The link between Parkinson’s disease and toxic chemicals
By Jane E. Brody
Michael Richard Clifford, a 66-year-old retired astronaut living in Cary, North Carolina, learned before his third spaceflight that he had Parkinson’s disease. He was only 44 and in excellent health at the time, and he had no family history of this disabling neurological disorder.
What he did have was years of exposure to numerous toxic chemicals, several of which have since been shown in animal studies to cause the kind of brain damage and symptoms that afflict people with Parkinson’s.
As a youngster, Clifford said, he worked in a gas station using degreasers to clean car engines. He also worked on a farm where he used pesticides and in fields where DDT was sprayed. Then, as an aviator, he cleaned engines readying them for test flights. But at none of these jobs was he protected from exposure to hazardous chemicals that are readily inhaled or absorbed through the skin.
Now Clifford, a lifelong nonsmoker, believes that his close contact with such substances explains why he developed Parkinson’s disease at such a young age. Several of the chemicals have strong links to Parkinson’s, and a growing body of evidence suggests that exposure to them may account for the great rise in the diagnosis of Parkinson’s in recent decades.
To be sure, the medical literature is replete with associations between people’s habits and exposures and their subsequent risk of developing various ailments, from allergies to heart disease and cancer. Such linkages do not — and cannot by themselves — prove cause and effect.
But sometimes the links are so strong and the evidence so compelling that there can be little doubt that one causes the other.
The link between cigarette smoking and lung cancer is a classic example. Despite tobacco industry claims that there was no definitive proof, the accumulation of evidence, both experimental and epidemiological, eventually made it impossible to deny that years of smoking can cause cancer even long after a person has quit.
The criteria that supported a cause-and-effect relationship between smoking and lung cancer included strength and consistency of the association; whether the link made biological sense; whether it applied especially or specifically to those exposed to the putative agent; and whether it was supported by experimental evidence.
Likewise, based on extensive evidence presented by experts in a new book, “Ending Parkinson’s Disease,” it seems shortsighted to deny a causative link between some cases of Parkinson’s disease and prior exposure to various toxic chemicals.
The book was written by Dr. Ray Dorsey, neurologist at the University of Rochester; Todd Sherer, neuroscientist at the Michael J. Fox Foundation for Parkinson’s Research; Dr. Michael S. Okun, neurologist at the University of Florida; and Dr. Bastiaan R. Bloem, neurologist at Radboud University Nijmegen Medical Center in the Netherlands.
The authors called the increasing prominence of Parkinson’s “a man-made pandemic.” Its prevalence has closely tracked the growth of industrialization and has increased sharply with the use of pesticides, industrial solvents and degreasing agents in countries throughout the world.
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. In fact, a 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.
Langston and 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, to 6.3 million from 2.6 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 worsening symptoms for decades, resulting in a huge burden on caregivers.
And the economic burden of Parkinson’s is huge, said 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 $26 billion more in indirect costs, she said.
Regular exercise, a healthy diet and efforts to prevent exposure to toxic chemicals can reduce the risk of Parkinson’s, even in people who must work with dangerous substances, Tanner said.