Diazinon is an organophosphate (OP) pesticide developed by Novartis in
the early 1950s. Government regulators are increasingly likely to restrict its
use because of concerns about neurotoxicity and data gaps.
What is diazinon?
Diazinon is an OP insecticide and acaricide (a
chemical which kills mites and ticks), which acts as a contact stomach and
respiratory poison. In common with other OPs, diazinon’s toxic action is
achieved by inhibiting acetylcholinesterase, an enzyme essential for normal
nerve impulse transmission.
It is used throughout the world to
control a wide range of sucking and chewing insects and mites on a range of
crops, including deciduous fruit trees, citrus fruit, bananas, vegetables,
potatoes, beet, sugar cane, coffee, cocoa, tea, tobacco, cotton, and rice. It is
also used to control agricultural soil-dwelling insects, and is applied as a
sheep dip to control ectoparasites such as sheep scab and blow fly strike(1).
Diazinon use in homes controls cockroaches, ants, and carpet beetles, and is in
insecticidal pet collars(2). Trade names for diazinon include Knox-out, Dianon
Production and use
Diazinon was originally developed by JR Geigy (now Novartis) in the early 1950s.
Manufacturers since include Aimco, Elf Atochem, Drexel, Makhteshim-Agan, Nippon
Kayaku, and Novartis(4). In many parts of the world, production
and usage figures are not publicly available. Between 1987 and 1997, annual
usage of diazinon in the US totalled about 6 million pounds (2.7 million kg). US
states with significant use include California, Texas and Florida(5). US
production figures for 1999 show 13.5 million pounds (6.1 million kg) of active
ingredient were produced for sale(6). Up to 75% of the diazinon used in the US
each year is for non-agricultural purposes, with 39% used by homeowners(7).
A review of diazinon in 1991 by the
UK’s Pesticide Safety Directorate raised concerns about sub-standard
production resulting in the hazardous by-products
monothiono-tetraethylpyrophosphate and sulfotepp(8).
In California during 1998, 900,596
pounds (408,871kg) of diazinon active ingredient were used in both agricultural
and some urban situations(9).
In the UK, diazinon has mostly been
used either as a sheep dip, or to control ants and cockroaches around the home.
Agricultural uses have been phased out since 1999 (see below). OP sheep dip use
peaked in 1986 when in total 186 tonnes were sold. By 1998 sales had decreased
to about 80 tonnes, largely because compulsory sheep dipping ended in 1992(10).
Since December 1999 OP sheep dip use has been suspended in the UK (see
The World Health Organisation (WHO) classifies diazinon as a class II
‘moderately hazardous’ pesticide. The acute oral LD50 (the dose required to
kill half a population of laboratory animals) for rats is 1,250 mg/kg, and for
mice it is 80-135 mg/kg(12).
Diazinon, as with other OPs and carbamates, poisons humans
and insects through its effects on nerve enzymes. Diazinon combines chemically
with the acetylcholinesterase enzyme and inactivates it. This enzyme is
essential for the control of nerve impulse transmission. Loss of
acetylcholinesterase allows the accumulation of acetylcholine, the substance
secreted by nerves that activates muscles, glands, and other nerves.
Accumulation of sufficient levels of acetylcholine at junctions between nerves
muscles will cause muscle contractions or twitching. Accumulation of
acetylcholine at junctions between nerves and glands results in gland secretion;
and accumulation between nerves in the brain causes sensory and behavioural
The main symptoms of acute diazinon
poisoning are headache, nausea, dizziness, pin-point pupils, blurred vision,
tightness in the chest, difficulty in breathing, muscle weakness or twitching,
difficulty in walking, vomiting, abdominal cramps, and diarrhoea. Effects on the
central nervous system may include confusion, anxiety, drowsiness, depression,
difficulty in concentrating, slurred speech, poor recall, insomnia, nightmares,
and a form of toxic psychosis resulting in bizarre behaviour(14). The US
Environmental Protection Agency (EPA) is concerned that poisoning due to
unrecognised dermal absorption, and other routes of exposure can easily be
misdiagnosed. This suggests that some individual cases of poisoning are
The US Poison Control Centre (PCC) has
recently analysed its nationwide operations for the years 1985 to 1992(16).
There were a total of 20,565 diazinon cases in the PCC database. Of these, 749
cases were occupational exposure – 519 (69.3%) included exposure to diazinon
alone, and 230 (30.7%) involved exposure to multiple chemicals, including
diazinon. There were a total of 10,079 adult non-occupational exposures –
9,060 (89.9%) involved the chemical alone and 1,019 (10.1%) concerned multiple
California is one of the few places in
the world that actively requires mandatory reporting of all occupational
pesticide poisoning incidents from doctors. The California Pesticide Illness
Surveillance Program (1982-1995) reported 521 cases where diazinon alone was
judged responsible for adverse health effects. Only cases with a definite,
probable, or possible relationship were reviewed. Diazinon ranked 5th as a cause
of systemic poisoning in California from 1990 to 1994(18).
In the UK there have been concerns
about the effects of OP sheep dips (including diazinon) since they replaced the
use of organochlorines in the mid-1980s.
Based on inhibition of the enzyme
acetylcholinesterase, the daily administered no-observed-adverse-effect-level
(NOAEL) for humans is 0.025 mg/kg body weight per day, according to WHO(19).
Other reports suggest no-effect doses have ranged from 0.02 mg/kg/day in humans
to 0.1 mg/kg/day in rats(20).
In sub-chronic and chronic toxicity
studies conducted in mice, rats and dogs, systemic toxicity occurred with
decreases in body weight and body weight gains(21).
There are also potential concerns about
breakdown products. In animals diazinon is converted to diazoxon (where the
sulphur molecule is substituted for oxygen), a compound that is a strong enzyme
Diazinon is not considered carcinogenic by agencies such as the International
Agency for the Research on Cancer, or the US EPA(23). However, use of diazinon
by farmers in Iowa and Minnesota has been linked to increased risk of
non-Hodgkins lymphoma, a rare form of cancer(24). Similar links were found in
the 1980s in Nebraska(25).
Since the mid 1980s the health effects from exposure
to low levels of OP sheep dip remain unclear and of great concern. Public
interest groups in the UK such as the OP Information Network and the Pesticide
Exposure Groups of Sufferers have campaigned for a ban on the use of diazinon
because of these issues. Diazinon has played a major role in OP sheep dip use,
along with propetamphos and chlorfenvinphos. Most methods of toxicological
analyses for OPs have concentrated on measuring the indirect biological effects
on humans, by measuring the degree of erythrocyte and plasma cholinesterase
activity. However, people can have a wide variation in cholinesterase levels due
to genetic factors and disease status, so measuring adverse effects can be
difficult without baseline data(26).
After years of concern, the UK
government set up a Committee on Toxicity (COT) Working Group on OPs. Its terms
of reference were: ‘To advise on whether prolonged or repeated low-level
exposure to OPs, or acute exposure to OPs at a lower dose than causing frank
intoxication, can cause chronic ill-health effects.’ The report went on to
‘advise regulatory agencies that any ill-effects remain unproven, although a
question remains over whether there may be a small group of individuals
particularly susceptible to OPs.’(27) Environmental groups say this ‘small
group’ may include about 1,000 sheep dippers(28).
Further work done at the Institute of
Occupational Medicine identified the main risk of adverse effects from OP sheep
dips as exposure to the concentrate. In December 1999 the government responded
by withdrawing all OP sheep dip concentrate containers from the market until the
introduction of containers which will minimise operator exposure to the OP(29).
Fate in the environment
A recent US paper reports that diazinon has been found in rivers across the US
including the Mississippi and the Rio Grande. Diazinon is one of the most
commonly detected insecticides in air, rain and fog(30).
Analysis by the Environment Agency of
Welsh rivers contaminated with OP and synthetic pyrethroid sheep dip revealed
that during 1999, 57% of 111 river sites monitored recorded positive results for
The primary wildlife concern with diazinon results
from its extremely high acute risk to birds. The acute oral LD50 for mallard
ducklings is 3.5 mg/kg, and for young pheasants the figure is 4.3 mg/kg(32). In
the US it has caused more documented avian deaths than any other pesticide
except carbofuran(33). The majority of incidents on known sites have occurred on
lawns and other turf, particularly on golf courses. In one US incident, diazinon
applied at two pounds active ingredient per acre rate on turf caused the death
of some 85 wigeons (a type of duck) after just 30/40 minutes of feeding(34).
The Food and Agriculture Organisation advises against
using WHO class II pesticides like diazinon in developing countries, yet it is
widely used in for example Senegal and Indonesia(35).
There is evidence that fish poisoning
by pesticides occurs in Lake Victoria in East Africa. Between October 1997 and
February 1999 six people were convicted and given prison sentences ranging from
six months to 20 years after being caught on Lake Victoria with pesticides
(endosulfan and diazinon)(36).
Diazinon is found in a range of fruit and vegetables
including pears, soyabean, grain, strawberries, beans and tomatoes. The
Consumers Union in the US is concerned that tolerance levels (maximum residue
levels) for diazinon are too high, especially for children(37). In the UK,
residues of the OPs diazinon and propetamphos are regularly found in sheep.
Surveillance carried out in 1999 on 643 samples of kidney fat showed 20
contained OP residues, with diazinon detected in the range 21-150 µg/kg(38).
The UK government is currently reviewing most uses of
anticholinesterase pesticides (OPs and carbamates) (excluding sheep dips and
other veterinary, and human medicines). In the case of diazinon, agricultural
approvals were revoked in April 1999 because of a lack of data support by the
manufacturers. Existing stocks can continue to be used for two years(39). The
use of diazinon as a sheep dip is currently suspended (see above).
The US also has an ongoing Review Process for all OPs to make
sure that older pesticides such as diazinon meet current scientific and
The manufacturers are not supporting
diazinon at the European Union level and it may be withdrawn by 2003(40).
Earlier this year the US EPA issued a Data-Call-In
notice for a developmental neurotoxicity study for all OPs(41). Specifically for
diazinon, it also announced data requirements for estimation of dermal and
inhalation exposure for workers using the product(42). The US EPA has further
listed a number of environmental fate data gaps including monitoring in
reservoirs and lakes, drinking water supplies, air rain and fog; usage
information in non-agricultural areas; freshwater fish acute toxicity study(43).
A UK review of diazinon in 1991
identified a number of data gaps including a need for operator studies using
emulsifiable concentrates, and for additional wildlife studies(44).
Public interest groups around the world have raised
concerns about the use of OPs in general, and diazinon in particular. In July
this year the Consumers Union called on the US EPA to phase out all uses of
diazinon within four years. It is concerned about exposure levels for children,
and the threat to wildlife, especially many different species of birds. For many
years, groups in the UK such as the OP Information Network and the Pesticide
Exposure Groups of Sufferers have campaigned for a ban on the use of diazinon
because of its impacts on sheep dippers.
After 50 years of use regulatory action
over data gaps may see the end of diazinon. (By David Buffin)
1. C.D.S. Tomlin, The Pesticide Manual, British Crop Protection Council,
2. Diazinon, Environmental Health Criteria 198, International Programme on
Chemical Safety, World Health Organisation, Geneva, 1998, p12.
3. Op. cit. 1.
4. Op. cit. 1.
5. Preliminary Human Health Risk Assessment Diazinon, US Environmental
Protection Agency, Office on Prevention, Pesticides and Toxic Substances, 12
April 2000, p20.
7. Preliminary Environmental Risk Assessment for Diazinon, US Environmental
Protection Agency, 5 May 1999, p10.
8. Diazinon, Evaluation Document, No. 35, Pesticides Safety Directorate, UK,
9. Trends in Use in Cholinesterase Inhibiting Pesticides, California
Department of Pesticide Regulations, 2000,
10. Monitoring of pesticides in the environment, Report of the Pesticides in
the Environment Working Group, Environment Agency, Bristol, 2000, p23.
11. Government Announces Four Point Plan on Organophosphates, MAFF press
release, 455/99, 20 December 1999.
12. WHO Classification of Pesticides by Hazard 1998-1999, International
Programme on Chemical Safety, WHO/IPCS/98.21.
13. Review of Diazinon Incident Reports, US EPA, Office on Prevention,
Pesticides and Toxic Substances, 2 July 2000, p2.
14. Ibid, p3.
15. Op. cit. 13, p3.
16. Op. cit. 13, p18.
17. Op. cit. 13, p22-23.
18. Op. cit. 13, p28.
19. Op. cit. 2, p107.
20. Diazinon, Extoxnet, US, April 1992.
21. Op. cit. 5, p7.
22. Op. cit. 20.
24. K.B. Cantor, et. al., Pesticides and other risk factors for
non-Hodgkin’s Lymphoma among men in Iowa and Minnesota, Cancer Research,
1992, 52, pp2447-2455.
25. S.H. Zahm, et. al., A case-control study on non-Hodgkin’s lymphoma and
agricultural factors in Eastern Nabraska, American Journal of Epidemiology,
1988, 128, p901.
26. K.J.M. Niven, et. al., Occupational hygiene assessment of exposure to
insecticides and the effectiveness of protective clothing during sheep
dipping operations, Insitute of Occupational Medicine, February 1994.
27. Committee on Toxicity Working Group on OPs, 1998.
28. OP Sheep Dips and Human Health, seminar proceedings, National Farmers
Union, UK, 2/6/95. p11.
29 Government announces four point plan on OPs, MAFF press release, 20
30. Op. cit. 7, p11.
31. Sheep Dip in Wales - 2000, Environment Agency, UK, 9 May 2000.
32. Op. cit. 1.
33. Op. cit. 7, p16.
34. Op. cit 7, p16.
35. The Pesticides Trail, The Pesticides Trust [now PAN UK], January 1995.
36. Assessing the Controls on Pesticide Residues in Fish Coming From Lake
Victoria, Final Report, European Commission, DG24, 6 December 1999.
37. Adam Goldberg, Edward Groth and Charles Benbrook, letter to US EPA from
Consumers Union, 17 July 2000.
38. Annual Report on Surveillance for Veterinary Residues in 1999,
Veterinary Medicines Directorate, UK, 2000, p92.
39. Review of Anticholinesterase Compounds - Phase II, letter from the
Pesticide Safety Directorate, 14 April 1999.
41. Op. cit. 5, p8.
42. Op. cit.5, p119.
43. Op. cit, 7, 26-28.
44. Op. cit. 8.
[This article first appeared in Pesticides News No.49, September 2000,