Paraquat is a rapidly acting herbicide that kills green plant tissue on contact. The Pesticide Action Network reports that paraquat is commercially produced by Zeneca and sold for agricultural use in formulations that are between 24 and 36 percent, and for home and garden use as a 2.5 percent solution. Paraquat is categorized as a "restricted use" herbicide, and only licensed applicators may use it in the United States.
Exposure
Inhalation exposure to paraquat is rare because it does not evaporate into the air, and the droplets are too large to be inhaled in agricultural spray applications. Most fatalities from paraquat are a result of accidental or intentional ingestion. Paraquat causes lung toxicity whether the route of exposure is via ingestion, inhalation or through the skin. The paraquat profile, published by the Extension Toxicology Network, reports that in humans the lethal ingestion dose is 35 mg/kg, and that a maximum of 3.5 mg per hour can be absorbed either by inhalation or through the skin without producing toxicity.
Acute Lung Toxicity
Although paraquat is toxic to other organs, death primarily results from respiratory failure. An article in a 2009 issue of "BioScience Trends" states that the lungs selectively accumulate paraquat because of a selective cellular uptake system. Paraquat destroys the areas of the lungs responsible for gas exchange, leading to rapid respiratory failure. Permanent lung scarring can result if a non-lethal dose of paraquat is ingested.
Chronic Lung Toxicity
There is no evidence of chronic toxicity caused by paraquat in humans. The Environmental Protection Agency reports that chronic lung inflammation and lung lesions occurred in animals exposed to chronic doses of paraquat. There was no evidence of carcinogenicity in the animal studies. Paraquat was extensively applied to marijuana plants in Mexico in the 1970s. A study published in the "American Journal of Public Health" reported that no clinical cases of acute or chronic paraquat poisoning were reported in people who smoked paraquat-contaminated marijuana. However, the authors of this study did note that it is unlikely that pot smokers who were ill would report their illness to public health authorities. The most likely consequence of chronic exposure to paraquat in humans is progressive scarring of lung tissue.
References
- Centers for Disease Control and Prevention: Facts About Paraquat
- Pan-UK Pesticide Action Network: Paraquat
- BioScience Trends: Protective Effect of Montelukast on Paraquat-Induced Lung Toxicity in Rats
- Extension Toxicology Network: Pesticide Information Profile
- Environmental Protection Agency: Paraquat Dichloride
