WORLD HEALTH ORGANIZATION FOOD AND AGRICULTURE
ORGANISATION MONDIALE DE LA SANTE ORGANISATION POUR L'ALIMENTATION
ORIGINAL : ENGLISH
DATA SHEETS ON PESTICIDES No. 4 Rev.1
Primary use: Herbicide
Secondary use: None
Chemical group: Bipyridyl
Data sheet No. 4, Rev.1 (8/78)
It must be noted that the issue of a Data Sheet for a
particular pesticide does not imply endorsement of the pesticide by
WHO or FAO for any particular use, or exclude its use for other
purposes not stated. While the information provided is believed to
be accurate according to data available at the time when the sheet
was compiled, neither WHO nor FAO are responsible for any errors or
omissions, or any consequences therefrom.
The issue of this document does Ce document ne constitue pas une
not constitute formal publication. Il ne doit faire
publication. It should not be l'objet d'aucun compte rendu ou
reviewed, abstracted or quoted résumé ni d'aucune citation sans
without the agreement of the l'autorisation de l'Organisation
Food and Agriculture des Nations Unies pour
Organization of the United l'Alimentation et l'Agriculture
Nations or of the World Health ou de l'Organisation Mondiale de
Organization. la Santé.
1. GENERAL INFORMATION
1.1 COMMON NAME: Paraquat (ISO)
1.1.1 Identity: 1,1'-dimethyl-4.4'-bipyridilium ion. It should be
stated which anion is resent (e.g. paraquat dichloride).
Synonyms: Local synonyms:
1.2 SYNOPSIS - Paraquat is a bipyridyl herbicide, highly toxic to man
on oral ingestion; its toxic effect in mammals is due largely to
damage to lung alveoli. It is a severe eye and moderate skin
irritant, but is not significantly absorbed through intact skin.
Absorption of spray mist can occur but does not appear to be of
1.3 SELECTED PROPERTIES
1.3.1 Physical characteristics - Available as the dimethyl sulfate or
the dichloride. White crystalline solids; the dimethyl sulfate
is deliquescent. Both have m.p. ca 300°C with decomposition.
Concentrated solutions corrode steel, tinplate, galvanized iron
It must be noted that the issue of a Data Sheet for a particular
pesticide does not imply endorsement of the pesticide by WHO or FAO for
any particular use, or exclude its use for other purposes not stated.
While the information provided is believed to be accurate according to
data available at the time when the sheet was compiled, neither WHO nor
FAO are responsible for any errors or omissions, or any consequences
The issue of this document does not constitute formal publication.
lt should not be reviewed, abstracted or quoted without the
agreement of the Food and Agriculture Organization of the
United Nations or of the World Health Organization.
Ce document ne constitue pas une publication. If ne doit faire l'objet
d'aucun compte rendu ou résumé ni d'aucune citation sans I'autorisation
de l'Organisation des Nations Unies pour l'Alimentation et
l'Agriculture ou de l'Organisation Mondiale de la Santé.
1.3.2 Solubility - Water at 20°C about 700 g/l; slightly soluble in
alcohol, insoluble in most other organic solvents.
1.3.3 Stability - Stable in acid and neutral solutions, unstable in
alkaline solutions. Decomposes in ultra-violet light.
Inactivated by anionic surface-active agents and by inert clays.
Rapidly inactivated on contact with soil.
1.3.4 Vapour pressure (volatility) - Not measurable: nonvolatile.
1.4 AGRICULTURE, HORTICULTURE AND FORESTRY
1.4.1 Common formulations - Aqueous solutions of the dichloride
containing 200 g/l of the cation, together with anticorrosive and
surface-active agents. A formulation without surface active
agents is used as an aquatic herbicide.
Mixtures containing 100-200 g/l paraquat with diquat (80-90 g/l)
or a residual herbicide are available.
Also formulated as water-soluble granules containing 25 g/kg
paraquat + 25 g/kg diquat.
There is an FAO specification for the aqueous salt solution.
1.4.2 Susceptible pests - Green plant tissue generally, on contact and
in the presence of light. Used particularly to control broad-
leaved weeds and grasses.
1.4.3 Use pattern - As contact herbicide before and after crop
emergence on plantation and vegetable crops, in orchards, for
aquatic weed control, stubble clearing and pasture renovation.
Main uses are for weed control around trees in orchards and
plantations and, by directed application, between rows of growing
crops, and as cotton defoliant and dessicant on various crops,
particularly potato haulm and sugar cane. Application rates
usually range from 250 to 1500 g/ha. Up to 2200 g/ha is used for
grass and stubble clearing.
1.4.4 Unintended effects - Damage can occur to bulbs in very sandy
soil. Not harmful to wildlife or soil processes when used
1.5 PUBLIC HEALTH PROGRAMME - Not used.
1.6 HOUSEHOLD USE - The granule formulation (25 g/kg paraquat + 25
g/kg diquat) is used for weed control in home gardens. Liquid
formulations for dilution before use are sometimes marketed.
2. TOXICOLOGY AND RISKS
2.1 TOXICOLOGY - MAMMALS
2.1.1 Absorption route - May be absorbed through the gastrointestinal
tract. Paraquat is not absorbed to any great extent by intact
skin and there is no evidence of significant absorption from
2.1.2 Mode of action - After a latent period, produces marked
congestion of the lungs with oedematous fluid in many of the
alveoli and excess macrophages in others. Paraquat may also
produce severe kidney damage giving rise to renal failure.
2.1.3 Excretion products - Oral administration of paraquat dichloride
to rats resulted in 94% excretion in the faeces and 6% in the
urine within 48 hours.
2.1.4 Toxicity, single dose
Oral: LD50 rat (M): 100 mg/kg
LD50 rat (F): 110 mg/kg
Dermal: LD50 rat (M): 80 mg/kg
LD50 rat (F): 90 mg/kg
Inhalation: LC50 (four hours) rabbit, dichloride, 6.4 mg/m3
Most susceptible species - Guinea-pig, oral LD50 30 mg/kg.
Man appears to be a highly susceptible species.
2.1.5 Toxicity, repeated dose
Oral: Daily oral doses of 20 (mg/kg)/day to sheep for five days
resulted in the death of all animals within two weeks. At 10
(mg/kg)/day for five days, one out of six sheep died while 5
(mg/kg)/day for 14 days resulted only in listless animals.
Similar effects were observed in cattle.
Dermal: Rabbits were given daily percutaneous doses of paraquat.
At 14.5 (mg/kg)/day, two out of three animals died within 20
days. At 7.3 (mg/kg)/day there were no deaths but there was some
consolidation in lung alveoli. The no-effect level was 2.8
(mg/kg)/ day. In another study, one out of five rabbits died
when a daily percutaneous dose of 1.5 (mg/kg)/day was
administered under an impervious layer for 20 days.
Inhalation: Repeated daily six-hour exposure of rats to
paraquat aerosols over a three week period produced signs of lung
irritation but no deaths at 0.4 µg/m3.
Cumulation of compound: Does not appear to accumulate in
2.1.6 Dietary studies
Short-term: No information.
Long-term: In a 26-27 month feeding study of paraquat
dichloride to dogs there was increased mortality and lung changes
at 125 mg/kg diet (3.125 (mg/kg)/day) but no effect at 50 mg/kg
(1.25 (mg/kg)/day). No adverse effects were observed at a
dietary level of 250 mg//kg (12.5 (mg/kg)/day) of paraquat
dichloride (the maximum level) fed to rats over a two-year
2.1.7 Supplementer studies of toxicity
Rat: No increase in tumour incidence at a maximum dietary level
of 250 mg/kg diet (12.5 (mg/kg)/day) for two years.
Reproduction studies: A multi-generation study in rats has
shown that 100 mg/kg paraquat in the diet did not interfere with
the reproduction of three successive generations.
Rat: A single intraperitoneal injection of 6.5 mg/kg of
paraquat on day 6 of gestation produced a high incidence of
costal cartilage malformation in the embryos. This defect was
not noted when injections were given on days 7 to 14 of
Grazing studies: Paraquat, when ingested as a residue in
herbage, has been reported to present no toxicological hazard to
2.1.8 Modifications of toxicity: No special features reported.
2.2 TOXICOLOGY - MAN
2.2.1 Absorption - See 2.1.1
Ingestion has proved to be the main cause of poisoning with this
compound. One fatal case of percutaneous absorption has been
2.2.2 Dangerous doses
Single: The fatal dose in adults is estimated to be 10-15 ml of
the 20 g/l concentrate (i.e., 30-50 mg/kg). However, it has been
suggested that the ingestion of 3 g is the maximum compatible
Repeated: No information.
2.2.3 Observations of occupationally exposed workers - No reported
incidence of serious systemic toxic effects from plant workers
engaged in the manufacture of paraquat. Irritation of skin and
mucous membranes, severe irritation of the eye and effects on
finger-nails have resulted from careless use.
2.2.4 Observations on exposure of the general population - No
2.2.5 Observations of volunteers - No information available.
2.2.6 Reported mishaps - There are no known outbreaks of poisoning by
paraquat. There have, however, been numerous cases, mostly with
a fatal outcome. About half of these have been accidents, the
others suicides. It has been suggested that the incidence of
mortality from accidental ingestion of paraquat is 50%. In 40%
of all fatal cases the interval between ingestion and death has
been more than a week.
2.3 TOXICITY TO NON-MAMMALIAN SPECIES
2.3.1 Fish - Not hazardous: rapidly absorbed by aquatic plants and
inactivated in mud.
2.3.2 Birds - Not highly toxic. No hazard under normal conditions of
2.3.3 Other species - Toxic to bees, but method of use avoids risk.
3. FOR REGULATORY AUTHORITIES - RECOMMENDATIONS ON REGULATION OF
3.1 RECOMMENDED RESTRICTIONS ON AVAILABILITY
(For definition of categories, see introduction).
Liquid formulations 10% or more, category 4.
Solids over 25% category 4, all other formulations, category 5.
3.2 TRANSPORTATION AND STORAGE
All formulations in categories 3 and 4 - Should be transported
or stored in clearly labelled rigid and leak-proof containers.
No food or drink should be transported or stored in the same
compartment. Storage should be under lock and key, and secure
from access by unauthorized persons and children.
Formulations in category 5 - Should be transported or stored in
clearly labelled leakproof containers away from food.
All formulations in categories 3 and 4 - Protective clothing
should be provided for those handling concentrates. Adequate
washing facilities should be available close at hand. Eating,
drinking and smoking should be prohibited during handling and
before washing after handling.
Formulations in category 5 - No facilities other than those
needed for the handling of any chemical are required.
3.4 DISPOSAL AND/OR DECONTAMINATION OF CONTAINERS - Containers must
either be burned or crushed and buried below topsoil. Containers
may be decontaminated (for method see paragraph 4.3 or sheet 4).
Decontaminated containers should not be used for food and drink.
3.5 SELECTION, TRAINING AND MEDICAL SUPERVISION OF WORKERS
All formulations in categories 3 and 4 - Training of workers in
techniques to minimize contact essential.
Formulations in category 5 - Warning of workers to avoid contact
3.6 ADDITIONAL REGULATIONS RECOMMENDED IF DISTRIBUTED BY AIRCRAFT
All formulations - Pilots and loaders should receive special
training in application methods. Use of flagmen not recommended.
Flagmen, if used, should wear overalls and be located well away
from the dropping zone.
All formulations in categories 3 and 4 - Minimum cautionary
statement - Paraquat is a toxic substance. It is poisonous if
swallowed and highly irritating to the eyes if splashed into
them. Avoid skin contact; wear protective gloves while mixing
and wear protective clothing while mixing and using the material.
Wash thoroughly with soap and water after using. Keep the
material out of reach of children and well away from foodstuffs,
animal feed and their containers.
Formulations in category 5 - Minimum cautionary statement -
This formulation contains paraquat which is a toxic substance.
It is poisonous if swallowed and highly irritating to the eyes if
splashed into them. Keep the material out of reach of children
and well away from foodstuffs, animal feed and their containers.
3.8 RESIDUES IN FOOD
3.8.1 Maximum residue levels (tolerances) - The Joint FAO/WHO Meeting
on Pesticide Residues has recommended maximum residue levels.
3.9 SPECIAL NOTE ON PARAQUAT - While poisoning is frequently fatal,
it usually only results from misuse of paraquat, i.e. by
accidental or deliberate ingestion. The hazard can be diminished
by limiting the maximum concentrations of the chemical as
4. PREVENTION OF POISONING IN MAN AND EMERGENCY AID
4.1 PRECAUTIONS IN USE
4.1.1 General - Paraquat is a bipyridyl herbicide, highly toxic to man
by oral ingestion, its toxic effect in mammals is due largely to
the damage that it produces to lung alveoli. It is a severe eye
and moderate skin irritant but is not absorbed to any great
extent by intact skin; there is no evidence of significant
absorption from spray mist.
4.1.2 Manufacture and formulations
ACGIH - 0.5 mg/m3.
Closed systems and forced ventilation may be required to reduce
as much as possible the exposure of workers to the chemical.
4.1.3 Mixers and applicators - When opening the container and when
mixing, protective impermeable boots, clean overalls, gloves and
a face mask should be worn. Mixing, if not mechanical, should
always be carried out with a paddle of appropriate length. When
spraying tall weeds or during aerial application a face visor
should be worn as well as an impermeable hood, clothing, boots
and gloves. The applicator should avoid working in spray mist
and avoid contact with the mouth. Particular care is needed when
equipment is being washed after use. All protective clothing
should be washed immediately after use, including the insides of
gloves. Splashes must be washed immediately from the skin or eyes
with large quantities of water. Before eating, drinking or
smoking, hands and other exposed skin should be washed.
4.1.4 Other associated workers (including flagmen in aerial operations)
- Persons exposed to paraquat and associated with its application
should wear protective clothing and observe the precautions
described in 4.1.3 under "mixers and applicators".
4.1.5 Other populations likely to be affected - With good agricultural
practice subject to 4.2 below, other populations should not be
exposed to hazardous amounts of paraquat.
4.2 ENTRY OF PERSON INTO TREATED AREAS - No restrictions.
4.3 SAFE DISPOSAL OF CONTAINERS AND SPILLAGE - Containers should be
emptied in a diluted form into a deep pit. The empty container
may be decontaminated by rinsing two or three times with water
and scrubbing the sides. An additional rinse should be carried
out with 5% sodium hydroxide solution which should remain in the
container overnight. Impermeable gauntlets should be worn during
this work and a soakage pit should be provided for the rinsings.
Decontaminated containers should not be used for food and drink.
Spillage of paraquat and its formulations should be removed by
covering the area with soil and rinsing with large quantities of
4.4 EMERGENCY AID
4.4.1 Early symptoms of poisoning - Early symptoms of poisoning may
include epigastric discomfort and vomiting as well as general
malaise and weakness. There may be irritation of the mouth,
pharynx and oesophagus with local burning. With very large doses
there may be excitement and convulsions.
4.4.2 Treatment before person is seen by a physician if these symptoms
appear following exposure - If swallowed, vomiting should be
induced. A high fluid intake should be maintained, the patient
kept at rest and sent to hospital immediately. In cases of
contamination of skin or clothing, wash the affected skin with
soap and water, if available, and flush the area with large
quantities of water.
5. FOR MEDICAL AND LABORATORY PERSONNEL
5.1 MEDICAL DIAGNOSIS AND TREATMENT OF CASES OF POISONING
5.1.1 General information - A bipyridyl herbicide of moderately high
acute toxicity which may be absorbed through the intact skin as
well as by inhalation. The main hazard, however, is absorption
by oral intake. Paraquat owes its toxic effect largely to the
delayed damage that it produces on the lung alveoli. In rats it
is excreted largely in the faeces but after absorption can be
readily detected in the urine. The extent to which it persists
in the tissues is still unclear.
5.1.2 Symptoms and signs - Initial symptoms of poisoning may be
epigastric discomfort, diarrhoea and vomiting along with general
malaise and weakness. There may be irritation of the mouth,
pharynx and oesophagus with local burning. After one or two days
signs of tissue and possibly liver damage will appear, if
appreciable quantities have been swallowed. After one to two
weeks there may be dyspnoea with pulmonary oedema leading to
massive pulmonary fibrosis and death due to respiratory
insufficiency. With very large doses there may be excitement and
5.1.3 Laboratory - The presence of paraquat in the urine is indicative
of absorption of the compound. Urinary levels should be measured
at frequent intervals. Blood levels are very low and do not
provide a satisfactory method for determining the extent of
5.1.4 Treatment - If the pesticide has been ingested it is imperative
that a prompt effort be made to remove as much paraquat as
possible before absorption takes place so as to supplement its
elimination via the kidneys. Gastric lavage should be carried
out with care because of the possible oesophageal injury. At
least 500 ml of a 7% bentonite (colloidal aluminium silicate)
suspension should be introduced into the stomach within one to
two hours after the paraquat has been ingested. The suspension
is prepared by triturating bentonite with glycerine and adding
water to a final concentration of 7% bentonite and 10% glycerine.
Fuller's earth 30% can be used in place of bentonite. As
paraquat is freely excreted by the renal glomeruli but is
reabsorbed in the tubules, forced diuresis is of benefit in
hastening excretion. Haemodialysis and peritoneal dialysis may be
indicated if there is evidence of renal failure. Additional
treatment may include immunosuppressive therapy and prednisone 60
mg and cyclophosphomide 3 mg/kg per day have been recommended to
try to prevent the lung lesions. Oxygen may be necessary if
cyanosis or dypsnoea occurs but there is some evidence that its
effect may be harmful.
5.1.5 Prognosis - The prognosis in cases of paraquat poisoning is very
poor. In 40% of cases death has occurred more than a week after
ingestion. Progressive respiratory embarrassment may occur five
to 10 days after taking the paraquat, sometimes after a period
of apparent recovery. Once the lung changes become evident
chances of recovery are practically nil.
5.1.6 References of previously reported cases - The following
references give methods of treatment used in cases of poisoning:
Kerr, F., Patel, A. R., Scott, P. D. R. & Thompsett, S. L. (1968)
Brit. med. J., 3, 290-291
McDonagh, B. J. & Martin, J. (1970) Arch. Dis. Childh., 45,
Clinicopathological Conference (1971) Scot. med. J., 16, 407
Malone, J. D. G., Carmody, M., Keogh, B. & O'Dwyer, W. F. (1971)
J. Irish med. Ass., 64, 69
5.2 SURVEILLANCE - Levels of paraquat in the urine provide the most
readily available method for indicating absorption of paraquat.
However, actual levels cannot be correlated with the severity of
intoxication because recovery is probably also dependent on the
volume of urine excreted and therefore the total amount of
paraquat eliminated from the body. By way of guidance the
highest concentration of paraquat found in the urine of spray
workers was 0.32 mg/l and the average was well below 0.1 mg/l.
In poisoning cases it has been found that recovery may occur if
the peak level is below 200 mg/l.
5.3 LABORATORY METHODS
References only are given.
5.3.1 Detection and assay of compounds - Detection of paraquat depends
upon reduction to the free radical with sodium dithionite. In
alkaline solution a stable blue colour is then formed which may
be measured spectrophotometrically. For determination in urine
see Thompsett (1970) and Berry & Grove (1971). (Thompsett also
describes determination in other body fluids and tissues.)
Residues in food crops can be determined by the method of
Calderband & Yuen (1965) (see also Pack, 1967); later
modifications are suitable for determinations in meat, milk and
animal tissues (Plant Protection Ltd., 1972).
5.3.2 Other tests in cases of poisoning - None.
Thompsett, S. L. (1970) Paraquat poisoning, Acta. Pharmacol. Toxicol.,
Berry, D. J. & Grove, J. (1971) The determination of paraquat (1,1'-
dimethyl-4.4'-bipyridilium cation) in urine, Clin. chim. Acta,
Calderband, A. & Yuen, S. H. (1965) An ion-exchange method for
determining paraquat residues in food crops, Analyst, 90, 99
Pack, D. E. (1967) In: Zweig, G., ed., Analytical Methods for
Pesticides, Plant Growth Regulators and Food Additives,
Academic Press, New York and London, vol. V, p. 473
Plant Protection Limited (1972) Details of the methods are available
from Plant Protection Limited, Fernhurst, Hazlemere, Surrey,
England (Personal communication)