As with smoking, which doesn’t cause cancer in all smokers, most cases of Parkinson’s are likely to reflect an interaction between environmental exposures and genetic predisposition. But also as with cancer and smoking, criteria that strongly suggest a cause-and-effect relationship apply as well to chemical exposure and the development of Parkinson’s disease. In fact, a pioneering study in California by Dr. Caroline Tanner and Dr. William Langston of more than 17,000 twin brothers, both fraternal and identical, suggested that environmental factors outranked genetics as a cause of Parkinson’s.

Thirty years ago, researchers at Emory University showed that rats developed classic features of Parkinson’s when given rotenone, then a popular household insecticide that is still used by fisheries to eliminate invasive species. When the researchers examined the rats’ brains, they found a loss of nerve cells that produce dopamine, the same damage that afflicts people with Parkinson’s.

Dr. Langston and Dr. Tanner later showed that farmers who used rotenone and paraquat, among other pesticides, were more than twice as likely to develop Parkinson’s as those who did not use these chemicals. In laboratory studies, the Parkinson-associated chemicals have been shown to injure nerve cells.

Although Parkinson’s is most likely to afflict older people, its rise has far exceeded the aging of the population. In just 25 years, from 1990 to 2015, the number of people afflicted globally more than doubled, from 2.6 million to 6.3 million, and is projected to reach 12.9 million by 2040.

The disease is progressive, characterized by tremors, stiffness, slow movements, difficulty walking and balance problems. It can also cause loss of smell, constipation, sleep disorders and depression. While there are medications that can alleviate symptoms, there is as yet no cure. People can live with gradually worsening symptoms for decades, resulting in a huge burden on caregivers.

And the economic burden of Parkinson’s is huge, said Dr. Tanner, now a neurologist and environmental health scientist at the University of California, San Francisco. In 2017, it resulted in about $25 billion in direct medical costs and another $26 billion in indirect costs, she said.

In addition to preventing exposure to toxic chemicals, Dr. Tanner said that regular exercise and a healthy diet can reduce the risk of Parkinson’s even in people who were occupationally exposed to nerve toxins.

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