An industrial chemical once widely used in dry cleaning solutions, paints and adhesives has been linked to a six-fold increased risk of Parkinson's disease.
Scientists made the connection after analysing data on 99 sets of twins. One twin in each pair had Parkinson's and the other did not.
The US researchers studied information about the twins' lifetime exposures to six solvents previously associated with Parkinson's disease (PD) in the medical literature.
A significant link was found with one, trichloroethylene (TCE). Exposure to the chemical increased the likelihood of developing Parkinson's disease by more than six times.
Dr Samuel Goldman, from the Parkinson's Institute in Sunnyvale, California, said: "Our study confirms that common environmental contaminants may increase the risk of developing PD, which has considerable public health implications."
The findings were reported in the journal Annals of Neurology.
TCE is now largely banned around the world but continues to be used as a degreasing agent. The chemical is known to contaminate soil, groundwater and air. In the US, millions of pounds of TCE are still released into the environment each year.
Symptoms of Parkinson's may occur up to 40 years after exposure to TCE, said the scientists. The disease affects motor nerves resulting in limb tremors, slowed movement and speech, and muscle rigidity.
Dr Michelle Gardner, research development manager at the charity Parkinson's UK, said: "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.
"Low levels of this solvent and other solvents are found in the environment from industrial and other emissions but this study only examined heavy exposure at work to the solvent. Further larger-scale studies on populations with more defined exposures are needed to confirm the link."