© Stuart M Bennett 2001

What is chlorpyrifos?
Chlorpyrifos is one of about 100 organophosphate (OP) insecticides on the market today. It is used to kill insect pests by disrupting their nervous system. Chlorpyrifos has an advantage over other products in that it is effective against a wide range of plant-eating insect pests.

Iupac Formula: O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate so if you really need to look it up in a chemistry book that is what you look for.

Chemical Formula: C9H11Cl3NO3PS

Structure of Chlorpyrifos

The oral LD50 for chlorpyrifos in rats is 95 to 270 mg/kg. The LD50 for chlorpyrifos is 60 mg/kg in mice, 1000 mg/kg in rabbits, 32 mg/kg in chickens, 500 to 504 mg/kg in guinea pigs, and 800 mg/kg in sheep.

How it Works:
Chlorpyrifos and other insecticide OPs are inhibitors of acetylcholinesterase (ACh-ase) as are also carbamates, which is an enzyme vital to the nervous systems of animals and humans. The transmission of impulses across certain nerve junctions (synapses), (including, in humans, those of the autonomic nervous system) involves the release of a transmitter chemical, acetylcholine (ACh). The stimulant effect on ACh is rapidly cancelled by ACh-ase activity. The inhibiting effect of OPs on ACh-ase results in sustained high levels of ACh with consequent serious and widespread disruption of nervous activity.
Symptoms of acute chlorpyrifos poisoning in humans include headache, nausea, dizziness, muscle twitching, weakness, increased sweating and salivation, and occur when cholinesterase activity has been reduced by about 50%. Unconsciousness, convulsions, and death can result with sufficient exposure. These symptoms are common to all organophosphate insecticides with delayed symptoms one to four weeks after exposure of numbness, tingling, weakness and cramping in the lower limbs which can progress into paralysis.

Looking carefully at the molecular structures above, and working outwards from the Phosphorous atom (P), it can be seen that there is a similarity in structure even though some of the groups and side chains are different. It is no surprise therefore that a lot of the side effects experienced with nerve gas poisoning (Tabun and Sarin are nerve gases) are also experienced with insecticide poisoning.

Effects on the central nervous system may include confusion, drowsiness, depression, difficulty concentrating, slurred speech, insomnia, nightmares, and a form of toxic psychosis resulting in bizarre behaviour.

Chlorpyrifos poses a risk of serious damage to eyes, and is irritating to skin. Poisoning via the skin can easily be misdiagnosed suggesting some cases of occupational exposure are missed. In humans, chlorpyrifos and its principal metabolites are eliminated rapidly. After a single oral dose, the half-life of chlorpyrifos in the blood appears to be about 1 day.

Chronic effects:
The adverse effects of OPs are currently the subject of much debate in the UK. A recent government report concluded that their potential to cause ill health following long-term low-level exposure remains unknown and subject to controversy.

Repeat or prolonged exposure to chlorpyrifos may result in the same effects as acute exposure, including the delayed symptoms. Other effects reported on workers repeatedly exposed include impaired memory and concentration, disorientation, severe depression, 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. Human volunteers who ingested 0.1 mg/kg/day of chlorpyrifos for 4 weeks showed significant plasma cholinesterase inhibition.

Some information here is courtesy of Pesticide News.

Back to main Insecticide Page