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
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.
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.
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
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.
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
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