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People exposed to regular low doses of pesticides on the job, such as farmers and fishermen, suffer from sharply higher rates of Parkinson's disease, according to new research.

The study, the largest and most detailed of its kind, found that workers with chronic exposure to pesticides and herbicides had a 70-per-cent higher rate of Parkinson's.

Alberto Ascherio, an associate professor of epidemiology at the Harvard School of Public Health, said the new research does not prove a definitive link but gives credence to the belief that pesticides may be responsible for some cases of the neurodegenerative illness.

Parkinson's disease is characterized by tremors, stiffness, slowness of movement and difficulty with balance. The most recognized symptom is an involuntary trembling of the hand. Parkinson's usually strikes people over 60, but can occur at a younger age.

Symptoms occur when neurons in a part of the brain called the substantia nigra die. Usually, these cells produce a chemical known as dopamine. Dopamine allows smooth, co-ordinated function of the body's muscles and movement. When approximately 80 per cent of the dopamine-producing cells are damaged, the symptoms of Parkinson's disease appear.

Tests on laboratory animals have shown that exposure to some chemicals, notably the commonly used herbicide MPTP, can damage cells that produce dopamine.

Dr. Ascherio said, however, that further research is required to determine the types of chemicals and the levels of exposure that increase risk.

To conduct the latest study, which was published in this week's edition of the Annals of Neurology, Harvard researchers examined data from more than 140,000 people involved in an ongoing study by the American Cancer Society.

Among that large group, nearly 8,000 people reported exposure to pesticides, including almost 2,000 farmers, ranchers and fishermen.

Dr. Ascherio noted, however, that farmers and non-farmers (such as gardeners who regularly used pesticides) had similar levels of risk so it appears to be pesticides that increase risk, not other aspects of farming.

Men exposed to pesticides were more likely to develop Parkinson's than were women, but it is not clear why.

In the new study, researchers also looked at whether exposure to other occupational hazards -- including asbestos, coal dust, acids and solvents -- increased the risk of developing Parkinson's but found none.

The new research was funded, in part, by the Michael J. Fox Foundation. The Canadian-born actor suffers from Parkinson's disease, as does legendary boxer Muhammad Ali.

In an unrelated study, published last week in the journal Science, scientists at the Whitehead Institute in Cambridge, Mass., reported that they have figured out how to repair a pathway that, when obstructed, causes Parkinson's symptoms.

They were even able to restore normal neurological function in certain animals, offering a glimmer of hope that surgery may some day ease symptoms. The study also provides new insight on the basic causes of the disease.

Parkinson's is particularly frustrating because there are no therapies available that alter the fundamental clinical course of the condition.

An estimated 100,000 Canadians are living with Parkinson's disease, according to the Parkinson Society of Canada.