Vermont is the first US state to ban the weedkilling pesticide paraquat, backed by lawmakers who cited concerns about research showing the chemical substantially increases the risk of the incurable brain ailment known as Parkinson’s disease.

Phil Scott, the governor, signed the legislation on Tuesday. The new law takes effect on 1 November, though it contains a provision allowing state regulators to issue special permits for paraquat use on fruit-producing tree orchards, berries and other “small fruit” crops up until 31 December 2030.

The law also calls for annual data reporting on any use of paraquat in the state, and a state-funded study on recommendations for alternatives to the use of paraquat for farmers.

Some lawmakers expressed concern that a ban will hurt the competitiveness of Vermont farmers if the chemical is still allowed for use in other states, and said more research is needed to fully prove a link between paraquat and Parkinson’s.

But others said the scientific research was well established and the risks of not banning the chemical outweigh the risks of keeping it on the market.

Early versions of the law pointed to multiple studies by the National Institutes of Health have demonstrated that paraquat exposure substantially increases the risk of Parkinson’s disease in those exposed to the herbicide. Lawmakers also noted that other NIH studies have linked paraquat to non-Hodgkin lymphoma and childhood leukemia.

“There are so many factors that are pointing to the correlation … between paraquat use and Parkinson’s,” Michelle Bos-Lun, a state representative, said in a 13 May House committee hearing about the bill. “We have to do something to phase this out. Our job is to support farmers and to support all Vermonters. My belief is that paraquat is causing harm to both.”

Ray Dorsey, a neurologist who directs research into environmental causes of brain diseases at Atria Health and Research Institute, said Vermont’s action was “another step toward preventing this largely man-made disease”. He said that many countries had banned paraquat and said it “is long overdue for the US to do the same”.

Though Vermont is the first to pass a ban, lawmakers in multiple states have introduced legislation to ban paraquat and several federal lawmakers have also called for bans on the chemical.

Paraquat has been used in the United States since 1964 as a tool to kill broadleaf weeds and grasses. Though banned in several countries, it is one of the most widely used herbicides in the United States. Paraquat is used in growing soybeans, cotton and corn as well as in growing grapes, pistachios, peanuts and many other crops.

The chemical is known to be extremely dangerous to anyone who ingests even a small amount, and regulators have issued warnings and placed restrictions on its use because of poisoning risks. But whether or not it causes Parkinson’s disease has been a matter of fierce debate.

Syngenta, the key maker of paraquat for decades, has maintained that there is no credible evidence of a connection between the disease and its weedkiller. But numerous scientific studies have found that paraquat damages cells in the brain in ways that can lead to Parkinson’s, and more than 8,000 lawsuits are pending in US courts over the Parkinson’s allegations. The company has settled several cases without admitting liability before they went to trial and has been negotiating to settle a bulk of the ongoing cases.

In April, the company said it would stop making paraquat and would no longer attempt to sell any paraquat products in the US and other countries, though generic versions are available from other suppliers.

“If there is a chance that we could reduce the harm by making this step, it feels like that is a responsible thing to do,” Bos-Lun said in the committee hearing. “When I think about the people I’ve known who’ve had Parkinson’s, we want to do everything we can to avoid that. Every case that we could prevent, that would be a blessing.”

The number of people suffering from Parkinson’s has more than doubled in the last 25 years, according to the World Health Organization (WHO), and incidences are expected to continue to rise.

Parkinson’s symptoms develop when dopamine-producing neurons in the brain degenerate. The disease is progressive, leaving people without the ability to speak and move normally.

A trove of internal corporate records that date back several decades show that Syngenta and its predecessor corporate entities have wrestled with concerns about paraquat links to Parkinson’s and other chronic health issues for decades.

The Guardian revealed many of Syngenta’s internal files on paraquat in 2022 and 2023. The files show the company employed an array of secretive tactics to hide or downplay research linking its chemical to the disease, discredit independent research and mislead or otherwise influence regulatory decisions to protect paraquat sales.

“The evidence linking paraquat exposure to increased Parkinson’s risk is strong and has been building for decades,” said Dan Feehan, chief policy and government affairs officer for the Michael J Fox Foundation for Parkinson’s Research (MJFF), a key group pushing for paraquat bans.

“Other states should follow Vermont’s lead – and the federal government has an obligation to protect all Americans with a national paraquat ban.”

This story is co-published with the New Lede, a journalism project of the Environmental Working Group