Chlorpyrifos was originally marketed in the US in the
1960s and is now one of the word's leading insecticides. More recently concerns
about related health effects have led to restrictions on its use.
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
Chlorpyrifos (trade name Dursban, Lorsban and
others) is primarily produced by the US multinational DowElanco. Other
manufacturers are Aimco, Agriphar, Excel, Ficom, Gharda, Lupin, Montari (all
India), Frunol (Germany), Jin Hung (South Korea), Point Enterprises
(Switzerland) Luxembourg and Makhteshim-Agan, (both Israel). DowElanco protects
the compound from generic competition vigorously and in 1995 received
compensation form Micro Flo who, DowElanco claimed, had used DowElanco data to
obtain US registration without paying for the data(1).
Uses and usage
Chlorpyrifos is the world's leading
insecticide in volume terms(2). It was first reported in the scientific
literature in 1966, and was originally developed by Dow Chemical Co. (now Dow
Elanco)(3). This OP is used extensively in the US, in agriculture, and both
inside and outside the home environment. In agriculture, it is used primarily on
corn (maize), alfalfa and cotton.
Figures on usage are not generally
available. In the US, the Journal of Pesticide Reform indicated that
during the mid-1990s, 9-12 million pounds (4-5.5 million kg) were used annually
in non-agricultural situations in over 17% of households. Agricultural usage
estimates vary even more with annual application anywhere between 10 and 21
million pounds (4.5-10 million kg)(4).
In the UK, chlorpyrifos is more
commonly used in agriculture, although no really accurate figures are available
for use in the home.
During 1997 about 100,000 kg were used
on about 50 different agricultural crops. Those most heavily dosed in descending
order were grassland, wheat, spring barley, desert apples and culinary apples(5).
Chlorpyrifos is registered for household use against ants and wasps, and by
professional users for use in food storage areas against insects(60.
The acute oral LD50 (the dose required to kill
half of a population of laboratory test animals) for chlorpyrifos is between
135-165 mg/kg for rats(7). It is classified by the World Health Organisation as
a Class II, 'moderately hazardous' pesticide(8).
Chlorpyrifos and other insecticide OPs
are inhibitors of anticholinesterase (ACh-ase), an enzyme vital to the nervous
systems of animals and humans. The transmission of impulses across certain nerve
junctions (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.
Chlorpyrifos is one of the leading
causes of acute insecticide poisoning in the US, according to the US
Environmental Protection Agency (EPA)(9).
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(10) (see
also PN34 fact sheet on OPs).
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
Chlorpyrifos poses a risk of serious
damage to eyes, and is irritating to skin(12).
Poisoning via the skin can easily be misdiagnosed suggesting some cases of
occupational exposure are missed(13).
The dermal LD50 for rabbits is about
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(15).
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
In 1996, the Multiple Chemical
Sensitivity Referral and Resources in the US cited cases of over 450 adults and
children who were poisoned by the pesticide in their home or workplace. Their
most common symptoms were chronic headaches, nausea and vomiting, breathing
difficulties, vision problems, neuromuscular pains and multiple chemical
In 1997, the US EPA announced a Risk
Reduction Plan for Chlorpyrifos which included restrictions on its use
because of a number of alleged ill-health effects cases. They included 22
reports of alleged nervous system disorders and 35 cases that alleged
sensitivity to the chemical(18) (see
also Restriction below).
According to the US EPA, it is not known
whether chlorpyrifos can affect reproduction or cause birth defects in people,
although these potential problems have been the subject of much debate(19). In
1996, a national coalition of US environmentalists(20) called on the EPA to
issue emergency regulations restricting the use of Dursban because of concerns
that it could cause birth defects.
An independent scientific study
reported four cases of serious and disabling birth defects seen in children
whose mothers were exposed to Dursban during the first three months of their
pregnancy. These children have deformed heads, faces, eyes and genitals - and
require constant care. The report concluded that "exposure to Dursban is
consistent with a common teratogenic (birth defect) agent. The production of
similar defects in animals exposed to Dursban and its components supports this
The US EPA classification indicates that
chlorpyrifos shows no evidence of carcinogenicity(22). However, according to the
Journal of Pesticide Reform, xylenes, used as solvents in some
chlorpyrifos-containing products, have caused increased rates of leukemia among
exposed workers. Xylenes may also be co-carcinogens and increase the number of
skin cancers caused by other carcinogens in laboratory animals(23).
Effects on the immune system
Recent research has identified immune system
abnormalities in individuals following chlorpyrifos exposure. Higher than usual
frequencies of allergies and sensitivities to antibiotics together with atypical
abundances of certain types of lymphocytes (decreases in T cells and increases
in CD26 cells) were found in patients one to five years after chlorpyrifos
exposure. Increased expression of CD26 cells is associated with autoimmunity,
where an individual's immune system acts against itself, rather than against
Recent US research showed that if a child
played in his/her home a week after chlorpyrifos application, there was a danger
that he/she would be overexposed to chlorpyrifos. The researchers were still
finding chlorpyrifos residues on toys two weeks after application(25). Based on
the findings of this and other research, the estimated chlorpyrifos exposure
levels from indoor spraying for children are about 21-119 times above the
US-recommended reference dose of 3 mg/kg/day
from all sources(26).
A national survey of pesticides and their
metabolites in the US found that the primary chlorpyrifos breakdown product,
3,5,6-trichloro-2-pyridinol was the second most commonly detected chemical in
Chlorpyrifos is regularly detected in
UK fruit and vegetables. The latest report for 1997 shows that one sample out of
15 of Spanish celery was detected at 0.06 mg/kg which exceeded the maximum
residue limit of 0.05 mg/kg(28).
Chlorpyrifos is relatively non-persistent in
the environment. However, aquatic invertebrates, particularly crustaceans and
insect larvae are sensitive to exposure. The LC50s for these species are
generally less than 0.1 mg/litre(29).
Restrictions on use
In 1997, the EPA, in conjunction with
manufacturers, designed policies to reduce exposure in the home, especially to
children. They agreed to eliminate chlorpyrifos concentrates that require
mixing, limit household consumer use to ready-to-use products, and prohibited
use in inappropriate areas (toys, curtains and furnituture)(30).
In common with other OPs, chlorpyrifos usage
has raised a number of concerns during the 1990s. Most of the examples of
ill-health problems originate from the US. However chlorpyrifos is used widely
throughout the world, and it is likely that many US exposure conditions can be
related to the rest of the world.
1. Agrow's Top Twenty Five, PJB Publications,1997.
3. CDS Tomlin, The Pesticide Manual, BCPC, 1997,
4. Caroline Cox, Chlorpyrifos, Part 1: Toxicology,
Journal of Pesticide Reform, 1994, Vol. 14, No. 4, p15.
5. Pers. Comm., Miles Thomas, Head, Pesticide Survey
Group, Central Science Laboratory, Sand Hutton, York, 1998.
6. OPs and Sheep Dips, the Pesticides Trust [now PAN
7. Op. cit 3.
8. The WHO Recommended Classification of Pesticides by
Hazard 1996-1997, International Programme on Chemical Safety, WHO/IPCS/96.3.
9. Review of Chlorpyrifos Poisoning Data, US EPA,
10. Extension Toxicology Network (EXTOXNET)
Chlorpyrifos, Oregan State University, US, 1993.
11. Op. cit. 9.
12. R. Whitehead, The UK Pesticide Guide, CAB
13. Op. cit.9.
14. Op. cit. 3.
15. Official Group on OPs Report to Ministers, MAFF
publications, 1998, p23.
16. Op. cit. 10.
17. Researchers Link Common Household Insecticide with
Serious Birth Defects and MCS, MCS Referal and Resources, press release, 20
18. Risk Reduction Plan for Chlorpyrifos, US EPA,
press release, June 1997.
19. Chlorpyrifos, Agency for Toxic Substances and
Disease Registry, US Department of Health and Human Services.
20. Op. cit. 17.
21. Janette Sherman, Chlorpyrifos (Dursban) Associated
Birth Defects, International Journal of Occupational Medicine and
Toxicology, 1995, Vol. 4., No. 4:417-431.
22. List of chemicals evaluated for carcinogenic
potential, US EPA, 1994.
23. Op. cit. 4, p18-19.
24. JD Thrasher R, Madison et. al., Immunologic
abnormalities in humans exposed to chlorpyrifos, Archives of Environmental
Health, 1994, Vol. 14, p18. Also op. cit 4.
25. S. Gurunathan, et. al., Accumulation of
Chlorpyrifos on Residual Surfaces and Toys Accessible to Children,
Environmental Health Perspectives, 1998, Vol. 106:9-16.
26. Devra Lee Davis and Karin Ahmed, Exposures for
Indoor Spraying of Chlorpyrifos Pose Greater Health Risks to Children than
Currently Estimated, Environmental Health Perspectives, Vol. 106:299-301.
27. Caroline Cox, Chlorpyrifos, Part 2: Human
Exposure, Journal of Pesticide Reform, 1994, Vol.15, No1, p14.
28. Annual Report of the Working Party on Pesticide
Residues, 1997, MAFF Publications, 1998.
29. Mace Barron and Kent Woodburn, Ecotoxicology of
Chlorpyrifos, Reviews of Environmental Contamination and Toxicology, 1995,
30. Op. cit 18.
[This article first
appeared in Pesticides News No.41, September 1998, p18-19]