Parkinson's disease risk may be higher among those exposed to an industrial solvent, an international team of researchers has found.

Studies have reported that exposure to solvents may increase risk of Parkinson's disease (PD) but research on specific chemicals is limited. 

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The contaminant TCE has been detected in up to 30 per cent of drinking water supplies in the U.S. (Elizabeth Conley/Detroit News/Associated Press)

Dr. Caroline Tanner, of the Parkinson's Institute in Sunnyvale, Calif., and other researchers spoke to 99 sets of twins — in each case one twin had Parkinson's and the other didn't.

The twins were interviewed about their work history and hobbies.

People with Parkinson's may have limb tremors, slowed movement, muscle stiffness, and speech impairment.

There was more than a six-fold increase in the risk of Parkinson's among those exposed to trichloroethene (TCE), the researchers reported in Monday's issue of the Annals of Neurology.

"Our findings require replication in other populations with well-characterized exposures, but the potential public health implications are considerable," the study's authors concluded.

"One remarkable observation made in all the reports linking TCE exposure with [Parkinson's] is the very long time lag [10 to 40 years] between exposure and clinical disease. 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."

Phasing out TCE use

The study focused on occupational exposures. TCE was a common solvent used in dry-cleaning solutions, adhesives, paints and carpet cleaners.

TCE is banned for use as a general anesthetic, skin disinfectant, and coffee decaffeinating agent in the U.S. but is still widely used as a degreasing agent that can contaminate drinking water supplies. A town with TCE-contaminated water was featured in the film A Civil Action.

In September, the U.S. Environmental Protection Agency declared TCE carcinogenic to humans.

The researchers also looked at other solvents, namely perchloroethylene (PERC) and carbon tetrachloride (CCl4). Both "tended towards significant risk of developing the disease."

PERC is also used in dry cleaning and as a degreasing agent. CC14 was used historically in the manufacture of chlorofluorocarbon refrigerants.

The investigators from the U.S., Toronto Western Hospital, Germany and Argentina found no statistical link with three other solvents — toluene, xylene and n-hexane.

The study was the first to show that TCE may be associated with an increased risk of developing Parkinson's, Dr Michelle Gardner, research development manager at Parkinson's UK, said in joining the authors in calling for more research to confirm the link.

"It is important to highlight that many of the previous uses of this solvent have been discontinued for safety reasons over 30 years ago and that safety and protection in work places where strong chemicals such as this solvent are used has greatly improved in recent years," Gardner said in a release.

The researchers noted limitations of the study, such as the small number of subjects, and relying on people to recall exposures decades ago. They also relied on proxy information from relatives for a large proportion of subjects.

The research was funded by the U.S. National Institute of Neurological Disorders and Stroke, the Michael J. Fox Foundation, Parkinson’s Unity Walk, the Valley Foundation, and James and Sharron Clark. Several of the study's authors reported ties to pharmaceutical companies.