The Link Between Parkinson’s Disease and Toxic Chemicals

Michael Richard Clifford, a 66-year-old retired astronaut living in Cary, N.C., learned before his third spaceflight that he had Parkinson’s disease. He was only 44 and in excellent health at the time, and 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, Mr. 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 Mr. Clifford, a lifelong nonsmoker, believes that his close contact with these various 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 very well account for the dramatic 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.

Sometimes, though, the links are so strong and the evidence so compelling that there can be little doubt that one causes the other.

The link of cigarette smoking to 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 four 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 dramatically with the use of pesticides, industrial solvents and degreasing agents in countries throughout the world.

“Over the last 25 years,” the authors noted, “the prevalence rates for Parkinson’s, adjusted for age, increased by 22 percent for the world, by 30 percent for India, and by 116 percent for China.”

Furthermore, they added, men, who are more likely to work in occupations that expose them to industrial products linked to the disease, have a 40 percent greater risk than women of developing it.

But no one is being spared a potential risk. Among other exposures, a solvent called trichloroethylene, or TCE, linked to Parkinson’s, is so widespread in the American environment that nearly everyone has been exposed to it. It contaminates up to 30 percent of the country’s drinking water and, because it evaporates readily, it can enter homes undetected through the air.

Yet a proposed ban on the use of TCE was postponed indefinitely in 2017 by the Environmental Protection Agency, as has a ban on the nerve toxin chlorpyrifos, an insecticide linked to Parkinson’s that is widely used on crops and golf courses.

Another prominent toxin, the pesticide paraquat, can increase the risk of Parkinson’s by 150 percent. It has been banned by 32 countries, including China, but not by the United States, where use on agricultural fields has doubled during the last decade, the authors noted. Both TCE and paraquat were banned years ago in the Netherlands, and the incidence of Parkinson’s there has since declined.

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.