What Science Says About Paraquat-Based Weed Killers and Parkinson’s Disease

Manufacturers hid scientific studies that connected the use of weed and grass killers containing paraquat dichloride to the risk of developing Parkinson’s disease. But thankfully, independent researchers were not deterred by chemical companies’ denials, and continued to examine these associations. Today’s studies show what manufacturers of paraquat dichloride-containing herbicides already knew decades ago: Paraquat exposure is associated with the development of Parkinson’s disease.¹

Studies Paraquat Manufacturers Ignored

Companies that produced paraquat-dichloride herbicides were among the first organizations to study the link between these weed killers and Parkinson’s disease. Staff researchers uncovered numerous alarming correlations. When subjected to paraquat, rabbits showed signs of weakness and ataxia and rodents displayed stiff movements and tremors. Documents revealed that long-term exposure could potentially impair the central nervous system, and internal articles explored links between the herbicide’s use and Parkinson’s cases. Instead of doubling down on the science, the producers of paraquat-containing herbicides hid the evidence and suppressed the studies.

An uptick in Parkinson’s disease cases within the farming community renewed the need to study these associations. Now, everyone is listening to what scientists—not the herbicide brands—say about paraquat exposure.

Developing Parkinson’s Disease

Most experts agree that Parkinson’s disease results from genetic and environmental factors but only a handful of cases have been attributed to genetics alone. One study in California analyzed more than 17,000 fraternal and identical twin brothers and the conclusion suggested that environmental factors had a greater influence on Parkinson’s than genetics.² This raises the potential for a Parkinson’s diagnosis after repeated direct contact with paraquat dichloride-containing herbicides, which are major contributors to developing the condition.

Parkinson’s disease is incurable and affects a person’s ability to control their body movements and cognitive capabilities, and even affects one’s sense of smell. Despite evidence that contact with paraquat herbicides increases a person’s risk of Parkinson’s by 2.5 times, it remains on the market and manufacturers continue to profit.

Links Between Parkinson’s Disease and Paraquat Exposure

Farmers with a history of direct contact with paraquat display common symptoms of Parkinson’s disease, including stiff muscles, tremors, and gait issues, even before a formal clinical diagnosis. The chemicals in paraquat disrupt the biological blockers that normally prevent damage to the central nervous system. Many laboratory studies of rodent models and epidemiologic studies offer insights into chronic exposure pathways for paraquat that lead to Parkinson’s disease.

Inhalation of Paraquat

While it is accepted that inhaled airborne pollutants can travel from the lungs to the brain, research on whether this is true of paraquat has been lacking until recently. A 2021 study³ demonstrated that breathing in concentrates that contain paraquat causes the chemicals to travel through the olfactory bulb, bypass the blood-brain barrier, and enter all brain regions. Research connects inhaling weed killers like paraquat with olfactory impairment, which often indicates the onset of Parkinson’s disease.

Dermal Exposure to Paraquat and Absorption

As an herbicide, paraquat creates chemical reactions in plant cells to kill the vegetation. When low doses are absorbed into human cells, this causes oxidative stress, which has been linked to Parkinson’s disease. Researchers⁴ have replicated this reaction in mice. The result was a loss of dopaminergic neurons, a hallmark of Parkinson’s disease that causes motor issues. Observations of rats exposed to paraquat dichloride showed slowed, rigid movement, and reduced strength—known effects of reduced neurotransmitters. Id. When paraquat is absorbed by a person’s skin, it also sets off chemical reactions in the body that damage cells, trigger inflammation, and compromise protective barriers. Continuous dermal exposure to paraquat is linked to the onset of Parkinson’s disease. Dermal absorption impacts the blood brain barrier, which is a membrane that separates the brain from the bloodstream. Magnetic resonance imaging (MRI) has confirmed structural changes in the brain after repeated dermal contact with paraquat.

Despite all of the scientific evidence linking the weed killer paraquat and Parkinson’s disease, the manufacturers insist these compounds are safe to use as directed. Choosing profits over safety has continued for decades. Anyone directly exposed to paraquat and diagnosed with Parkinson’s disease has the right to join the lawsuit to help hold these companies accountable.

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¹See Paraquat Papers Media Library, which can be found here.

²Warner, T.T. and Schapira, A.H.V. (2003), Genetic and environmental factors in the cause of Parkinson’s disease. Ann Neurol., 53: S16-S25.

³See Merrill T. Anderson, et.al., Paraquat Inhalation, a Translationally Relevant Route of Exposure: Disposition to the Brain and Male-Specific Olfactory Impairment in Mice. Toxicol Sci. 2021 Feb 26.

⁴See A.I Brooks, C.A Chadwick, H.A Gelbard, D.A Cory-Slechta, H.J Federoff, Paraquat elicited neurobehavioral syndrome caused by dopaminergic neuron loss, Brain Research, Volume 823, Issues 1–2, 1999.