November 14th, 2011
06:45 PM ET
Exposure to a man-made chemical known as trichloroethylene, or TCE, is associated with a sixfold increased risk of developing Parkinson's disease, according to a new study published Monday in the Annals of Neurology. TCE is a common organic contaminant that pollutes groundwater, soil, and air.
The study also found that exposure to another man-made chemical similar to TCE, known as perchloroethylene or tetrachloroethylene, or PERC, is associated with a tenfold increased risk of Parkinson's. Both chemicals are found in metal degreasers, metal cleaners, paint, spot removers, and carpet-cleaning fluids.
"The fact that we were able to find a six-to-tenfold increased risk in exposure I think is very meaningful," says Dr. Samuel M. Goldman, an associate professor of clinical research at The Parkinson's Institute and the lead author of the study.
Chemicals and solvents like TCE have been anecdotally linked to Parkinson's disease before but according to Goldman, no epidemiologic study has been done to verify the relationship until now.
To conduct the study, Goldman and his team identified six specific solvents previously suspected to be related to the development of Parkinson's, two of which were TCE and PERC. They then reached out to 99 all-male pairs of twins, each composed of one twin with Parkinson's and one without. The twins were all male because they were part of the National Academy of Sciences/National Research Council World War II Veteran Twins Registry that was founded in the 1960s using military records.
Goldman and his team interviewed the twins using detailed job-specific questionnaires to gauge the likelihood of each person being exposed to the predetermined solvents.
"We designed these extremely detailed interviews so that we didn't have to rely on the memory or the knowledge of the respondent," explains Goldman.
For example, if one of the study participants said he used to work as an aircraft mechanic in the 1950s, Goldman and his team would question the participant about the different duties or machinery the job involved.
"We know the geographic locations where [each participant] worked, the decade, and what they did so we can say, 'OK – we know that with someone who worked in the 1950s, in a plant with air plane engines, and they were working with the degreasing process, there is a high likelihood that person was exposed to TCE.'"
By working with twins, Goldman and his team were able to account for genetic and lifestyle factors and focus on the job differences between each brother, one of whom had Parkinson's. They found that exposure to TCE, PERC, and to a lesser extent another chemical known as carbon tetrachloride, were all associated with an increased risk of the neurodegenerative disease.
Goldman says that single finding could have major public health implications given how ubiquitous these chemicals, particularly TCE, are in the environment.
"These results need to be replicated," says Goldman. "Even though we have this single epidemiologic study, it's something that needs to be studied quickly."
That sense of urgency is reinforced by the Environmental Protection Agency's recent decision to classify TCE as a known human carcinogen. In an email to CNN, a spokesman for the EPA says the agency believes there is no "acceptable" level of the chemical in groundwater because of its designation as a carcinogen. However, the agency has set a maximum standard of five parts per billion (ppb) to help measure and enforce the amount of TCE in water supplies around the country.
"Clean water is critical to the health and prosperity of every American community and a fundamental concern to every American family," the EPA spokesman wrote to CNN.
The findings in Goldman's study support an emerging line of thought among Parkinson's researchers that the disease is caused by a combination of genetic and environmental factors.
"In Parkinson's disease, research has been pointing in the direction supporting the notion that genetics loads the gun, and the environment pulls the trigger," says Dr. Michael S. Okun, medical director of the National Parkinson Foundation.
"This may make the investigation of pesticides, toxins, and trauma very important to understanding what leads to this disease."
Goldman agrees and says more research needs to be done to identify potential environmental triggers like TCE.
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