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in 1996. EXTOXNET no longer updates this information, but it may be useful
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E X T O X N E T
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University of Idaho, and the University of California at Davis
and the Institute for Environmental Toxicology, Michigan State
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EXTOXNET primary files maintained and archived at Oregon State
Revised June 1996
Trade and Other Names:
Product names include Crisquat, Cyclone, Dextrone, Dexuron,
Gramoxone Extra, Herbaxone, Ortho Weed and Spot Killer, and
Sweep. The compound may be found in formulations with many other
herbicides, including simazine and diquat dibromide.
Paraquat is a highly toxic compound in EPA toxicity class I.
Products containing it must be is labeled with the Signal Words
DANGER - POISON. Paraquat is a Restricted Use Pesticide (RUP).
RUPs may be purchased and used only by certified applicators.
quaternary nitrogen compound
Paraquat is a quaternary nitrogen herbicide widely used for
broadleaf weed control. It is a quick-acting, nonselective
compound, that destroys green plant tissue on contact and by
translocation within the plant. It has been employed for killing
marijuana in the U.S. and in Mexico. It is also used as a crop
desiccant and defoliant, and as an aquatic herbicide.
- Acute toxicity: Paraquat is highly toxic
via ingestion, with reported oral LD50 values of 110 to
150 mg/kg in rats, 50 mg/kg in monkeys, 48 mg/kg in cats,
and 50 to 70 mg/kg in cows [8,87]. The toxic effects of
paraquat are due to the cation, and the halogen anions
have little toxic effects . The dermal LD50 in
rabbits is 236 to 325 mg/kg, indicating moderate toxicity
by this route [58,87]. The 4-hour inhalation LC50 is
greater than 20 mg/L for the technical grade of the
compound . It causes skin and eye irritation in
rabbits (severe for some of the formulated products) and
also has caused skin sensitization in guinea pigs in some
formulations . Effects due to high acute exposure to
paraquat may include excitability and lung congestion,
which in some cases leads to convulsions, incoordination,
and death by respiratory failure . If swallowed,
burning of the mouth and throat often occurs, followed by
gastrointestinal tract irritation, resulting in abdominal
pain, loss of appetite, nausea, vomiting, and diarrhea
. Other toxic effects include thirst, shortness of
breath, rapid heart rate, kidney failure, lung sores, and
liver injury . Some symptoms may not occur until days
after exposure. Persons with lung problems may be at
increased risk from exposure. Many cases of illness
and/or death have been reported in humans. The estimated
lethal dose (via ingestion) for paraquat in humans is 35
mg/kg . A maximum of 3.5 mg/hour could be absorbed
through the dermal or respiratory route without damage
- Chronic toxicity: As indicated above,
repeated exposures may cause skin irritation,
sensitization, or ulcerations on contact [58,87]. In
animal studies, rats showed no effects after being
exposed for 2 years to paraquat at doses of 1.25
mg/kg/day . Dogs, however, developed lung problems
after being exposed for 2 years at high doses (above 34
mg/kg/day) . In a study of 30 workers spraying
paraquat over a 12-week period, approximately one-half
had minor irritation of the eyes and nose . Of 296
spray operators with gross and prolonged skin exposure,
55 had damaged fingernails as indicated by discoloration,
nail deformities, or loss of nails .
- Reproductive effects: In a long-term rat
study at doses up to 5 mg/kg/day, no adverse reproductive
effects were reported . However, paraquat dichloride
injected intraperitoneally at 3 mg/kg/day on days 8 to 16
of gestation increased fetal mortality in rats . Hens
given high levels of paraquat in their drinking water for
14 days produced an increased percentage of abnormal eggs
. It is unlikely to cause reproductive effects in
humans at expected exposure levels.
- Teratogenic effects: Offspring of mice
dosed with high doses of paraquat during the
organ-forming period of pregnancy had less complete bone
development than the mice given lower doses .
Offspring of rats given similar treatment showed no
developmental defects at any dose, but fetal and maternal
body weights were lower than normal . Other studies
of paraquat using rabbits and mice have shown no
teratogenic effects . The weight of evidence suggests
that paraquat does not cause birth defects at doses which
might reasonably be encountered.
- Mutagenic effects: Paraquat has been
shown to be mutagenic in microorganism tests and mouse
cell assays . It was unclear what levels of exposure
are necessary to produce these effects.
- Carcinogenic effects: Mice fed paraquat
dichloride for 99 weeks at high levels did not show
cancerous growths . Rats fed high doses for 113
(male) or 124 weeks (female) developed lung, thyroid,
skin, and adrenal tumors . Thus, the evidence
regarding carcinogenic effects of paraquat is
- Organ toxicity: Paraquat affects the
lungs, heart, liver, kidneys, cornea, adrenal glands,
skin, and digestive system.
- Fate in humans and animals: Paraquat is
not readily absorbed from the stomach, and is even more
slowly absorbed across the skin. Oral doses of paraquat
in rats are excreted mainly in the feces, while paraquat
injected into the abdomen leaves through urine . In
the stomach and gastrointestinal tract, paraquat
metabolites may be more readily absorbed than the parent
compound, but their identities and toxicities are unknown
. Paraquat may concentrate in lung tissue, where it
can be transformed to highly reactive and potentially
toxic forms . In one study, farm animals excreted
over 90% of the administered paraquat within a few days.
It was slightly absorbed and metabolized in the
gastrointestinal tract. Milk and eggs contained small
amounts of two paraquat metabolites .
- Effects on birds: The compound is
moderately toxic to birds, with reported acute oral LD50
values of 981 mg/kg and 970 mg/kg in bobwhite and
Japanese quail, respectively . The reported 5- to
8-day dietary LC50 value for the compound is 4048 ppm in
- Effects on aquatic organisms: Paraquat
is slightly to moderately toxic to many species of
aquatic life, including rainbow trout, bluegill, and
channel catfish [58,8]. The reported 96-hour LC50 for
paraquat is 32 mg/L in rainbow trout, and 13 mg/L in
brown trout . The LC50 for the aquatic invertebrate
Daphnia pulex is 1.2 to 4.0 mg/L . In rainbow trout
exposed for 7 days to paraquat, the chemical was detected
in the gut and liver, but not in the meat of the fish.
Aquatic weeds may bioaccumulate the compound. In one
study, 4 days after paraquat was applied as an aquatic
herbicide, weeds sampled showed significant residue
levels . At high levels, paraquat inhibits the
photosynthesis of some algae in stream waters .
- Effects on other organisms: Paraquat is
nontoxic to honey bees .
- Breakdown in soil and groundwater:
Paraquat is highly persistent in the soil environment,
with reported field half-lives of greater than 1000 days
[11,58]. The reported half-life for paraquat in one study
ranged from 16 months (aerobic laboratory conditions) to
13 years (field study) . Ultraviolet light,
sunlight, and soil microorganisms can degrade paraquat to
products which are less toxic than the parent compound.
The strong affinity for adsorption by soil particles and
organic matter may limit the bioavailability of paraquat
to plants, earthworms, and microorganisms [11,58]. The
bound residues may persist indefinitely and can be
transported in runoff with the sediment. Paraquat is not
significantly mobile in most soils . That which does
not become associated with soil particles can be
decomposed to a nontoxic end product by soil bacteria
. Thus, paraquat does not present a high risk of
groundwater contamination. Of 721 groundwater samples
analyzed, only one contained paraquat, at a concentration
of 20 mg/L .
- Breakdown in water: Paraquat will be
bound to suspended or precipitated sediment in the
aquatic environment, and may be even more highly
persistent than on land due to limited availability of
oxygen. It had a half-life in a laboratory stream water
column of 13.1 hours . In another study, paraquat
dichloride was stable for up to 30 days . In a third
study of low levels in water, paraquat had a half-life of
23 weeks .
- Breakdown in vegetation: Paraquat
dichloride droplets decompose when exposed to light after
being applied to maize, tomato, and broad-bean plants.
Small amounts of residues were found in potatoes treated
with paraquat as a desiccant, and boiling the potatoes
did not reduce the residue .
- Appearance: Paraquat salts are
colorless, white, or pale yellow crystalline solids,
which are hygroscopic and odorless .
- Chemical Name:
- CAS Number: 1910-42-5
- Molecular Weight: 257.20
- Water Solubility: 700,000 mg/L @ 20 C
- Solubility in Other Solvents: Dichloride
salt is sparingly soluble in lower alcohols 
- Melting Point: Decomposes @ 300 C 
- Vapor Pressure: Negligible @ room
temperature (paraquat dichloride) 
- Partition Coefficient: 4.4683 
- AdsorptionCoefficient: 1,000,000
- ADI: 0.004 mg/kg/day 
- MCL: Not Available
- RfD: 0.0045 mg/kg/day 
- PEL: Not Available
- HA: 0.03 mg/L (lifetime) 
- TLV: 0.1 mg/m3 (8-hour) (respirable
Zeneca Ag Products
1800 Concord Pike
Wilmington, DE 19897
- Phone: 800-759-4500
- Emergency: 800-759-2500
References for the information in this PIP can be found in
Reference List Number 10
information in this profile does not in any way replace or
supersede the information on the pesticide product labeling or
other regulatory requirements. Please refer to the pesticide