What Is Paraquat? The Herbicide Linked to Parkinson's Disease

What Is Paraquat?

Paraquat dichloride is one of the most widely used herbicides in the United States, applied across millions of acres of farmland each year to control weeds and grasses in crops ranging from soybeans and corn to cotton and wheat. Sold commercially under the trade name Gramoxone by agrochemical giant Syngenta, paraquat has been registered for use by the U.S. Environmental Protection Agency (EPA) since 1964.^[1]

Despite its agricultural utility, paraquat carries an extreme toxicity profile. According to the Agency for Toxic Substances and Disease Registry (ATSDR), paraquat is highly toxic even in very small amounts. A single sip can be fatal in humans, and there is no antidote for paraquat poisoning.^[3] This extraordinary danger has led more than 32 countries — including the European Union, China, Brazil, and Switzerland — to ban or severely restrict paraquat, while the United States continues to allow its use under increasingly restrictive conditions.

In recent years, scientific attention has shifted from paraquat's acute toxicity to its long-term health consequences. A growing body of peer-reviewed research has established a compelling link between chronic, low-level paraquat exposure and the development of Parkinson's disease, a progressive and incurable neurological disorder. This connection has triggered a wave of litigation against Syngenta and co-defendant Chevron Phillips Chemical, with thousands of cases now consolidated in federal court.

How Paraquat Damages the Body

Paraquat's extreme toxicity stems from its chemical mechanism of action. Once absorbed into the body — whether through skin contact, inhalation of spray mist, or accidental ingestion — paraquat undergoes a redox cycling process that generates large quantities of reactive oxygen species (ROS), also known as free radicals. These unstable molecules attack cell membranes, proteins, and DNA, causing widespread cellular destruction.^[3]

The lungs are typically the primary site of acute paraquat toxicity because lung tissue concentrates the chemical. Even at sublethal doses, paraquat can cause pulmonary fibrosis — scarring of the lung tissue — that leads to progressive respiratory failure. The kidneys and liver are also commonly affected.^[7]

Of particular relevance to the ongoing litigation, researchers have identified that paraquat's oxidative damage mechanism specifically targets dopamine-producing neurons in a region of the brain called the substantia nigra. The destruction of these neurons is the hallmark pathological feature of Parkinson's disease. NIEHS researchers have described paraquat as a chemical model for studying Parkinson's disease precisely because its neurotoxic effects so closely mirror the natural progression of the condition.^[2]

Chronic vs. Acute Exposure

There are two distinct exposure scenarios relevant to the paraquat lawsuits. Acute poisoning — typically from accidental ingestion or a major spill — can kill within days to weeks. Chronic low-level exposure, which is what agricultural workers and farm residents typically experience, does not produce immediate symptoms. Instead, the cumulative oxidative damage to brain neurons may silently progress for years or decades before the motor symptoms of Parkinson's disease become apparent. This latency period is one reason why establishing causation in paraquat lawsuits requires careful medical and occupational history documentation.

EPA Regulation and Gramoxone

The EPA has registered paraquat dichloride for use in the United States since 1964, and it continues to be one of the most heavily used herbicides in American agriculture. The EPA's paraquat page notes that the agency has taken a series of risk-mitigation steps over the years, including restricting paraquat to certified applicators in 1978 — meaning only licensed individuals may legally purchase or use it.^[1]

In 2016, the EPA strengthened its paraquat regulations by requiring additional personal protective equipment, engineering controls, and closed-system transfer technology to prevent accidental exposure during mixing and loading operations. The agency also mandated training requirements and updated label language. Despite these measures, critics and public health advocates argue that the restrictions fall far short of what is needed, and that the EPA's continued registration of paraquat stands in stark contrast to the bans enacted by more than 32 other countries.^[1]

Syngenta markets paraquat in the United States primarily under the Gramoxone brand name, though generic formulations are also registered. The herbicide is applied to a wide range of crops including soybeans, corn, cotton, sugarcane, potatoes, and many fruits and vegetables. Its fast-acting, non-selective properties — it kills virtually all green plant material on contact — make it particularly valued in no-till farming systems, where it replaces mechanical tillage for weed control before planting.

The Parkinson's Disease Connection

The scientific case linking paraquat exposure to Parkinson's disease has been building since the 1980s, when researchers noticed the structural similarity between paraquat and a neurotoxin called MPTP that was known to cause rapid-onset Parkinson's-like symptoms. Subsequent laboratory and epidemiological research has substantially strengthened this connection.^[4]

A landmark study published in the American Journal of Epidemiology and analyzed in a widely cited meta-analysis found that individuals exposed to paraquat had approximately 2.5 times the risk of developing Parkinson's disease compared to unexposed individuals.^[4] The NIH/NIEHS meta-analysis pooled data from multiple studies and concluded that the association between paraquat exposure and Parkinson's disease was statistically robust and biologically plausible.

The NIH's Agricultural Health Study, a large prospective cohort study of licensed pesticide applicators and their spouses in Iowa and North Carolina, has been particularly influential. This study tracked tens of thousands of farmers over many years and found consistent associations between paraquat use and elevated Parkinson's disease risk, even after controlling for other potential confounders.^[2]

The Parkinson's Foundation estimates that nearly 1 million Americans are currently living with Parkinson's disease, and approximately 90,000 new cases are diagnosed each year in the United States.^[6] The disease causes tremors, rigidity, slowness of movement, and balance problems that progressively worsen over time. There is currently no cure. The devastating impact of Parkinson's disease — combined with evidence that a preventable chemical exposure may be contributing to tens of thousands of cases — forms the moral and legal foundation of the ongoing paraquat litigation.

Who Is at Risk?

The populations most directly at risk from paraquat exposure are agricultural workers and their families. Licensed pesticide applicators who mix, load, and apply paraquat are at the highest risk, but research has also documented elevated Parkinson's risk among individuals who live near farms where paraquat is applied, suggesting that airborne drift can carry the chemical beyond the immediate application area.^[8]

At particular risk are:

• Licensed pesticide applicators and agricultural workers who directly handle paraquat
• Farm laborers who work in fields during or shortly after paraquat application
• Residents of farming communities where paraquat is commonly used
• Individuals involved in mixing and loading paraquat (a particularly high-exposure activity)
• Workers in landscaping, groundskeeping, and right-of-way maintenance who may use paraquat-containing products
• Family members of farm workers who may experience secondary exposure through contaminated clothing

OSHA's paraquat guidance identifies agricultural workers as the primary occupational risk group and notes that proper personal protective equipment — including chemical-resistant gloves, protective eyewear, and respirators — is essential for anyone who must work with the chemical.^[7] However, research suggests that even workers who follow label instructions may experience meaningful cumulative exposure over years of use.

The Legal Landscape

The paraquat lawsuit litigation is one of the largest and most consequential mass tort proceedings currently active in the U.S. federal court system. Thousands of plaintiffs — primarily agricultural workers and farm residents diagnosed with Parkinson's disease — have filed claims against Syngenta (manufacturer of Gramoxone) and Chevron Phillips Chemical (a prior registrant of paraquat).

These cases have been consolidated in a Multidistrict Litigation (MDL) proceeding designated MDL 3004, presided over by Judge Nancy Rosenstengel in the Southern District of Illinois. The MDL process allows for coordinated pretrial proceedings — including discovery and expert witness challenges — that can be applied across all cases, improving efficiency and consistency.

Plaintiffs allege that Syngenta and Chevron knew or should have known about paraquat's link to Parkinson's disease and failed to adequately warn users. Internal corporate documents produced during discovery are expected to be central to establishing what the defendants knew and when. If the litigation proceeds to trial or reaches a global settlement, the paraquat MDL could result in one of the largest mass tort resolutions in recent history.

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