People with long-term, low-level exposure to pesticides have a 70 percent higher incidence of Parkinson's disease than people who have not been exposed much to bug sprays, U.S. researchers reported on Monday.

Such workers include mostly farmers, ranchers and fishermen, the researchers report in the July issue of Annals of Neurology.

Their study supports previous research that suggests pesticides can be linked with Parkinson's, which is caused by the destruction of key brain cells, the team at the Harvard School of Public Health said.

"The findings support the hypothesis that exposure to pesticides is a risk factor for Parkinson's disease," they wrote.

Alberto Ascherio, an associate professor of nutrition and epidemiology and colleagues examined data from a 2001 American Cancer Society survey of 143,325 people.

The researchers contacted those people who reported they had been diagnosed with Parkinson's.

The American Cancer Society was studying factors for cancer risk and all the people had reported on eating and lifestyle habits and environmental exposures.

More than 5,200 men and 2,600 women reported exposure to pesticides. After adjusting for age, sex, and other risk factors for Parkinson's disease, the researchers found a 70 percent higher incidence of the disease among these nearly 8,000 people than among people who reported no exposure.

More men than women said they had been exposed to pesticides and those reporting exposure were more likely to report their occupation as farmer, rancher or fisherman, the researchers said.

People who had other jobs and who reported pesticide exposure most likely were using the chemicals at home or while gardening, the researchers speculated.

Exposure to asbestos, chemicals, acids, solvents, or coal or stone dust was not associated with a higher risk, the researchers said.

"Future studies should seek to identify the specific compounds associated with risk," the researchers said. A class of chemicals called organophosphates has been linked with Parkinson's risk in other studies.

There is no cure for Parkinson's, which starts off with tremors and ends up paralyzing and often killing patients. Globally, it is estimated 6.3 million people have Parkinson's, more than a million in the United States alone.