California phases out paraquat, pesticide linked to Parkinson’s Disease, amid growing legal and scientific pressure

• California has initiated a two-year phase-out of paraquat after manufacturers voluntarily cancelled their registrations, making it the second U.S. state to ban the herbicide.
• The state’s Department of Pesticide Regulation cited preliminary findings of thyroid impacts, birth defects and significant risks to birds and mammals.
• Scientific studies have linked paraquat exposure to a doubled risk of Parkinson’s disease, with internal corporate documents revealing decades of industry concern.
• The EPA continues to face criticism for its handling of the chemical, having re-approved it in 2021 despite a 2019 recommendation to ban most aerial applications.
• Over 8,000 lawsuits are pending against Syngenta and Chevron from farm workers alleging Parkinson’s disease from paraquat exposure.

The end of an era for a deadly herbicide

California, the nation’s largest agricultural producer and top user of the herbicide paraquat, has begun a formal phase-out of the chemical, a move that marks a significant shift in U.S. pesticide policy. The California Department of Pesticide Regulation (DPR) announced Monday that pesticide manufacturers have voluntarily cancelled their registrations for products containing paraquat, triggering a two-year statewide sell-off period. This decision follows a preliminary state review that found the chemical poses unacceptable risks to human health and wildlife and comes as mounting scientific evidence and thousands of lawsuits link the herbicide to Parkinson’s disease.

The phase-out is the result of a reevaluation process initiated in November 2024, mandated by state law. While the DPR’s review did not confirm a causal link to Parkinson’s, it identified potential associations with thyroid damage and birth defects and determined that existing restrictions may not adequately protect birds and other wildlife. Rather than supply additional safety data requested by regulators, the companies behind the products—including major manufacturer Syngenta—chose to cancel their registrations, with the last one effective August 6.

A history of concern and corporate maneuvering

The decision in California is the culmination of decades of concern that has been consistently downplayed by the chemical industry. Internal documents, unsealed as part of ongoing litigation, reveal that as early as 1958, a scientist for Syngenta’s predecessor, Imperial Chemical Industries, flagged central nervous system problems in lab animals exposed to a related substance. Further internal memos from the 1980s, involving Chevron, noted an “extraordinarily high correlation” between Parkinson’s and paraquat use. Chevron exited the business in 1986, with internal documents indicating worries about long-term liability.

Syngenta has repeatedly denied any credible link between paraquat and Parkinson’s. However, court-ordered disclosure of thousands of internal papers shows the company launched campaigns to discredit scientists who found connections, including efforts to keep Dr. Deborah Cory-Slechta, a leading researcher, off EPA advisory panels. The company also replicated independent studies using alternative counting methods that avoided measuring paraquat levels in the brain, allowing it to claim its tests found no definitive link.

State action sparks call for federal ban

With California’s action, the political and regulatory landscape for paraquat has shifted dramatically. Vermont became the first state to ban the chemical earlier this year, and legislators in Pennsylvania, Michigan and New Jersey have introduced similar bills. Public health advocates argue that state-by-state action is insufficient and have renewed calls for the Environmental Protection Agency (EPA) to act federally. The EPA has re-approved paraquat as recently as 2021 for another 15 years, despite a 2019 recommendation from its own scientists to ban most aerial applications.

The EPA has announced plans for a roundtable discussion on the chemical’s safety, but critics accuse the agency of delaying and deferring to industry-friendly data. A bipartisan bill, the Paraquat Prevention Act, has been introduced in Congress to ban the chemical nationwide.

A preventable tragedy

Paraquat is already banned in over 70 countries and is so toxic that a single sip can be lethal, with no antidote. The World Health Organization classifies it as highly hazardous. For agricultural communities and farmworkers who have been exposed for decades, the news from California is a long-awaited victory. But for the thousands of individuals who have filed suit claiming they developed Parkinson’s disease after exposure, the federal government’s continued inaction remains a source of frustration. The question now is whether the EPA and Congress will follow California’s lead or allow the chemical to remain on the market elsewhere, leaving the safety of farmworkers and communities dependent on their zip code. The story of paraquat is a cautionary tale of how corporate influence can overshadow public health, and how state-level action can force a national conversation.

Sources for this article include:

TheNewLede.org

CDPR.ca.gov

TheGuardian.com