PhotoA study involving twins shows new evidence that on-the-job exposure to chemical solvents raises the risk of Parkinson's disease.

Researchers analyzed the occupational histories of twins in which one twin developed the neurodegenerative disorder, and assessed that twin's likelihood of exposure to six chemicals previously linked to Parkinson's.

Of the six chemicals investigated, researchers concluded that two common chemical solvents, trichloroethylene (TCE) and perchloroethylene (PERC), are significantly linked to development of this disease.

Parkinson's disease is a movement disorder caused by the loss of brain cells that produce a molecule called dopamine. The primary symptoms of Parkinson's are tremor, stiffness, slowed movement and impaired balance, and as these symptoms progress, patients may also develop difficulty walking, speaking or completing other activities of daily living.

"The potential importance is great, since both solvents persist in the environment and are commonly used," said Samuel Goldman, M.D., M.P.H., at the Parkinson's Institute in Sunnyvale, Calif. "Parkinson's was sixfold more common in twins exposed to TCE, and ninefold more common in twins exposed to TCE or PERC."

Tenfold increase

There was also a trend toward a tenfold increase in Parkinson’s disease in twins exposed to PERC alone.

Genes play a role in Parkinson’s disease, but fewer than 10 percent of cases are due to a single gene mutation, and not all people with these mutations develop Parkinson’s, suggesting that environmental factors also contribute to the likelihood of developing the disease.

The latest study, supported in part by the National Institute of Neurological Disorders and Stroke (NINDS), a part of the National Institutes of Health, appears in the Nov. 14, 2011 issue of Annals of Neurology.

The researchers collected the histories of 99 pairs of twins in which one of the pair developed Parkinson’s and the other twin did not. Since twins are so genetically similar, twin studies are especially useful in identifying environmental influences in disease. Of the 99 pairs, half were genetically identical twins, and half were fraternal twins.

The study team assessed the twins' lifetime work and hobby activities, specifically inquiring about occupational tasks such as electrical work, industrial machinery repair, and dry cleaning, which would potentially expose people to chemicals previously linked to Parkinson's.

The researchers also collected information on head injuries, which are suspected to increase Parkinson’s risk, and smoking history, which is reported to decrease Parkinson’s risk.

Expert evaluators, unaware of which study subjects had Parkinson's, reviewed this information and calculated lifelong exposure to six chemicals: TCE, PERC, carbon tetrachloride, n-hexane, xylene and toluene. Of these, TCE and PERC posed a notable risk for developing Parkinson's.

In this study researchers looked only at occupational chemical exposure, and the association with job categories tended toward significance only for the industrial machinery repairer and industrial worker categories. However, the chemicals evaluated here are found outside industrial settings as well.

PERC is the leading chemical used in garment dry cleaning. TCE is the most frequently reported organic groundwater contaminant, was once used as general anesthetic and coffee decaffeinating agent, and is still used widely as a metal degreasing agent.

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