Paraquat is the trade name for N,N′-dimethyl-4,4′-bipyridinium
dichloride, is the organic compound with the formula [(C5H4N)2]Cl2. It is
classified as a viologen, a family of redox-active heterocycles of similar
structure. This salt is one of the most widely used herbicides. It is
quick-acting and non-selective, killing green plant tissue on contact. It is
also toxic to human beings and animals. It is linked to development of
Parkinson's disease. The name is derived from the para positions of the
quat ernary nitrogens. A variety of related compounds are used in textiles.
Production Pyridine is coupled by treatment with
sodium amide followed by oxidation. The resulting 4,4'-bipyridine, which is then
methylated with chloromethane to give the desired compound:
History Although first synthesized in 1882,
paraquat's herbicidal properties were not recognized until 1955. Paraquat was
first manufactured and sold by ICI in early 1962, and is today among the most
commonly used herbicides. The European Union approved the use of
paraquat in 2004 but Sweden, supported by Denmark, Austria, and Finland,
appealed this decision. In 2007, the court annulled the directive authorizing
paraquat as an active plant protection substance stating that the 2004 decision
was wrong in finding that there were no indications of neurotoxicity associated
with paraquat and that the studies about the link between paraquat and
Parkinson's disease should have been considered.
Herbicide use The key characteristics that distinguish
the non-selective contact herbicide paraquat from other active ingredients
used in plant protection products are: It kills a wide range of annual grasses
and broad-leaved weeds and the tops of established perennial weeds.
It is very fast-acting. It is rain-fast within minutes of
application. It is partially inactivated upon contact
with soil. These properties led to paraquat being
used in the development of no-till farming. Current research into no-till
farming using mulching techniques as a substitute for herbicide application are
producing good results In the United States, paraquat is
available primarily as a solution in various strengths. It is classified as
"restricted use," which means that it can be used by licensed applicators
only. In the European Union, paraquat has been forbidden since 2007.
Reactivity and mode of action Paraquat interferes with electron
transfer, a process that is common to all life. It is an electron acceptor in
redox and radical reactions. As an herbicide, paraquat acts by
inhibiting photosynthesis. In light-exposed plants, it accepts
electrons from photosystem I and transfers them to molecular oxygen. In
this manner, destructive reactive oxygen species are produced. In forming these
reactive oxygen species, the oxidized form of paraquat is regenerated, and is
again available to shunt electrons from photosystem I to start the cycle again.
Paraquat is often used in science to catalyze the formation of reactive
oxygen species, more specifically, the superoxide free radical. Paraquat will
undergo redox cycling in vivo, being reduced by an electron donor such as
NADPH, before being oxidized by an electron receptor such as dioxygen to
produce superoxide, a major ROS. Weed resistance management
Problems with herbicide resistant weeds may be addressed by applying herbicides
with different modes of action, along with cultural methods such as crop
rotation, in integrated weed management systems. Paraquat, with its distinctive
mode of action, is one of few chemical options that can be used to prevent and
mitigate problems with weeds that have become resistant to the very widely used
non-selective herbicide glyphosate. One example is the "Double Knock" system
used in Australia. Before planting a crop, weeds are sprayed with glyphosate
first, then followed seven to ten days later by a paraquat herbicide. Although
twice as expensive as using a single glyphosate spray, the "Double Knock"
system is an important resistance management strategy widely relied upon
by farmers. Nevertheless, herbicide resistance has been seen for both
herbicides in Western Australia. A computer simulation showed that with
alternating annual use between glyphosate and paraquat, only one field
in five would be expected to have glyphosate-resistant annual ryegrass
after 30 years, compared to nearly 90% of fields sprayed only with glyphosate.
A "Double Knock" regime with paraquat cleaning-up after glyphosate was
predicted to keep all fields free of glyphosate resistant ryegrass for at
least 30 years. "Paraquat pot"
During the late 1970s, a controversial program sponsored by the US government
sprayed paraquat on cannabis fields in Mexico. Since much of this cannabis was
subsequently smoked by Americans, the US government's "Paraquat Pot" program
stirred much debate. Perhaps in an attempt to deter people from using
cannabis, representatives of the program warned that spraying rendered the crop
unsafe to smoke. However, a 1995 study found that "no lung or other injury in
cannabis users has ever been attributed to paraquat contamination". Also a
United States Environmental Protection Agency manual states: "... toxic effects
caused by this mechanism have been either very rare or nonexistent. Most
paraquat that contaminates cannabis is pyrolyzed during smoking to dipyridyl,
which is a product of combustion of the leaf material itself and presents little
toxic hazard." Toxicity
Pure paraquat, when ingested, is highly toxic to mammals, including humans,
potentially leading to acute respiratory distress syndrome. Although there are no
specific antidotes, fuller's earth or activated charcoal is an effective
treatment if taken in time. There have been some successful cases of using
cyclophosphamide to treat paraquat poisoning. Death may occur up to 30 days
after ingestion. Diluted paraquat used for spraying is less toxic; thus, the
greatest risk of accidental poisoning is during mixing and loading paraquat for
use. 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 for acute inhalation
effects. However, the EPA has determined that particles used in agricultural
practices are well beyond the respirable range and therefore inhalation toxicity
is not a toxicological endpoint of concern. Paraquat is toxic by the oral
route and moderately toxic 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 respectively for these effects.
Even a single swig, immediately spat out, can cause death from fibrous tissue
developing in the lungs, leading to asphyxiation.
One of the characters in the infamous British public information film Apaches
dies hours after accidentally swallowing a small amount; the paraquat in the film
is contained in a receptacle similar to a whisky bottle.
According to the Centers for Disease Control, ingesting paraquat causes
symptoms such as liver, lung, heart, and kidney failure within several days to
several weeks that can 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. Long-term exposures to paraquat would most likely
cause lung and eye damage, but reproductive/fertility damage was not
found by the United States Environmental Protection Agency in their review.
= Use in suicide and murder= A large majority of fatalities from
paraquat poisoning are suicides, which occur mostly in developing countries.
For instance, in Samoa from 1979–2001, 70 percent of suicides were by paraquat
poisoning. Trinidad and Tobago is particularly well-known for its
incidence of suicides involving the use of Gramoxone. In southern Trinidad,
particularly in Penal, Debe from 1996–1997, 76 percent of suicides were
by paraquat, 96 percent of which involved the over-consumption of alcohol
such as rum. Fashion celebrity Isabella Blow committed suicide using paraquat in
2007. Paraquat is widely used as a suicide agent in third-world countries
because it is widely available at low cost. Further, the toxic dose is low.
Campaigns exist to control or even ban paraquat, and there are moves to
restrict its availability by requiring user education and the locking up of
paraquat stores. The indiscriminate paraquat murders,
which occurred in Japan in 1985, were carried out using paraquat as a poison.
Paraquat, as the weedkiller Gramoxone, was used in the UK in 1981 by Susan
Barber to poison the gravy of her husband Michael's pie. She was convicted
of murder in November 1982, maintaining throughout that she had not intended to
kill him. = Parkinson's disease=
In 2011, a US National Institutes of Health study showed a link between
paraquat use and Parkinson's disease in farm workers. A co-author of the paper
said that paraquat increases production of certain oxygen derivatives that may
harm cellular structures, and that people who used paraquat, or other
pesticides with a similar mechanism of action, were more likely to develop
Parkinson's. Paraquat-induced toxicity in rats has also been linked to
Parkinson's-like neurological degenerative mechanisms. A study by the
Buck Institute for Research on Aging showed a connection between exposure to
paraquat and iron in infancy and mid-life Parkinson's in laboratory mice.
Paraquat is structurally similar to MPP+, a known fast-acting inducer of
Parkinson's disease in primate brains. The chloride of MPP+ was sold under the
trade name Cyperquat. Paraquat also induces oxidative stress
in invertebrates such as Drosophila melanogaster. Paraquat-fed flies suffer
early-onset mortality and significant increases in superoxide dismutase
activity. References
Further reading Slade, P.. "The Fate of Paraquat Applied
to Plants". Weed Research 6: 158. doi:10.1111/j.1365-3180.1966.tb00876.x.
Smith, S. N.; Lyon, A. J. E.; Sahid, Ismail BIN. "The Breakdown of Paraquat
and Diquat by Soil Fungi". New Phytologist 77: 735.
doi:10.1111/j.1469-8137.1976.tb04668.x. External links
"Paradox Product". Sinon do Brasil. "Stop Paraquat". Erklärung von Bern.
"The Paraquat Information Center". Syngenta Crop Protection AG.
"NIOSH Pocket Guide to Chemical Hazards". Centers for Disease Control
and Prevention. Paraquat in the Pesticide Properties
DataBase Paraquat dichloride in the Pesticide
Properties DataBase