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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


A formerly used common name was DMTP (26). Trade names include Bay 29493, Baycid, Baytex, Dalf, DMTP, Entex, Lebaycid, Mercaptophos, Prentox Fenthion 4E, Queletox, S 1752, Spotton, Talodex, Tiguvon.


Fenthion is classified by the U.S. Environmental Protection Agency (EPA) as a Restricted Use Pesticide (RUP) due to the special handling warranted by its toxicity. All bird control products, as well as non-domestic, non-granular formulations of 70% and greater are RUPs (22). RUPs may be purchased and used only by trained certified applicators (4, 21). Fenthion may not be used on food crops (2). Products containing fenthion must bear the signal word "Warning" (26). Check with specific state regulations for local restrictions that may apply.


Fenthion is a contact and stomach insecticide used against many sucking, biting pests, especially fruit flies, stem borers, mosquitoes, and Eurygaster cereal bugs (26). In mosquitoes, it is toxic to both the adult and immature forms (larvae) (18). Once used extensively in the U.S. for controlling intestinal worms, fenthion no longer has FDA approval due to an excess number of poisoning deaths (19). Fenthion is available in dust, emulsifiable concentrate, granular, liquid concentrate, spray concentrate, ULV and wettable powder formulations (26).

While it is effective as an insecticide, it is also moderately toxic to mammals, and highly toxic to birds. Based on its high toxicity to birds, fenthion is used in various parts of the world for weaver bird control. Pest control operators have used it to control pigeons around public buildings, as well. For bird control, use is made of fenthion's contact action and its ready absorption through the skin. It is applied as a paste to roosting areas when utilized for such purposes (10). Fenthion has been tested in dogs and cows for possible control of parasites (25).

Fenthion 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.



Fenthion is moderately toxic if ingested, inhaled, or absorbed through the skin. It affects the central nervous, cardiovascular, and respiratory systems, and may irritate eyes and mucous membranes (14). As with all organophosphates, fenthion is readily absorbed through the skin. Skin which has come in contact with this material should be washed immediately with soap and water and all contaminated clothing should be removed. While symptoms of poisoning may be delayed in animals, in cases of human poisonings, symptoms have generally been immediate. Deaths are primarily due to respiratory failure (5). Several cases of intentional or accidental human poisonings via ingestion and/or dermal exposure are known (8, 19). Individuals with respiratory ailments, recent exposure to cholinesterase inhibitors or impaired cholinesterase production, or with liver malfunction may be at increased risk from exposure to coumaphos (27).

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, 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 (27).

Some organophosphates may cause delayed symptoms beginning 1 to 4 weeks after an acute exposure which may or may not have produced immediate symptoms. In such cases, numbness, tingling, weakness and cramping may appear in the lower limbs and progress to incoordination and paralysis. Improvement may occur over months or years, but some residual impairment will remain (27).

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 fenthion in rats is 180 to 298 mg/kg, 150 mg/kg in rabbits, 260 mg/kg in guinea pigs, and 88 to 145 mg/kg in mice (1, 2, 11, 13, 27). The dermal LD50 in rats is 330 to 1,000 mg/kg, and 500 mg/kg in mice (1, 26, 27).

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 inhalation 1-hour LC50 for fenthion is rats is 2.4 to 3.0 mg/l (27).


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 (27). Prolonged exposure to fenthion may result in flu-like symptoms, such as loss of appetite (anorexia), weakness, and malaise (14).

There was no evidence of weight loss or decreased food consumption in dogs that were given 50 mg/kg of fenthion in their diets for one year (24).

Reproductive Effects

Single injections of 40 or 80 mg/kg of fenthion into the abdominal cavities of pregnant female mice caused poisoning in the developing fetuses, particularly when administered on days 10 through 12 of gestation. There were significantly more abnormalities in the offspring of female mice that had received 40 mg/kg on days 8 or 10 of pregnancy. Fetuses were injured primarily by dosages that caused toxicity in the maternal mouse (8). No influence was seen on reproduction in other 3- generation studies of mice (1). After administration of 0.5 mg/kg/day for 30 days, the eggs laid by surviving mallards had markedly reduced fertility (20). Once in the bloodstream, fenthion may cross the placenta (27).

Teratogenic Effects

Some reduction in fetal weight occurred, but no defects were found in mice that were given intraperitoneal doses of up to 80 mg/kg of fenthion in single day or 3-day periods during the period of gestation in which organs are formed (17). Other tests on mice and rats did not show teratogenic effects from fenthion (1). No teratogenic effects were seen in five generations of mice that drank water containing 60 parts per million (ppm) fenthion (8).

Mutagenic Effects

Tests on mice did not show mutagenic effects from fenthion (1).

Carcinogenic Effects

The National Cancer Institute performed carcinogenicity tests on fenthion that indicated that this insecticide may be a carcinogen in male mice (12). However, no carcinogenic effects were observed in other two-year feeding studies of rats and mice (1).

Organ Toxicity

Fenthion primarily affects the nervous system through inhibition of cholinesterase, an enzyme required for proper nerve functioning. No delayed neurological effects were found in chickens with single and repeated applications of fenthion (1). Within four weeks of exposure to a dietary concentration of 250 ppm (or 12.5 mg/kg/day) of fenthion, rats showed weight loss without change in food intake, and inhibition of brain cholinesterase activity to about 15% of normal. A dose of 1 mg/kg caused inhibition of 60-70% of normal brain cholinesterase activity. A dose of 0.25 mg/kg caused slight brain cholinesterase-inhibition (8).

Fate in Humans and Animals

In animals, fenthion is quickly absorbed into the bloodstream through the digestive tract, lungs, and skin, and then broken down. Its breakdown products are eliminated through the urine and the feces in a three-day period (1, 7). A single dose of the insecticide has prolonged action, suggesting that much of it is stored and later released for metabolism (8). Fenthion has fat-loving, or 'lipophilic,' properties and tends to be deposited in fatty tissue (19). Fenthion and its metabolites were found in the fat of steers slaughtered 3 days after dermal application of fenthion (25).

When cows were given a dermal application of 9 mg radio-labeled fenthion per kg, 45-55% of the dose was excreted in the urine, 2.0 to 2.5% in the feces, and 1.5 to 2.0% was recovered in the milk (25).


Effects on Birds

Fenthion is highly toxic to birds (10). It is more toxic to fowl than to mammals (5). Acute symptoms of fenthion poisoning in birds include tearing of the eyes, foamy salivation, lack of movement, tremors, congestion of the windpipe, lack of coordination in walking, and an abnormally rapid rate of breathing or difficult breathing (20). Chickens developed leg weakness when they were fed 25 mg/kg doses of fenthion (15). The acute oral LD50 in poultry is 15 to 30 mg/kg (7, 13).

The LC50 for fenthion in mallards is 250 to 299 ppm, 180 to 220 ppm in pheasants, and 25 to 35 ppm or 60 mg/kg in bobwhites. In these tests, fenthion was included in diets of two-week-old birds for five days and was followed by untreated feed for three days (15, 26).

Effects on Aquatic Organisms

Fenthion is moderately toxic to fish (19). The 96-hour LC50 for fenthion in rainbow trout is 0.930 ppm or 0.83 mg/l, 1.160 ppm in carp, 1.540 ppm in largemouth bass, 2.404 ppm in fathead minnow, and 3.404 ppm in goldfish. Brown bullheads were not affected by the insecticide when it was applied to a California refuge at 0.01 pounds per acre (15, 26). Fenthion should not be applied for mosquito control in areas containing fish, shrimp, crabs, or crayfish (18).

Effects on Other Animals (Nontarget species)

Because fenthion is toxic to bees, flowering crops must not be sprayed (7, 18). The LD50 in domestic farm animals given fenthion by injection through the skin is 98 mg/kg. The intramuscular LD50 of domestic farm animals is 46 mg/kg (13).


Organophosphates tend to be persistent and to bioconcentrate in environmental systems (23).

Breakdown of Chemical in Soil

Fenthion adsorbs strongly to soil particles. This adsorption makes fenthion less likely to move, or leach, through the soil with water percolating through the ground (23). In soil, residues of fenthion persist for approximately 4 to 6 weeks (6).

Breakdown of Chemical in Water

This material should be kept out of lakes, streams, or ponds, and it should not be applied where runoff is likely to occur. Care should be taken not to contaminate water by cleaning fenthion application equipment or disposal of related wastes. In one study of its persistence in water, 50% of applied fenthion remained in river water 2 weeks later, while 10% remained after 4 weeks (9). Residue levels of 16 micrograms per liter (ug/l) of fenthion were found in the water from which dead birds were retrieved, in an area that had been sprayed 2 days earlier with recommended mosquito control levels of the insecticide (19).

Breakdown of Chemical in Vegetation

Fenthion is phytotoxic (or poisonous to plant material) to American linden, Hawthorn and sugar maple trees, and to certain rose varieties (19). It is not considered phytotoxic when used at recommended rates, although injury has occurred in certain varieties of apples and cotton. Plant foliage should not be sprayed when temperatures exceed 90 degrees F (18). Only about 10% of applied fenthion remained on rice plants after 6 hours. Almost half of the activity was found in the rice bran, 6.5% was in the husk, and 14.7% was in polished rice. Water soluble metabolites were found 14 days after fenthion application to rice plants (19).


Pure fenthion is a colorless liquid. Technical fenthion is a yellow or brown oily liquid with a weak garlic odor (1, 25).

Fenthion is subject to hydrolysis (1), and it is not compatible with highly alkaline materials (18). Fenthion is stable under normal temperatures and pressures, but may pose a moderate fire hazard if exposed to heat or flame. Containers may explode in the heat of a fire. In the presence of strong oxidizers, fenthion may pose a fire and explosion hazard. Thermal decomposition may release toxic oxides of carbon, sulfur and phosphorus (27).

Fenthion is sometimes mixed with paints (18). Fenthion must be stored in sealed, original containers, and kept in well-ventilated dry storage areas. It should be kept away from heat sources, flames, or sparking equipment, as its temperature must not exceed 25-30 degrees C. Containers should be stacked in a way that permits air circulation at the bottom and insides of piles. Storage areas should be an appropriate distance from inhabited buildings, animal shelters, and food storage. Fenthion containers must also be kept out of reach of unauthorized persons, children, and domestic animals (19). Household use of fenthion should be limited to trained personnel. Excessive wetting of plastic, tile, or rubber should be avoided. It should not be applied to young, stressed, or sick animals (18).

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 (28).

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.

Exposure Guidelines:

TLV-TWA: 0.2 mg/m3 (skin) (1)
OSHA TWA: 0.2 mg/m3 (skin) (27)
ACGIH TWA: 0.2 mg/m3 (skin) (27)

Physical Properties:

CAS #: 55-38-9
Specific gravity: 1.25 at 20 degrees C (26)
H20 solubility: 54-56 mg/L (1); nearly insoluble in water (26)
Solubility in other solvents: highly soluble in most organic solvents, including alcohols, ethers, esters, halogenated aromatic.
Soluble in petroleum ether (11).
Readily soluble in dichloromethane, 2-propanol, and toluene (26).
Melting point: 7.5 degrees C (26)
Boiling point: 87 degrees C (189 degrees F) at 0.01 mm Hg (pure) (8, 25)
Flash point: >180 degrees F (>82 degrees C) (27)
Vapor pressure: 3.7 x 10 to the minus 5 power mmHg at 20 degrees C (26); Negligible (27)
Kow: 4.091 (19)
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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