in Twins Discordant for Parkinson’s Disease
The epidemiologic, case-control study included 99 twin pairs discordant for Parkinson’s disease, recruited from the National Academy of Sciences/National Research Council World War II Veteran Twins Cohort. All participants were interviewed regarding their occupations and hobbies with the use of detailed job task–specific questionnaires. The researchers analyzed lifetime exposure to six compounds that have been anecdotally associated with Parkinson’s disease—TCE, PERC, CCl4, toluene, xylene, and n-hexane.
Dr. Goldman and colleagues found that ever exposure to TCE was associated with a significantly increased risk of Parkinson’s disease (odds ratio, 6.1). Odds ratios for PERC and CCl4 exposure were 10.5 and 2.3, respectively.
The most frequent job or hobby types linked to TCE exposure included electrician, dry cleaner, industrial machinery repairer, and health worker. Jobs and hobbies that were most common among those exposed to PERC were electrician, dry cleaner, and artist; jobs/hobbies most common among those exposed to CCl4 were electrician, mechanic, aircraft mechanic, painter, dry cleaner, and industrial worker.
“In this model, TCE caused selective dose-dependent loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc), and selective accumulation of α-synuclein protein in the dorsal motor nucleus of the vagus nerve and SNpc, a pattern consistent with human pathologic staging of Parkinson’s disease,” stated the researchers. “TCE also reduced mitochondrial complex 1 activity, similar to the neurotoxin 1-methyl-4-phenylpyridinium (MPP+), the insecticide rotenone, and the mitochondrial deficiency seen in typical Parkinson’s disease.”
Because twins share genes and the environment, the study’s design “is more resistant to confounding by unrecognized genetic and environmental factors than standard case-control designs, reducing the likelihood of spurious results,” the investigators pointed out. However, limitations include the small sample size and the fact that exposure inferences were based on retrospective recall.
“One remarkable observation made in all the reports linking TCE exposure with Parkinson’s disease is the very long time lag (10 to 40 years) between exposure and clinical disease,” Dr. Goldman’s group commented. “These observations suggest that exposure may trigger a degenerative cascade dependent on the passage of time, providing a critical window of opportunity to arrest the disease process before clinical symptoms are manifested.
—Colby Stong