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Molecular Structure -
Diazinon
Laboratory Testing: Before pesticides are registered by
the U.S. EPA, they must undergo laboratory testing for
short-term (acute) and long-term (chronic) health effects.
Laboratory animals are purposely given high enough doses
to cause toxic effects. These tests help scientists judge how
these chemicals might affect humans, domestic animals,
and wildlife in cases of overexposure.
- Diazinon is the common name for a synthetic organophosphate
pesticide first registered in the United States in 1956. Diazinon is
used as an insecticide, acaricide, and nematicide.1
- The diazinon Chemical Abstracts Service (CAS) registry number
is 333-41-5. The CAS name for diazinon is O, O-diethyl O-[6-
methyl-2-(1-methylethyl)-4-pyrimidinyl] phosphorothioate and
the International Union of Pure and Applied Chemistry (IUPAC)
name is O,O-diethyl O-2-isopropyl-6-methylpyrimidin-4-yl phosphorothioate.1,2 See the text box on Laboratory Testing.
- Pure diazinon is a colorless oil. Technical grade diazinon (≥90% active ingredient) is
an amber to brown liquid.1
- Vapor pressure1,3: 1.40 x 10-4, 8.4 x 10-5 mmHg at 20 °C
- Octanol-Water Partition Coefficient (Kow)1,3: 2.5 x 104; (log Kow)4: 3.3, 3.81
- Henry's constant may be determined by estimation or experimentally derived. Reported
values include: 1.4 x 10-6 atm·m3/mol and 1.13 x 10-7 atm·m3/mol, depending
on the technique used.3,5
- Molecular weight1: 304.3 g/mol
- Solubility (water)1,3,4,6: 0.04 g/L (40 mg/L) at 20 °C and 30 °C, however other values between
0.054 and 0.069 g/L (54 and 69 mg/L) have been reported in the 20-40 °C temperature range. Diazinon
is completely miscible in acetone, benzene, ethanol, toluene, xylene and is soluble in
petroleum oils.
- Soil Sorption Coefficient (Koc)3,5: Reported values range from 40 to 854 L/kgoc
- Diazinon is a non-systemic insecticide used in agriculture to control soil and foliage
insects and pests on a variety of fruit, vegetable, nut and field crops. Diazinon is also
used on non-lactating cattle in an insecticidal ear tag. Prior to the cancellation of all
residential uses by 2004, diazinon was used outdoors on lawns and gardens, indoors
for fly control and in pet collars designed to control fleas and ticks.7 Uses for individual diazinon products vary widely. Always
read and follow the label when applying pesticide products.
- Diazinon products are formulated as dusts, granules, liquids, concentrates, microencapsulations, wettable powders, seed
dressings and impregnated materials.1
- Signal words for products containing diazinon may range from Caution to Danger. The signal word reflects the combined
toxicity of the active ingredient and other ingredients in the product. See the pesticide label on the product and refer to
the NPIC fact sheets on Signal Words and Inert or "Other" Ingredients.
- Diazinon was one of the most widely used insecticides for household and agricultural pest control. In 2000, the United
States Environmental Protection Agency (U.S. EPA) announced an agreement with the registrants of diazinon to cancel all
residential uses of diazinon. Indoor uses were cancelled in 2002 and outdoor uses in 2004, leaving only agricultural uses
for diazinon.7
- Current agricultural uses of diazinon are limited to selected crops, and diazinon products (other than cattle ear tags) are
regulated as restricted use pesticides.1
- To find a list of products containing diazinon which are registered in your state, visit the website
http://npic.orst.edu/reg/state_agencies.html select your state then click on the link for "State Products."
Target Organisms
- Diazinon is a contact insecticide which kills by altering normal neurotransmission within the nervous system of target organisms.9
Diazinon inhibits the enzyme acetylcholinesterase (AChE), which hydrolyzes the neurotransmitter acetylcholine
(ACh) in cholinergic synapses and neuromuscular junctions. This results in abnormal accumulation of ACh in the nervous
system.10
- Diazinon shares a common mechanism of toxicity with other organophosphate insecticides such as chlorpyrifos,
malathion and parathion, thus, diazinon would not be effective against organophosphate-resistant insect populations.9,11
- Diazinon is metabolized within organisms to form diazoxon (sometimes referred to as "activation," see metabolism section
below), and diazoxon is a more potent cholinesterase (ChE) inhibitor compared to diazinon itself.12
Non-target Organisms
- Diazinon toxicity to non-target organisms is similar to the mode of action for target organisms.5 The enzyme AChE, which
removes the neurotransmitter ACh, is inhibited by diazinon.5 This inhibition interferes with normal neurotransmission in
cholinergic synapses and neuromuscular junctions of the nervous system.13 In wildlife, even slight ChE inhibition has the
potential to make animals more susceptible to environmental factors such as predation, which can in turn effect the survival
of the organism.9
- Non-target organisms can be exposed to diazinon by inhalation, ingestion and/or dermal exposure.1,13
Oral
- Diazinon has a low acute oral toxicity in male and female rats.
The acute oral LD50 for female rats was 1160 mg/kg and 1340
mg/kg for male rats.13 Other reported LD50 values (in mg/kg) include:
300-850 (rats), 80-135 (mice), 250-355 (guinea pigs), 130
(rabbits), 8 (hens), 3 (pheasants), 3.5 (ducks) and 100 (pigs).2,15
See the text boxes on Toxicity Classification and LD50/LC50.
LD50/LC50: A common measure of acute toxicity is the lethal
dose (LD50) or lethal concentration (LC50) that causes death
(resulting from a single or limited exposure) in 50 percent
of the treated animals. LD50 is generally expressed as the
dose in milligrams (mg) of chemical per kilogram (kg) of
body weight. LC50 is often expressed as mg of chemical
per volume (e.g., liter (L)) of medium (i.e., air or water) the
organism is exposed to. Chemicals are considered highly
toxic when the LD50/LC50 is small and practically non-toxic
when the value is large. However, the LD50/LC50 does not
reflect any effects from long-term exposure (i.e., cancer,
birth defects or reproductive toxicity) that may occur at
levels below those that cause death.
- Diazinon causes acute neurotoxicity in rats at sub-lethal oral
doses. One neurotoxicity study found a NOAEL of 0.25 mg/kg/
day based on plasma ChE inhibition.1 See the text box on NOAEL,
NOEL, LOAEL, and LOEL.
| TOXICITY CLASSIFICATION - DIAZINON |
|
High Toxicity |
Moderate Toxicity |
Low Toxicity |
Very Low Toxicity |
| Acute Oral LD50 |
Up to and including 50 mg/kg
(≤ 50 mg/kg) |
Greater than 50 through 500 mg/kg
(>50-500 mg/kg) |
Greater than 500 through 5000 mg/kg
(>500-5000 mg/kg) |
Greater than 5000 mg/kg
(>5000 mg/kg) |
| Inhalation LC50 |
Up to and including 0.05 mg/L
(≤0.05 mg/L) |
Greater than 0.05 through 0.5 mg/L
(>0.05-0.5 mg/L) |
Greater than 0.5 through 2.0 mg/L
(>0.5-2.0 mg/L) |
Greater than 2.0 mg/L
(>2.0 mg/L) |
| Dermal LD50 |
Up to and including 200 mg/kg
(≤200 mg/kg) |
Greater than 200 through 2000 mg/kg
(>200-2000 mg/kg) |
Greater than 2000 through 5000 mg/kg
(>2000-5000 mg/kg) |
Greater than 5000 mg/kg
(>5000 mg/kg) |
| Primary Eye Irritation |
Corrosive (irreversible destruction of
ocular tissue) or corneal involvement or
irritation persisting for more than 21 days |
Corneal involvement or other
eye irritation clearing in 8 -
21 days |
Corneal involvement or other
eye irritation clearing in 7
days or less |
Minimal effects clearing in less than 24 hours |
| Primary Skin Irritation |
Corrosive (tissue destruction into the
dermis and/or scarring) |
Severe irritation at 72 hours
(severe erythema or edema) |
Moderate irritation at 72
hours (moderate erythema) |
Mild or slight irritation at
72 hours (no irritation or
erythema) |
| The highlighted boxes reflect the values in the "Acute Toxicity" section of this fact sheet. Modeled after the U.S. Environmental Protection Agency, Office of Pesticide Programs, Label Review Manual, Chapter 7: Precautionary Labeling. https://www.epa.gov/sites/default/files/2018-04/documents/chap-07-mar-2018.pdf |
Dermal
- Diazinon is low in toxicity following dermal exposure. The acute dermal LD50 is greater than 2020 mg/kg.1,14
- Diazinon is a minimal eye irritant and slight dermal irritant based on tests using rabbits.1
- Diazinon is not a skin sensitizer based on tests in guinea pigs.1
Inhalation
- Diazinon has very low acute inhalation toxicity in rats. Acute inhalation studies with rats found the LC50 to be greater than
2.33 milligrams per liter over four hours (mg/L/4 hours).1,14
Signs of Toxicity - Animals
- Clinical signs of toxicity from acute exposure to diazinon include lacrimation, salivation, anorexia, coughing, miosis, urination
and/or defecation, dyspnea, bradycardia, abdominal pain and distress, as well as vomiting (emesis). Typically these
symptoms are followed by head and body tremors, muscle tetany, stiffness, weakness with paresis and paralysis. Tachycardia
and mydriasis are also possible with massive oral exposures. Central nervous system signs and symptoms may also
be present such as restlessness and/or hyperactivity, depressed respiration, anxiety, depression, clonic-tonic seizures, and
coma.16 Severe exposures can also lead to bradycardia and convulsions.17 These symptoms have been seen in mammals as
well as reptiles, fish and birds.9,16 Birds exposed to diazinon have also demonstrated wing spasms, wing drop, hunched back,
tenesmus, diarrhea, ptosis of eyelid, prostration, opisthotonos-like seizures or wing-beat convulsions.9
- The onset of acute symptoms can occur within minutes of exposure or be delayed as long as 12-24 hours after the exposure
(depending on the formulation and route of exposure) and the duration of symptoms can be several days or even
weeks.17
- Severe diazinon poisoning may cause acute pancreatitis in dogs and guinea pigs.18 In rats, 200 mg/kg of diazinon can cause
acute pancreatitis and histopathological changes in the liver.19
- Exposure to organophosphate insecticides such as diazinon (or a combination of diazinon and other organophosphate
insecticides) can occasionally lead to an intermediate syndrome.16,20 Intermediate syndrome can be seen in dogs, cats and
other animals as a result of prolonged stimulation of cholinergic receptors with ACh, leading to interference of normal
nerve signal transmission. Clinical signs of intermediate syndrome generally occur within hours to days after the exposure,
but may not appear for 3 to 10 days following exposure.20 Symptoms include anorexia, diarrhea, generalized weakness,
muscle tremors, abnormal posturing and behavior, depression and death.20
Signs of Toxicity - Humans
- Symptoms of acute diazinon exposure develop in minutes to hours following exposure, depending of the exposure pathway.
Initial symptoms include nausea, dizziness, salivation, headache, sweating, lacrimation, and rhinorrhea. The symptoms
can progress to vomiting, abdominal cramps, diarrhea, muscle twitching, weakness, tremor, and a lack of coordination. Dark
or blurred vision, anxiety and restlessness, as well as psychiatric symptoms such as depression, memory loss, and confusion
have been reported.13,21
- Because diazinon is fat soluble, there is potential for delayed toxicity if significant amounts of diazinon are stored in fatty
tissues.13
- Intermediate syndrome (see discussion in animal section above) generally occurs within 24-96 hours after exposure. Intermediate
syndrome in humans is characterized by difficulty breathing and muscular weakness, often in the face neck and
proximal limb muscles. Cranial nerve palsies and depressed tendon reflexes have also been reported.13,22
- Studies have suggested that exposure to some organophosphate pesticides can result in long-term neurological problems
including organophosphate-induced delayed neuropathy (weakness or paralysis as well as paresthesia in the extremities);13,21
however, reports of these symptoms following diazinon exposures are rare.23,24
- Human poisoning victims have shown increased levels of serum amylase and glucose as well as elevated urinary diastase
levels accompanied by symptoms considered to be indicative of acute pancreatitis.12,18
- Always follow label instructions and take steps to minimize exposure. If any exposure occurs, be sure to follow the First Aid
instructions on the product label carefully. For additional treatment advice, contact the Poison Control Center at 1-800-
222-1222. If you wish to discuss an incident with the National Pesticide Information Center, please call 1-800-858-7378.
Animals
- Laboratory rats were exposed to diets containing diazinon for 98 weeks
at doses up to 12.0 mg/kg/day. The NOAEL based on ChE inhibition was
0.005 mg/kg/day, and the LOAEL was 0.060 mg/kg/day. No other toxic
effects were observed in the rats.14 See the text box on NOAEL, NOEL,
LOAEL, and LOEL.
NOAEL: No Observable Adverse Effect Level
NOEL: No Observed Effect Level
LOAEL: Lowest Observable Adverse Effect Level
LOEL: Lowest Observed Effect Level
- Beagles were fed diets containing diazinon at doses up to 9.1 mg/kg/day for 52 weeks. At the highest doses tested, researchers
observed decreases in body weight gain, suppressed appetite and ChE inhibition. The NOAEL for ChE inhibition
was 0.0037 mg/kg/day and the LOAEL was 0.0200 mg/kg/day.14
- Rhesus monkeys were given diazinon oral doses of 0, 0.1, 1.0 and 10.0 mg/kg body weight for 34 days and then doses were
lowered to 0.05, 0.50, and 5.00 mg/kg body weight for the remaining 102 weeks. The highest dose groups showed tremors,
and soft stools were noted for the two highest doses. All treatment groups showed slightly decreased weight gain. The
NOAEL for the study was 0.5 mg/kg/day based on ChE inhibition.18
Humans
- Applicators with extensive occupational exposure to diazinon may have a slight asymptomatic reduction in ChE activity.25 See the text box on Exposure.
Exposure: Effects of diazinon on human health and the environment depend on how much
diazinon is present and the length and frequency of exposure. Effects also depend on the health of a person and/or certain environmental factors.
- Rats exposed to diazinon in the air (11.6 mg/m3) for six hours/day, five days per week for three weeks showed no gross or
histological damage to the adrenal gland.12
- Rats exposed orally to diazinon doses up to 212 mg/kg/day for 13 weeks demonstrated no treatment-related damage to
the adrenal gland. In another oral exposure study, rats fed doses up to 12 mg/kg/day for 98 weeks showed no histological
evidence of treatment-related effects on the adrenals, pituitary or thyroid glands.12
- Diazinon exposure has been found to cause pancreatic damage in rats by increasing oxidative stress.26 Beagles receiving
oral doses of 10 mg/kg/day for eight months showed evidence of pancreatic atrophy and interstitial fibrosis in males but
not females.12
- Diazinon is included in the draft list of initial chemicals for screening under the U.S. EPA Endocrine Disrupter Screening
Program (EDSP). The list of chemicals was generated based upon exposure potential, not based on whether the pesticide
is a known or likely potential cause of endocrine effects.27
Animals
- In order to test diazinon for the potential to cause cancer, researchers fed rats 98% pure diazinon at doses of 20 or 40 milligrams
per kilogram body weight per day (mg/kg/day) for 103 weeks. No evidence of carcinogenicity was found.14 In a
similar study, researchers exposed mice to daily oral doses of diazinon at 14 and 29 mg/kg/day for 103 weeks and found
no evidence of carcinogenicity.14,18
Humans
- The U.S. EPA has classified diazinon as "Group D - not classifiable as to human carcinogenicity" based on the lack of evidence
of carcinogenicity found in the mice and rat studies listed above.14 See the text box on Cancer.
Cancer: Government agencies in the United States and abroad have developed programs to evaluate the
potential for a chemical to cause cancer. Testing guidelines and classification systems vary. To learn more
about the meaning of various cancer classification descriptors listed in this fact sheet, please visit the
appropriate reference, or call NPIC.
- Several case-control studies have suggested possible links between exposure to diazinon and childhood cancers and non-
Hodgkin's lymphoma.12 In one study, families who used diazinon in their gardening were found to have an increased incidence
of childhood cancers.28 Another study found farmers using organophosphate insecticides, including diazinon,
had higher rates of non-Hodgkin's lymphoma compared to non-farmers.29 In both studies however, it was not possible to
conclusively attribute the increased cancer risk to diazinon exposure.12,30
Animals
- In a prenatal developmental toxicity study, pregnant rats were fed diazinon doses up to 100 mg/kg/day during days 6
through 15 of gestation. Pregnant rats experienced decreases in body weight gain at the highest dose. Increased fetal
weights, declines in the number of live fetuses and increased pre- and post-implantation losses were also found at the
highest dose.14
- Laboratory tests with pregnant rabbits fed diazinon (89.2% purity) at doses up to 100 mg/kg/day during days 6-18 of gestation
found maternal mortality at the highest dose tested. No effects on the fetuses were noted at any dose.14
- A multi-generational reproductive study exposed rats to diazinon for ten weeks before mating at doses up to 41.43 mg/kg/
day. Offspring of the treated animals were then mated to produce a second generation of rats. The pregnant offspring of
the animals receiving 6.69 mg/kg/day or more had decreased weight gain during pregnancy and their second-generation
offspring had increased pup mortality and decreased weight gain.14
- Beagles exposed orally to diazinon at doses ranging from 2.5 to 20.0 mg/kg/day for 8 months showed testicular atrophy
and arrested spermatogenesis in some animals from the high dose groups.12
- Pregnant hamsters orally exposed to diazinon doses of 0.125 and 0.250 mg/kg/day during fetal organ development showed
maternal cholinergic symptoms but no developmental effects were noted in the offspring.12
- Rat pups were found to be more sensitive to ChE inhibition compared to adult rats. Rat pups given a 75 mg/kg single oral
dose of diazinon showed a 75% brain ChE inhibition compared to 38% in adult rats.11
- Recent studies of neonatal exposure to diazinon in rats have shown that diazinon doses below those that elicit ChE response
can create lasting effects on neural cell development, ACh synaptic function and behavior.31
- Research suggests that ACh and AChE play multiple roles in developing nervous systems. It is not known how much alteration
is necessary or what effects may be observed as a result of altered ACh or ChE levels in the developing nervous system.11
Behavioral effects were confirmed in rat neurotoxicity studies indicating a need to further study the developmental
neurotoxicity of diazinon.10
Humans
- Studies suggest diazinon can be transferred to developing fetuses based on correlations between personal air sampling
devices worn by mothers during pregnancy and maternal and umbilical cord plasma diazinon concentrations.32
- No studies were found on the developmental or reproductive effects of diazinon in humans following inhalation, oral or
dermal exposure to diazinon.
Absorption
- Studies in rats have shown that diazinon given orally is rapidly absorbed. Following oral doses of 80 mg/kg diazinon, the average
time required for rats to absorb half the administered dose was 2.6 hours and maximum plasma concentrations were
found within 2.0 hours of exposure. The study found that about 35% of the oral dose was systemically bioavailable.10
- Rapid oral adsorption has also been documented in humans, beagles, goats, sheep and cows.12
- Human volunteers exposed to radio-labeled diazinon dermally for 24 hours absorbed little of the dose. Researchers found
about 2-4% of the applied dose was absorbed by the skin regardless of the site of application or the vehicle used to administer
the diazinon (lanolin or acetone).12,14
- In vitro studies using a glass flow-through cell with human cadaver skin indicate a higher dermal absorption rate of 14.1%;
however, the results were also more variable (±9.2%).10
Distribution
- Animal tissue studies and autopsy reports of human deaths from diazinon have shown that it is widely and rapidly distributed
throughout the body of humans, rats, sheep and cows.10,12
- Rats exposed orally to 23 mg/kg diazinon had detectable levels of diazinon in the blood, adipose tissue, muscle, liver and
brain. All levels peaked at day 4 except muscle and liver concentrations, which peaked at 12 and 8 days, respectively. Detectable
levels were no longer found in any samples after 30 days.12
- Much of the absorbed diazinon (89-99%) may be bound to plasma protein. Tissue binding likely also occurs, affecting the
time required to clear diazinon from the body.10
- Cows treated dermally with diazinon had trace amounts in their milk 24 hours after the application.33
- No studies were identified that investigated the distribution of diazinon following inhalation exposure in animals or humans.
Metabolism
- Diazinon metabolism has been studied in mice, rats, guinea pigs, dogs, hens, cattle and sheep.34 The primary metabolic
route was cleavage of the P-O-pyrimidine group followed by oxidation and dealkylation of the alkyl substituents on the
pyrimidine ring.34
- Diazinon is oxygenated to an intermediate compound, phosphooxythiran, which in turn undergoes desulfurization to form
diazoxon. Diazoxon is a more potent ChE inhibitor compared to diazinon itself.12 Diazoxon can be detoxified by paraoxanase
1/arylestase (PON1) enzymes35 and can be deactivated by hydrolysis, desulfurization and deoxygenation to form
2-isopropyl-4-methyl-6-hydroxypyrimidine (IMHP), diethylthiophosphate (DETP) and diethylphosphate (DEP).12
- In rats orally exposed to diazinon, liver metabolism reduced the systemically bioavailable amount of diazinon by 65% during
the first pass through the liver.10
- Diazinon is rapidly metabolized with most metabolites being excreted in the urine within 48 hours.34
Excretion
- Diazinon is rapidly excreted from the bodies of animals. Studies have reported blood half-lives for diazinon ranging from
2.5-5.0 hours.10
- Fifty percent of single 4 mg/kg oral doses of radio-labeled diazinon given to rats were excreted within 12 hours. Most (69-
80%) of the metabolites were found in the urine, and 18-25% were excreted in the feces.12
- In another study of rats orally exposed to radio-labeled diazinon, most diazinon was metabolized and excreted in the
urine (58.2% of dose in females and 93.3% in males) within 24 hours and small amounts (less than 2.5%) were excreted in
the feces. After seven days, less than 1% of the labeled dose remained in the tissues, with the highest levels found in the
blood.14
- Human volunteers who ate diazinon excreted 60% of the applied dose as urinary metabolites within two hours of the
exposure.36
- In one study, human volunteers were exposed dermally to diazinon for eight hours. After eight hours, most of the applied
diazinon (90%) was recovered from the application site with only 1% of the applied dose being recovered in the urine.36
Another human study found that dermal exposure to radio-labeled diazinon for 24 hours resulted in 3-4% of the administered
dose being excreted in the urine over seven days following the exposure.12
- Blood samples can be evaluated for decreased plasma pseudocholinesterase and red blood cell AChE levels. The enzyme
depression is usually apparent within a few minutes to hours after exposure and may persist for several days to weeks.13
The activity of plasma can be depressed in some people without exposure to diazinon or similar pesticides because about
3% of humans have a genetically determined low level of plasma pseudocholinesterase.13
- The alkyl phosphate metabolites of diazinon exposure can be detected in the urine within 48 hours of absorption of diazinon,
and may be useful in diagnosing low level exposures that would not result in cholinergic effects.13
- The diazinon metabolites DEP, DETP and IMHP have all been measured in the urine of humans as part of a national human
exposure assessment.37
- Diazinon toxicosis can be diagnosed in some animals by testing the ChE activity of heparinized whole blood. Other testing
that can be performed by laboratories to detect diazinon exposure in animals may include testing of stomach contents,
vomitus, hair, or suspected baits.16
Soil
- Diazinon released into the environment is moderately persistent
and moderately mobile.1,3 Studies of diazinon applied to several
soils show that it is not likely to adsorb to soils. One study looked at
25 soils and found diazinon to be mobile in 80% of the soils tested,
while another study found diazinon leached more in light-textured
soils with low organic matter content.9
- Soil metabolism studies report soil half-lives for diazinon ranging
from 21 to 103 days depending on the type of soil.4 In most tests,
diazinon was detected in soils at a maximum depth of 18 inches.1
See the text box on Half-life.
The "half-life" is the time required for half of the
compound to break down in the environment.
1 half-life = 50% remaining
2 half-lives = 25% remaining
3 half-lives = 12% remaining
4 half-lives = 6% remaining
5 half-lives = 3% remaining
Half-lives can vary widely based on environmental
factors. The amount of chemical remaining after a
half-life will always depend on the amount of the
chemical originally applied. It should be noted that
some chemicals may degrade into compounds of
toxicological significance.
- Microbial degradation in soils is the primary route of diazinon dissipation from the environment.9 Lab studies investigating
diazinon soil metabolism showed half-lives of 1-5 weeks for diazinon in non-sterile soils, and 6-12 weeks in sterile soils.3 The
soil half-life in sandy loam has been reported to be 37 days at pH 5.4 and 39 days at pH 7.8. The aerobic soil metabolism
half-life of diazinon was determined to be 37.4 days in one study5 and 38.0 days in another.5 Diazinon can also degrade
under anaerobic conditions.9
- Diazinon is also degraded in the environment by hydrolysis. Measured rates of hydrolysis were fastest under acidic conditions
(t½ = 12 days at pH 5).1 Diazoxon is the primary degradate of diazinon hydrolysis, however diazoxon is rapidly hydrolyzed
to oxypyrimidine which is more mobile in the environment compared to the parent compound.9
- In addition, diazinon can be broken down by photolysis with soil surface half-lives estimated to be between 17.3 and 37.4
hours.3
- Ultimately, diazinon applied to soils is broken down to carbon dioxide.34
Water
- The persistence and mobility of diazinon and its metabolites suggest the potential for groundwater contamination.1 Diazinon
has been detected in groundwater samples collected in the United States and Canada.12
- Diazinon was the most frequently detected insecticide in surface waters prior to the phase-out of urban uses in 2004.1,38
Since that time, diazinon concentrations have declined in 90% of sampled streams in the Midwestern and northeastern
United States, many showing declines of 50% or more during the summer months.38
- Diazinon degrades in water as a result of hydrolysis, especially under acidic conditions. In sterile water, diazinon was determined
to have a half-life of only 12 days in acidic water (pH = 5) and 138 days in neutral water (pH = 7).1
- Diazinon breakdown is faster at warmer water temperatures, degrading 2-4 times faster in water at 21 °C compared to
water at 10 °C.39
- Diazinon is stable to photolysis in water.1
- The major environmental degradate of diazinon is diazoxon, which rapidly hydrolyzes to oxypyrimidine under most circumstances.
Diazoxon has been found in field studies, but not in laboratory experiments looking at the environmental fate
of diazinon.9
- Oxypyrimidine is very mobile in the environment and has been measured up to 72 inches below the surface of soils.9 Oxypyrimidine
appears to be more persistent under at least some conditions compared with diazinon.5
- Diazinon is not expected to volatilize readily from water based on its vapor pressure and Henry's Law constant. However,
in some studies, as much as 50% of the applied diazinon volatilized from the water. Diazinon has been detected in rain at
concentrations up to 2 μg/L and in fog at concentrations of up to 76 μg/L.9
- In constructed wetlands designed to mitigate agricultural diazinon runoff, 43% of applied diazinon was found to be associated
with aquatic plant material, 34% with the water and 23% associated with aquatic sediments.39
Air
- Diazinon volatilizes into the air at room temperature. Atmospheric concentrations have been measured as high as 306 ng/m3 in air.9
- Diazinon is expected to exist in the particulate and vapor phases when released to the atmosphere.3
- Diazinon adsorbs light above 290 nm wavelength, thus may be subject to direct photolysis.3,12
- Diazinon undergoes transformation to diazoxon (a more potent ChE inhibitor compared to diazinon) in the atmosphere
with an estimated half life of 4 hours.12 Diazoxon has been detected in rain and fog.5
Plants
- Diazinon applied to soils can be absorbed by plant roots and translocated in plants.33
- Diazinon degrades rapidly in leafy vegetables, forage crops, and grass. Half-lives range from 2 to 14 days. Low temperature
and high oil content increase the persistence of diazinon in plants.33
- Diazinon metabolites in plants include the primary hydrolysis product, pyrimidinol, as well as hydroxypyrimidinol. Diazoxon
is also found in plants (and the measured pyrimidinol may result from the hydrolysis of diazoxon rather than diazinon
itself).34
Indoor
- Concentrations of diazinon were measured as high as 13 μg/m3 in indoor air when diazinon was registered for home use.40
Retail garden store indoor air diazinon levels were measured at 3.400 μg/m3 and pest control company buildings showed
concentrations ranging from 0.163 to 0.284 μg/m3.3
- Small amounts of diazinon applied outdoors can be carried indoor by air, dust, soil and pets creating the potential for exposure.41
Food Residue
- The 2006 United States Department of Agriculture (USDA) Pesticide Data Program (PDP) reports that 8102 samples of food
commodities were tested for diazinon residues and 8289 samples were tested for the metabolites of diazinon. Of all the
samples tested, diazinon residues were found in 49 samples (0.6% of the samples tested) in the range of 0.003-0.140 ppm,
concentrations well below the range of tolerances for those commodities (tolerances ranged from 0.50 to 0.75 ppm). A
diazinon metabolite was measured in one spinach sample at 0.005 ppm. No tolerances currently exist for diazinon metabolites
on food.42 Almost 600 raw and finished drinking water samples were also analyzed for the presence of diazinon and its
oxygenated metabolite; no measurable amount of either was found in any of the samples.42
Birds
- Diazinon is very highly toxic to birds, with acute oral LD50 values ranging from 1.44 (mallard duck) to 69.0 mg/kg (brownheaded
cow bird).1 Diazinon has been linked to a number of bird kills following outdoor applications.1,9
- Mallard ducks show reproductive toxicity as a result of chronic diazinon exposure. Concentrations of
16.3 mg/L in the diet resulted in a reduction in the number hatchlings reaching 14 days of age.5,9
- The intermediate degradate diazoxon is also very highly toxic to birds on an acute oral basis. Oxypyrimidine, the terminal
degradation product, is practically non-toxic to birds.5,9
Fish and Aquatic Life
- Studies investigating the toxicity of diazinon to freshwater fish have found diazinon to be moderately to highly toxic to
freshwater fish on an acute basis with LC50 values ranging from 90 to 7800 μg/L. Chronic exposure to diazinon in freshwater
fish has also been studied in brook trout with concentrations of 0.55 μg/L effecting growth and causing neurological
symptoms.1,5 Diazinon caused histopathological changes in the gill structure of bluegill (Lepomis macrochirus) at water
concentrations as low as 15 μg/L.43
- Diazinon is also moderately to highly toxic to estuarine and marine fish with acute LC50 values for marine and estuarine fish
ranging from 150 to 1500 μg/L.1 Salmon behavior and olfactory responses were altered at diazinon concentrations below
those known to be toxic (levels as low as 1.0 μg/L).5
- Diazinon water concentrations ranging from 0.2 to 5.2 mg/L have been associated with fish kills.44
- In bluegill sunfish exposed to 2 μg/L of radio-labeled diazinon, diazinon was found to bioaccumulate in fish tissues. Reported
fish bioconcentration factors are 542x for the edible portions of fish and 583x for the inedible portions.9 Depuration
(the loss of diazinon from the tissues) was rapid with 96% of the diazinon being eliminated from the fish tissues within 7
days.5 Other reported bioconcentration factors range from 4.9x in crayfish (Austropotamobius pallipes) to 200.0x in sheepshead minnow (Cyprinodon variegatus).3
- Diazinon has very high acute toxicity to freshwater aquatic invertebrates
with EC50 values ranging from 0.21 to 35.0 μg/L.5 No
adverse effects were observed in Daphnia magna chronically exposed
to diazinon at 0.17 μg/L. At 0.32 μg/L invertebrate mortality
increased.5 See the text box on EC50.
EC50: The median effective concentration (EC50) may be
reported for sublethal or ambiguously lethal effects. This
measure is used in tests involving species such as aquatic
invertebrates where death may be difficult to determine.
This term is also used if sublethal events are being
monitored.
Newman, M.C.; Unger, M.A. Fundamentals of Ecotoxicology; CRC Press, LLC.:
Boca Raton, FL, 2003; p 178.
- Diazinon is moderately to very highly toxic to marine and estuarine
invertebrates, with LC50 values ranging from 4.2 to >1000.0
μg/L. The acute No Observable Adverse Effect Concentration
(NOAEC) in shrimp was <2.7 μg/L. When shrimp were exposed
chronically, less than one sixth of that dose effected the growth
(weight) of the shrimp.1
- Mesocosm studies in ponds treated with diazinon weekly for 6 weeks resulted in concentrations ranging from 5.7 to 91.5
μ/L of pond water. Aquatic invertebrate populations were reduced at concentrations higher than 11 μg/L. The experiments
did not show any significant direct or indirect effects on fish, despite significant fluctuations in aquatic macroinvertebrate
populations.5
- Available data on the acute toxicity of diazinon to amphibians indicate amphibians are less sensitive to diazinon compared
to fish (96 hour LC50 = 7500 μg/L in yellow-legged frog) and that the metabolite diazoxon is roughly an order of magnitude
more toxic than diazinon (96 hour LC50 = 760 μg/L). The LD50 for terrestrial phase bullfrogs (R. catesbiana) was found to be
>2000 mg/kg.5
- No data currently exist on the toxicity of diazinon to vascular plants, however one study in green algae (Pseudokirchneriella
subcapitata) resulted in a 7-day LC50 of 3700 μg/L, suggesting that green algae are not very sensitive to the toxicity of
diazinon.5
- Studies in constructed wetlands suggest diazinon bound to aquatic sediments has a lower bioavailability compared to
aqueous diazinon.45
- Environmental degradates of diazinon include diazoxon and oxypyrimidine. Diazoxon is higher in toxicity to aquatic organisms
compared with the parent compound. However, diazoxon is short-lived in the environment and its concentrations
are expected to be low compared to the parent compound.5 Oxypyrimidine is practically non-toxic to freshwater fish and
invertebrates (LC50 >101 mg/L and 102 mg/L, respectively) and is practically non-toxic to green algae with an EC50 >109
mg/L.5
Terrestrial Invertebrates
- Diazinon is known to be highly toxic to bees and other beneficial insects following acute contact exposure. The acute LD50
for bees is 0.22 μg/bee.1
- Diazinon is slightly toxic to the earthworm Eisenia foetida with a reported LC50 of 130 mg/kg of soil.2,18
- The acute reference dose (RfD) for diazinon is 0.0025
mg/kg/day.1 The chronic RfD is 0.0002 mg/kg/day.1 See
the text box on Reference Dose (RfD).
Reference Dose (RfD): The RfD is an estimate of the quantity of
chemical that a person could be exposed to every day for the rest
of their life with no appreciable risk of adverse health effects. The
reference dose is typically measured in milligrams (mg) of chemical
per kilogram (kg) of body weight per day.
U.S. Environmental Protection Agency, Integrated Risk Information System, IRIS Glossary, 2009. https://www.epa.gov/iris/iris-glossary#r
- The U.S. EPA has classified diazinon as "Group D - not
classifiable as to human carcinogenicity" based on the
lack of evidence of carcinogenicity found in mice and rat
studies.14 See the text box on Cancer.
- The acute Population Adjusted Dose (aPAD) is 0.0025 mg/kg/day.1
- The chronic Population Adjusted Dose (cPAD) is 0.0002 mg/kg/day.1
- No drinking water standards exist for diazinon. The U.S. EPA has set a lifetime health advisory at 0.001 mg/L for diazinon in
drinking water, however this health guidance level is not enforceable.46
- The National Institute for Occupational Safety and Health (NIOSH) Recommended Exposure Limit (REL) is 0.1 mg/m3 (timeweighted
average).47
Date Reviewed: January 2009
Please cite as: Harper, B.; Luukinen, B.; Gervais, J. A.; Buhl, K.; Stone, D. 2009. Diazinon Technical Fact Sheet; National Pesticide
Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/archive/diazinontech.html.
