A new study confirms the link between on-the-job pesticide exposure and Parkinson's disease, and suggests that certain insecticides may be particularly risky.

Parkinson's disease is a degenerative brain condition in which dopamine-producing cells in the brain gradually die off or malfunction. Dopamine helps regulate movement, and as Parkinson's progresses, people have increasing difficulty walking, talking and performing simple tasks.

The exact cause of Parkinson's is unknown, but research indicates that a combination of genetics and environmental triggers — such as certain chemicals or viral infections — may be a work.

This latest study, published in the Annals of Neurology, bolsters evidence that occupational pesticide exposure may be one of those environmental triggers.

French researchers found that among nearly 800 adults with and without Parkinson's, agricultural workers exposed to pesticides — including insecticides, weed killers and fungicides — were at greater risk of the disease.

The risk climbed in tandem with the amount of time a worker was exposed, strengthening the case for a cause-effect relationship, according to the researchers, led by Dr. Alexis Elbaz of INSERM, the national French institute for health research in Paris.

"We showed that the risk increased with the number of years or hours of exposure," Elbaz told Reuters Health.

"We also found that in men, among the main groups of pesticides — fungicides, herbicides, insecticides — the association was the strongest for insecticides, and among insecticides, with organochlorine insecticides, which have been frequently used in France in the past."

In general, men exposed to organochlorine insecticides had more than double the risk of men with no exposure.

This finding is potentially important, Elbaz and his colleagues write, because organochlorines are "highly persistent" in the environment.

They add, however, that this study looked only occupational exposure to the insecticides. It is not yet clear whether lower-level exposure, in homes or the environment, affect Parkinson's risk.

Elbaz also noted that while on-the-job pesticide exposure was linked to a relatively higher Parkinson's risk in general, the absolute risk to any one person is still low.