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Beyond Pesticides Rating: Toxic

Trichlorfon, an organophosphate pesticide, is used to control a variety of indoor and outdoor insect pests in agricultural, residential and commercial sites ranging from chicken coupes to golf courses to residential homes. The U.S. Department of Agriculture was the first to register this pesticide in 1955. In 1995, U.S. Environmental Protection Agency’s (EPA) Reregistration Eligibility Document identified several data gaps on the effects of trichlorfon on human health and the environment. 

Mode of Action

Trichlorfon acts on the target insect as both a contact poison and as a stomach poison. Trichlorfon, like all organophosphates, are cholinesterase inhibitors, meaning that the chemical binds irreversibly to the active site of an essential enzyme for normal nerve impulse transmission, acetylcholine esterase (AchE), inactivating the enzyme. The insect, therefore, loses control over its own nervous system and is paralyzed. 


EPA has categorized trichlorfon as a toxicity category II pesticide (on a scale of I to IV, I being the highest toxicity rating), and is required to have the signal word DANGER printed on the label (U.S. EPA 1997). While trichlorfon is considered to be moderately toxic by ingestion and skin absorption, it is readily absorbed through the skin and can result in skin sensitization (allergies). Inhalation results in respiratory problems (shortness of breath, coughing,) and bloody or runny nose. Eye contact will cause bleeding, blurred vision, pain and pupil constriction. Within a few minutes to twelve hours after exposure, systemic effects such as vomiting, abdominal cramps and diarrhea will be experienced.  

The primary effects of trichlorfon are seen in the central nervous system, symptoms include: loss of reflexes, slurred speech, involuntary muscle contractions, severe depression, irritability, confusion, impaired memory, insomnia, psychosis and paralysis (ETN 2001). A 60-day study in rats fed 100mg/kg/day decreased cholinesterase activity to under half the normal level.  

Studies show that trichlorfon can cause abortions in rats and fetal abnormalities in mice, hamsters and rats. The insecticide is a mutagen in mice when given in small repeated doses. Long-term exposure to trichlorfon can also result in edema (excessive fluid) of the brain, liver damage, inflammation of the lungs and heart muscle changes (ETN 2001).  


Trichlorfon is classified as a “Likely” carcinogen for high doses of exposure and “Not Likely” for low doses of exposure in EPA’s Weight-of-Evidence categories. Carcinogenic effects were seen in rats and other available data suggests that this chemical does contribute to the production of tumors of the kidneys, lungs and mammary in animal studies (U.S. EPA 2000).  

Ecological Effects

Trichlorfon is toxic to wildlife. It is highly toxic to birds, cold and warm water fish, beneficial insects, non-target insects and aquatic invertebrates. The acute bird oral toxicity is 40-47 mg/kg. Symptoms of poisoning (i.e. regurgitation, trembling, wing-beat convulsions) have been seen as little as ten minutes after exposure. The acute toxicity to freshwater fish is between 1.67 and 180 ppm (parts per million) (ETN 2001). 

Environmental Fate

Organophosphates have a tendency to persist and bioconcentrate in ecosystems. Trichlorfon is very mobile in soils of different textures and organic contents, very soluble in water and does not absorb to soil particles. Therefore, it is likely to contaminate groundwater. Its half-life in aerobic soils varies from 1 to 27 days, depending on the fertility of the soil. Approximately 99% of trichlorfon applied to alkaline water conditions (pH 8.5) was broken down within two hours. Pond water that had acidic conditions (pH 5.0) stayed stable for two hours. However, one of the breakdown products of trichlofon is dichlorvos (DDVP), an insecticide, which stayed present in the water for approximately 526 days at 20 degrees Celsius (ETN 2001) 

Data Gaps

EPA concluded that the data available was insufficient in order to completely evaluate the effects of the pesticide on human health and the environment. Studies have been requested for estuarine or marine mollusk acute toxicity, the aquatic invertebrate life cycle, terrestrial field dissipation, worker exposure and field drift. More data is also needed to show the action of trichlorfon in water.  


Extension Toxicology Network (ETN). 2001. Pesticide Information Profiles: Trichlorfon. <http://pmep.cce.cornell.edu/profiles/extoxnet/pyrethrins-ziram/trichlorfon-ext.html>. 

U.S. EPA. 2000. List of Chemicals Evaluated for Carcinogenic Potential. Office of Pesticide Programs. Washington, DC. 

U.S. EPA. 1997. “Reregistration Eligibility Document.” Office of Pesticide Programs. Washington, DC. <http://www.epa.gov/oppsrrd1/REDs/index_h2z.html#T>.