Exposure to certain pesticides may increase the risk of developing Parkinson’s disease, especially in people who carry a specific genetic variation.

The study, published in the journal Neurology, shows that some pesticides inhibit an enzyme in the aldehyde dehydrogenase (ALDH) gene family. These enzymes are responsible for the breakdown and elimination of various toxins, including alcohol.

Those behind the study, including researchers at the David Geffen School of Medicine at the University of California, Los Angeles, said that people with an ALDH variation who were exposed to certain pesticides were two to five times more likely to develop Parkinson’s disease.

“These results show that ALDH inhibition appears to be an important mechanism through which pesticides may contribute to the development of Parkinson’s disease,” study author Dr. Jeff Bronstein said in a statement.

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Toxic Chemicals Tied to Parkinson’s

Previous research in Neurology suggested that rural living and exposure to certain insecticides were risk factors for developing Parkinson’s, an incurable disease that affects an estimated 10 million people worldwide.

Experts say that 11 pesticides keep ALDH from eliminating chemicals from the body, thus increasing a person’s risk of developing Parkinson’s, a degenerative neurological disorder that causes movement, thinking, and behavior problems.

Researchers discovered the connection between ALDH and pesticides while studying 360 people with Parkinson’s and 816 without, all of whom lived in three rural California counties.

Using information from the California Department of Pesticide Regulation, researchers assessed each person’s exposure to pesticides at work and at home. The 11 pesticides that inhibited ALDH fit into four categories: dithiocarbamates, imidazoles, dicarboximides, and organocholorines. These are used to kill body insects and crop-consuming fungi.

Researchers say people who were exposed to three or more of these pesticides at both work and home were 3.5 times more likely to develop Parkinson’s than those who weren’t as heavily exposed.

“In other words, having this gene variant [alone] does not make you more likely to develop Parkinson’s” Bronstein said.

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Those who were exposed to the pesticide benomyl—which is classified as a possible carcinogen by the U.S. Environmental Protection Agency (EPA)—had a 65 percent greater chance of developing Parkinson’s, while those exposed to dieldrin—an insecticide banned in most parts of the world—had a six-fold increased risk.

According to the EPA, deildrin also weakens the human immune system, and it has been linked to birth defects, cancer, and kidney damage. Although banned for the majority of uses in the U.S. since 1987, traces of the chemical are still found in American soil and water.

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DDT Exposure and Alzheimer’s Disease

Earlier this year, an article in Neurology showed that exposure to another banned organochlorine pesticide, DDT, was linked to an increased risk of developing Alzheimer’s disease.

Researchers found that residue from the pesticide was tied to the gene variant ApoE-ε4, the greatest known genetic risk factor for late-onset Alzheimer’s disease.

In other research, DDT has been linked to increased rates of diabetes, developmental problems, miscarriages, and certain cancers. Last year, one study linked it to an increase in the likelihood of obesity in third-generation children.

While both studies require further investigation, experts agree that they illustrate how environmental factors can contribute to neurological problems later in life.