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Extension Toxicology Network

A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Michigan State University, Oregon State University, and University of California at Davis. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.


Publication Date: 9/93


Trade names include AC 3422, Alkron, Alleron, Aphamite, Corothion, E-605, ENT 15108, Ethyl parathion, Etilon, Fosferno 50, Niran, Orthophos, Panthion, Paramar, Paraphos, Parathene, Parawet, Phoskil, Rhodiatox, Soprathion, Stathion and Thiophos. The common name thiophos is used in the former USSR.


Because of its high toxicity and risks of exposure to agricultural workers and to birds, and in response to the manufacturers' request, EPA in January 1992 announced the cancellation of all uses of parathion on fruit, nut and vegetable crops. The only uses retained are those on alfalfa, barley, corn, cotton, sorghum, soybeans, sunflowers and wheat. Further, to reduce exposure of agricultural workers, parathion may be applied to these crops only by commercially certified aerial applicators and treated crops may not be harvested by hand. EPA intends to cancel all uses of parathion in the near future (12, 13).

Parathion is one of the most acutely toxic pesticides registered by the EPA. Because of its highly toxic nature, parathion is classified as a Restricted Use Pesticide (RUP) (1, 3). RUPs may be purchased and used only by certified applicators. Products containing parathion must bear the signal word "Danger" (3).


Parathion is a broad spectrum, organophosphate pesticide used to control many insects and mites (5, 8). It has non-systemic, contact, stomach and fumigant actions (5, 8). It has a wide range of applications on many crops against numerous insect species (1). Parathion is available in dust, emulsion concentrate, granular, ULV liquid, and wettable powder formulations (3).

Parathion is one of a class of insecticides referred to as organophosphates. These chemicals act by interfering with the activities of cholinesterase, an enzyme that is essential for the proper working of the nervous systems of both humans and insects. Please refer to the Toxicology Information Brief on cholinesterase-inhibition for a more detailed description of this topic.



Parathion is highly toxic by all routes of exposure. Human fatalities have been caused by ingestion, dermal adsorption, and inhalation of parathion (2). As with all organophosphates, parathion is readily absorbed through the skin (1). Skin which has come in contact with this material should be washed immediately with soap and water and all contaminated clothing should be removed. Persons with cardiovascular, liver or kidney diseases, glaucoma, or central nervous system abnormalities may be at increased risk from exposure to parathion. High environmental temperatures or exposure of the chemical to visible or UV light may increase its toxicity (11).

Parathion may cause thickening and roughening of the skin (hyperkeratinization). It does not cause sensitization (allergies). Parathion is not irritating to the eyes. Splashing parathion into an eye may cause constriction of the pupil, making it difficult to determine the path of moving objects. Organophosphates are used to treat diseases of the eye, like glaucoma. It is possible, though, that they cause cataracts to form (6).

The organophosphate insecticides are cholinesterase inhibitors. They are highly toxic by all routes of exposure. When inhaled, the first effects are usually respiratory and may include bloody or runny nose, coughing, chest discomfort, difficult or short breath, and wheezing due to constriction or excess fluid in the bronchial tubes. Skin contact with organophosphates may cause localized sweating and involuntary muscle contractions. Eye contact will cause pain, bleeding, tears, pupil constriction, and blurred vision. Following exposure by any route, other systemic effects may begin within a few minutes or be delayed for up to 12 hours. These may include pallor, nausea, vomiting, diarrhea, abdominal cramps, headache, dizziness, eye pain, blurred vision, constriction or dilation of the eye pupils, tears, salivation, sweating, and confusion. Severe poisoning will affect the central nervous system, producing incoordination, slurred speech, loss of reflexes, weakness, fatigue, involuntary muscle contractions, twitching, tremors of the tongue or eyelids, and eventually paralysis of the body extremities and the respiratory muscles. In severe cases there may also be involuntary defecation or urination, psychosis, irregular heart beats, unconsciousness, convulsions and coma. Death may be caused by respiratory failure or cardiac arrest (12).

The amount of a chemical that is lethal to one-half (50%) of experimental animals fed the material is referred to as its acute oral lethal dose fifty, or LD50. The oral LD50 for parathion is 2 to 30 mg/kg in rats, 5 to 25 mg/kg in mice, 8 to 32 mg/kg in guinea pigs, 10 mg/kg in rabbits, 0.93 mg/kg in cats, and 3 to 5 mg/kg in dogs (2, 3, 11). The dermal LD50 in rats is 6.8 to 50 mg/kg (2, 3), in mice is 19 mg/kg, in guinea pigs is 45 mg/kg, and in rabbits is 15 mg/kg (11). The lowest dosage with toxic effects (TDlo) in humans is 240 ug/kg (less than 0.1 ounce).

The lethal concentration fifty, or LC50, is that concentration of a chemical in air or water that kills half of the experimental animals exposed to it for a set time period. The 4-hour inhalation LC50 for parathion in rats is 84 mg/m3 (11).


Repeated or prolonged exposure to organophosphates may result in the same effects as acute exposure including the delayed symptoms. Other effects reported in workers repeatedly exposed include impaired memory and concentration, disorientation, severe depressions, irritability, confusion, headache, speech difficulties, delayed reaction times, nightmares, sleepwalking and drowsiness or insomnia. An influenza-like condition with headache, nausea, weakness, loss of appetite, and malaise has also been reported (12).

One study found that dietary doses of 50 ppm (about 2.5 mg/kg/day) produced toxic symptoms, growth retardation and death in rats. In another feeding study, dietary doses of 2.5 mg/kg/day for 2-years had no effect on rats, while doses of 5 mg/kg/day produced only slight signs of toxicity and growth retardation, but no deaths (2).

Reproductive Effects

Once in the bloodstream, parathion may cross the placenta (7, 11). Repeated feedings to female rats before mating resulted in adverse effects on the reproductive system (11). In lab animals, such as rats and mice, several effects are seen. Fewer pups are born to dams fed parathion. These pups have reduced birth weight and do not tend to survive as well as normal pups (IARC Monographs, 16).

Teratogenic Effects

While parathion is toxic to the fetus, it does not cause birth defects (2).

Mutagenic Effects

Dietary doses of parathion failed to produce dominant lethal effects in mice (2).

Carcinogenic Effects

Parathion is a possible carcinogen (14).

Organ Toxicity

Parathion primarily affects the nervous system through inhibition of cholinesterase, an enzyme required for proper nerve functioning. In humans poisoned with parathion, an increase in brain weight occurs. It is not understood why this happens (Z. Rechtsmed 90 (3):173 - 189. 1983). Dogs have changes in their livers (NRC Drinking Water and Health 1977). Delayed neurotoxicity is not a problem with parathion (Toxicol. 23 (4):267-279. 1982).

Fate in Humans and Animals

Parathion is readily absorbed into the bloodstream from the skin, lungs or gut (7). The vapor pressure of parathion is so low that breathing the vapor alone is not a likely source of poisoning. Breathing dusts, or aerosols, may be extremely dangerous (6). Parathion is rapidly distributed through the body. The liver metabolizes parathion into the active metabolite: paraoxon. It is paraoxon that actually inhibits the cholinesterase. Paraoxon is further metabolized to compounds such as paranitrophenol which is readily excreted in the urine. Parathion may be stored in fat. Unlike the organochlorine pesticides (DDT), the organophosphates (parathion) are rapidly broken down once they are mobilized from the fat stores (6).


Effects on Birds

Parathion is extremely toxic to birds (4) such as mallards, pigeons (5), quail, sparrows and grouse (4). It is less toxic to pheasants (5). The LD50 for parathion in bobwhite quail is 6 mg/kg (3), 3 mg/kg in pigeons, and 2.1 mg/kg in ducks (Hdbk Acute Tox. Chem. Fish & Aquatic Inverts. 1980). (NIOSH RTECS Online File # 84/8406).

Effects on Aquatic Organisms

Parathion is moderately toxic to fish and aquatic invertebrates (like crayfish, snails and worms) (5, 8, 16). The 96-hour LC50 for parathion in fish in general is 1.43 mg/l (3). The 96-hour LC50 in trout is 1.6 mg/l, 1.8 mg/l in goldfish, 2.7 mg/l in catfish, 0.3 mg/l in mosquito fish, and 0.02 mg/l in bluegill (Hdbk Acute Tox. Chem. Fish & Aquatic Inverts. 1980). (NIOSH RTECS Online File # 84/8406).

Effects on Other Animals (Non-target species)

The 24-hour LD50 for parathion in honeybees is 0.07 to 0.10 ug/bee when applied topically (3). The LD50 for parathion in mule deer is 22-44 mg/kg (Hdbk Acute Tox. Chem. Fish & Aquatic Inverts. 1980). (NIOSH RTECS Online File # 84/8406).

Some fat storage of parathion does occur. However, upon release from fat storage, parathion is rapidly broken down and eliminated. Bioconcentration of parathion is low to moderate. There is no evidence of bioaccumulation of parathion in cattle, sheep or rabbits (10).


Breakdown of Chemical in Soil and Groundwater

Parathion has little or no potential for groundwater contamination (3). It binds tightly to soil particles and is degraded by biological and chemical processes within several weeks. Degradation is faster in flooded soil. Residues of parathion can persist for many years, but usually remain in the upper 6 inches of soil. Photodegradation may occur on soil surfaces (10).

Sunlight can convert parathion into the active metabolite paraoxon, which is more toxic than parathion. The breakdown of parathion in soil or water increases with increasing (more alkaline) pH. Soil microorganisms, sunlight, plants and water all break parathion down.

Breakdown of Chemical in Water

In open water, parathion will usually disappear within a week, mainly by adsorption to suspended particles and bottom sediments. Adsorbed parathion is subject to degradation by microorganisms and chemical hydrolysis. The half-life for photo-degradation of parathion in water is 1 to 10 days. Increasing the pH (alkalinity), increases the rate of breakdown (10).

Breakdown of Chemical in Vegetation

Following spray applications, parathion residues on foliage will decay with a half-life of 1 day, reaching low levels in a week or two (10). In orange groves, the half-life of parathion is as long as one month. Usually, it is closer to one-two weeks. Most crops tolerate parathion very well. Only at high application rates do apples, cucumbers, and tomatoes suffer from parathion usage (8).


Pure parathion is a pale yellow liquid with a faint odor of garlic at temperatures above 6 degrees C. Technical parathion is a deep brown to yellow liquid (2, 11). Parathion hydrolyzes slowly at pH 7 or below, but is otherwise stable at normal temperatures (3). At temperatures above 120 degrees C, parathion decomposes and may develop enough pressure to cause containers to explode. Thermal decomposition may release toxic gases such as diethylsulfide, sulfur dioxide, carbon monoxide, carbon dioxide, phosphorus pentoxide, and nitrogen oxides (3). Parathion poses a fire and explosion hazard in the presence of strong oxidizers. It may attack plastics, rubber and coatings (11).

Persons who work with organophosphate materials for long periods of time should have frequent blood tests of their cholinesterase levels. If the cholinesterase level falls below a critical point, no further exposure should be allowed until it returns to normal (15).

Protective clothing must be worn when handling fenthion. Before removing gloves, wash them with soap and water. Always wash hands, face and arms with soap and water before smoking, eating or drinking.

After work, remove all work clothes and shoes. Shower with soap and water. Wear only clean clothes when leaving the job. Wash contaminated clothing and equipment with soap and water after each use. Keep contaminated work clothes separate from regular laundry.

The National Fire Protection Agency ratings for parathion include:

A. health: 4 = a few whiffs of the fumes could prove fatal; normal fire fighting gear is inadequate to protect against any exposure to the skin.

B. flammability: 1 = solids which must be preheated to burn, but which are combustible. It may be dangerous to use water to extinguish burning parathion.

C. reactivity: 2 = normally unstable materials which will react violently (with water). Also, it is potentially explosive when mixed with water (18).

Exposure Guidelines:

Air concentrations of 20 mg/m3 or higher are immediately dangerous to life or health (11).

OSHA TWA (skin): 0.1 mg/m3 (11)
ACGIH TWA (skin): 0.1 mg/m3 (11)
NIOSH Recommended TWA (skin): 0.05 mg/m3 (11)
Odor threshold: 0.04 ppm (11)

Physical Properties:

CAS #: 56-38-2
Specific gravity: 1.26 (3)
Viscosity: 15.30 CPS at 25 degrees C (11)
Solubility in water: 12.4 mg/l at 25 degrees C (3); 24 ppm (11)
Solubility: Soluble in alcohols, animal & vegetable oils, aromatic hydrocarbons, esters, ethers, n-hexane, dichloromethane, 2-propanol, toluene and ketones.
Insoluble in kerosene, petroleum ether, or spray oil (2, 3, 11, 19).
Boiling point: 375 degrees C (707 degrees F) at 760 mm Hg (19)
Melting point: 43 degrees F (6 degrees C) (11)
Flash point: 174 degrees C, decomposes rapidly above 120 degrees C (3); >200 degrees F (>93 degrees C) (11)
Vapor pressure: 8.9 x 10 to the minus 6 mm Hg at 20 degrees C (3); 4 x 10 to the minus 5 mm Hg at 20 degrees C (11)
Oil: water partition coefficient:
Chemical class/use: organophosphate insecticide


Miles, Inc.
Crop Protection and Animal Health Div.
PO Box 4913
Kansas City MO 64120

Review by Basic Manufacturer:

Comments solicited: November, 1992
Comments received:


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  13. _____. 1991 (Dec.). Notice of Voluntary Cancellation of Parathion Registrations Except for Use on Field Crops. US EPA, Washington, DC.
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