Paraquat is the trade name for N,N'-Dimethyl-4,4'-bipyridinium dichloride, a viologen. Paraquat is used as a quaternary ammonium herbicide. It is extremely poisonous to humans if swallowed. Other members of this class include diquat, cyperquat, diethamquat, difenzoquat, and morfamquat. All of these are easily reduced to the radical ion, which generates superoxide radical that reacts with unsaturated membrane lipids.
The European Union allowed Paraquat in 2004. Sweden, supported by Denmark, Austria, and Finland, brought the European Union commission to court. On 11 July 2007 the court annulled the directive authorising Paraquat as an active plant protection substance.
Being a herbicide, paraquat protects crops by controlling a wide range of annual and certain perennial weeds that reduce crop yield and quality by competing with the crop for water, nutrients, and light.
The key characteristics that distinguish the non-selective contact herbicide paraquat from other active ingredients used in plant protection products are:
In the United States, paraquat is available primarily as a liquid in various strengths. It is classified as "restricted use," which means that it can be used only by licensed applicators. As with many chemicals, caution must be exercised during use.
In the European Union, paraquat has been forbidden since July 11th 2007.
In acute toxicity studies using laboratory animals, paraquat has been shown to be highly toxic by the inhalation route and has been placed in Toxicity Category I (the highest of four levels) for acute inhalation effects. However, the EPA has determined that particles used in agricultural practices (400 to 800 μm) are well beyond the respirable range and therefore inhalation toxicity is not a toxicological endpoint of concern. Paraquat is toxic (Category II) by the oral route and moderately toxic (Category III) by the dermal route. Paraquat will cause moderate to severe eye irritation and minimal dermal irritation, and has been placed in Toxicity Categories II and IV (slightly toxic) for these effects.
Even a single swig immediately spat out can cause death from fibrous tissue developing in the lungs leading to asphyxiation.
According to the Center for Disease Control, ingesting paraquat causes symptoms such as liver, lung, heart, and kidney failure within several days to several weeks that could lead to death up to 30 days after ingestion. Those who suffer large exposures are unlikely to survive. Chronic exposure can lead to lung damage, kidney failure, heart failure, and oesophageal strictures. Accidental deaths and suicides from paraquat ingestion are relatively common. For example, there have been 18 deaths in Australia from paraquat poisoning since 2000.
Paraquat-induced toxicity in rats has also been linked to Parkinson's-like pathological degenerative mechanisms. A study by the Buck Institute shows a connection between exposure to paraquat and iron in infancy and mid-life Parkinson's in laboratory mice.
Long term exposures to paraquat would most likely cause lung and eye damage, but reproductive/fertility damage was not found by the EPA in their review. Some suspect a possible link to a greater incidence of Parkinson's disease.
A large majority (93%) of fatalities from paraquat poisoning are cases of intentional self-administration, ie, suicides. In third world countries, paraquat is a "major suicide agent" (Dinham, B. 1996: Active Ingredient fact sheet, Paraquat, in: PAN UK: Pesticide News No. 32, p. 20-1: http://www.pan-uk.org/pestnews/actives/paraquat.htm, 24.06.2003).
For instance, in Samoa from 1979-2001, 70% of suicides were by paraquat poisoning. In Southern Trinidad from 1996-1997, 76% of suicides were by paraquat.
The reason paraquat is such a widely used suicide agent in third-world countries is due to its widespread availability, low toxic dose (10 mLs or 2 teaspoons is enough to kill) and relative cheapness. There are campaigns to control or even ban paraquat outright, and there are moves to restrict its availability by requiring user education and the locking up of paraquat stores.
However, independent bodies have studied paraquat in this use. Jenny Pronczuk de Garbino, stated: "no lung or other injury in marijuana users has ever been attributed to Paraquat contamination". On this topic, D.P. Morgan states in a United States Environmental Protection Agency publication that: "Smoking Paraquat-contaminated marijuana does not result in lung damage as the herbicide is pyrolyzed to dipyridyl (which does not present a toxic hazard) during smoking".