This document provides an overview of pesticide classification and toxicity. It begins by defining pesticides and tracing their historical development. It then describes three methods of classifying pesticides based on mode of entry, function/target organism, and chemical composition. Several classes of pesticides are discussed in depth, including insecticides, herbicides, fungicides and rodenticides. Specific pesticides are highlighted and their mechanisms of toxicity and health effects are summarized. The document concludes by distinguishing chemical pesticides from natural or biological pesticides.

1. JIMMA UNIVERSITY
INSTITUTE OF HEALTH SCIENCE
SCHOOL OF MEDICAL LABOTATORY SCIENCE
Toxicology and Therapeutic Drug Monitoring Course
Assignment On Toxic Effect of Pesticides
By: Hindiya Abdurahman and
:Reshad Nuredin

2. Out line of presentation
Objectives
Introduction
Toxicological classification of pesticides
Insecticides
Herbicides
Rodenticides
Fungicides
Summary
References

3. Objectives
At the ends of this presentation students should be able to;
Define pesticides and its application
Discuss toxicological classification of pesticides
Explain mechanism of toxicity of different classes of
pesticides
Discuss different types of bio pesticides
List toxicity level of pesticides

4. Introduction
• A pesticide is a substance or mixture of substances intended
for preventing, destroying, repelling, or lessening the
damage of any pest.
• The Chinese used arsenic to control insects, the early
Romans used common salt to control weeds and sulfur to
control insects.
• In the 1800s, pyrethrin was found to have insecticidal
properties.
• The roots of certain Derris species were used by the Chinese
and by South American natives as a fish poison.

5. Cont…
• However, it was not until the 1900s that the compounds we
identify today as having pesticidal properties came into
being.
• Petroleum oils, distilled from crude mineral oils, were
introduced in the 1920s to control scale insects and red
spider mites.
• The 1940s saw the introduction of the chlorinated
hydrocarbon insecticides such as DDT and the phenoxy acid
herbicides such as 2,4 - D).
• Triazine herbicides, such as atrazine, introduced in the late
1950s, dominated the world herbicide market for years.

6. Cont…
• Synthetic pyrethrins or pyrethroid insecticides (e.g.,
resmethrin) became and continue to be widely used
insecticides due to their low toxicity, enhanced persistence
compared to the pyrethrins, and low application rates.

7. TOXICOLOGICAL CLASSIFICATION OF PESTICIDES
• Pesticide is a common term that characterizes several classes
of insecticides, herbicides, fungicides, rodenticides, wood
preservatives, garden chemicals and household disinfectants
that are used to either to kill or protect from pests.
• These pesticides differ in their physical, chemical and
identical properties from one class to other.
• Therefore, it is worthy to classify them based their properties
and study under their respective groups.
• They are categorized into various classes depending on the
needs.

8. Cont…
• Presently, there are three most popular method of pesticides
classification suggested by Drum.
• These three popular methods of pesticides classes comprises:
(i) classification based on the mode of entry
(ii) classification based on pesticide function and the pest
organism they kill
(iii) classification based on the chemical composition of
the pesticide.

9. Classification Based on Mode of Entry
Systemic pesticides
• Systemic pesticides are pesticides which are absorbed by
plants or animals and transfer to untreated tissues.
• Systemic herbicide moves through the plant and can reach
to untreated areas of leaves, stems or roots.
• They can effectively penetrate in the plant tissues and move
through plant vascular system to kill specific pests.
• Examples of systemic pesticides include 2,4-
Dichlorophenoxyacetic acid (2, 4-D) and glyphosate.

10. Cont…
Non-systemic (Contact) pesticides
• The non-systemic pesticides are also called contact
pesticides as it acts on target pests when they come in
contact.
• Pesticides must come into physical contact with the pest to
be effective.
• These pesticides do not necessarily penetrate the plant
tissues and consequently not transported through the plant
vascular system.
• Examples of contact pesticides are paraquat and diquat
dibromide.

11. Cont…
Stomach poisoning and stomach toxicants
• Stomach poisoning pesticide enters the pest’s body through
their mouth and digestive system and causes death by
poisoning.
• Stomach poisons are acquired during feeding of pests, when
they ingest the insecticide applied in the leaves and other
parts of the plant.
• These insecticides kill the vector by destroying the midgut
(or stomach) of the larvae
• Example: Malathion.

12. Cont…
Fumigants
• Fumigants are such pesticides which acts or may kill the
target pests by producing vapor.
• These pesticides form poisonous gases when applied.
• These pesticides in vapor form enter the body of pests via
their tracheal system (respiratory) through spiracles and
causes death by poisoning.
• Fumigants are used to remove stored product pests from
fruits, vegetables and grains.

13. Classification Based on Pesticide Function
and Pest Organism They Kill
• pesticides are classified based on target pest’s organism and
pesticides are given specific names to reflect their activity.
• The group names of these pesticides arise from the Latin
word cide (meaning kill or killer) that are used as suffix after
corresponding name of pests they kill.
• Not necessarily, all pesticides end with word-cide.

14. Table: Pesticide classification by target pests
Type of pests Target pests/Function Example
Insecticides Kill insects and other arthropods Aldicarb
Fungicides Kill fungi (including blights,
mildews, molds, and rusts)
Azoxystrobin
Bactericides Kill bacteria or acts against bacteria Copper
complexes
Herbicides Kill weeds Atrazine
Rodenticides Control mice and other rodents Warfarin
Acaricides Kill mites Bifenazate
Repellents Repel pests by its taste or smell Methiocarb
Molluscicides Inhibit or kill mollusc’s i.e. snail’s
usually disturbing growth of plants
or crops
Metaldehyde

15. Insecticides
• Insecticides play a most relevant role in the control of insect
pests.
• Most of the chemical insecticides in use today are
neurotoxicants, and act by poisoning the nervous systems of
the target organisms.
• Thus, insecticides are mostly not species-selective with
regard to targets of toxicity, and mammals, including
humans, are highly sensitive to their toxicity.
• As a class, insecticides have higher acute toxicity toward
nontarget species compared to other pesticides.

16. Fig. : Classification of insecticides

17. Cont…
Organochlorine
• Organochlorines pesticides (also known as chlorinated
hydrocarbons) are organic compounds attached with five or
more chlorine atoms.
• Most of them were widely used as insecticides for the
control of a wide range of insects.
• Most common examples of these pesticides includes: DDT,
lindane, endosulfan, Aldrin, diel Drin and chlordane.

18. Cont…
Mechanisms of Action
• Organochlorine compound chemicals act on the nervous
system to produce adverse effects.
• This class of chemicals is thought to act by the interference
with cation exchange across the nerve cell membranes
resulting in hyperactivity of the nerves.
• Benzene hexachloride compounds (BHCs) (lindane and
related compounds) are examples of isomers that produce
different effects on the nervous system.
• he γ isomer, also referred to as lindane, causes severe
convulsions with rapid onset,

19. Cont…
Health Effects of Organochlorine Insecticides
• The principal adverse effect associated with over-exposure to
organochlorine insecticides is nervous system hyperactivity.
• Early symptoms seen in chlorinated insecticide intoxications,
such as with DDT, include
• hyperesthesia’s and paresthesias of the face and limbs
• Dizziness
• nausea and vomiting
• tremor, and mental disturbances
• Myoclonic movement and convulsions are sometimes seen
in severe cases of poisoning.

20. Cont…
Organophosphates
• Organophosphate pesticides are considered to be one of the
broad spectrum pesticides which control wide range of pests
due to their multiple functions.
• Organophosphorus insecticides are more toxic to vertebrates
and invertebrates as cholinesterase inhibitors leading to a
permanent overlay of acetylcholine neurotransmitter across a
synapse.
• These pesticides are also biodegradable, cause minimum
environmental pollution and are slow pest resistance.
• Some of the widely used organophosphorus insecticides
include parathion, malathion and diaznon.

21. Cont…
Signs and Symptoms of Toxicity and Mechanism of
Action OP
• The primary target for OPs is AChE.
• Inhibition of AChE by OPs causes accumulation of
acetylcholine at cholinergic synapses, with overstimulation
of cholinergic receptors of the muscarinic and nicotinic type.
• As these receptors are localized in most organs of the body,
a “cholinergic syndrome” ensues, which includes
• increased sweating and salivation, profound bronchial secretion,
bronchoconstriction, miosis, increased gastrointestinal motility,
diarrhea, tremors, muscular twitching, and various central nervous
system effects.

22. Cont…
Treatment of Poisoning
• Upon OP poisoning, prompt treatment is essential.
• In case of dermal exposure, contaminated clothing should
be removed, and the skin washed with alkaline soap.
• Atropine represents the cornerstone of the treatment for OP
poisoning.
• Oximes, such as pralidoxime (2-PAM) are also used in the
therapy of OP poisoning.

23. Cont…
Carbamates
• Structurally, Carbamates are similar to organophosphates.
• However, they differ in their origin. Organophosphates are
derivatives of phosphoric acid, while carbamates derived
from carbamic acid.
• The mechanism of toxicity of carbamates is analogous to
that of OPs, in that they inhibit AChE.
• However, inhibition is transient and rapidly reversible,
because there is rapid reactivation of the carbamylated
enzyme in the presence of water.

24. Cont…
• The sign and symptoms of carbamate poisoning are the same
as observed following intoxication with OPs, and include
• miosis,
• urination,
• diarrhea,
• salivation,
• muscle fasciculation, and CNS effects.
• However, differently from OPs, acute intoxication by
carbamates is generally resolved within a few hours.
• Some of the widely used insecticides under this group
include carbaryl, carbofuran, propoxur and aminocarb.

25. Herbicides
• Herbicides control weeds and are the most widely used class
of pesticides.
• This class of pesticide can be applied to crops using many
strategies to eliminate or reduce weed populations.
• These include:
• preplant incorporation
• preemergent applications and
• post emergent applications.

26. Cont…
Chlorophenoxy
• 2,4-dichlorophenoxy acetic acid (2,4-D) and 2,4,5-
trichlorophenoxy acetic acid (2,4,5-T) are the most
commonly recognized Chlorophenoxy herbicides.
• Some of the commonly used chlorophenoxy herbicides
include Banvel (dicamba), Weedone (2,4-D), and Basagran
M (MCPA).
Figure: 2,4-Dichlorophenoxyacetic acid (2,4-D).

27. Cont…
• The primary routes of exposure to chlorophenoxy herbicides
are dermal and inhalation.
• Chlorophenoxy compounds act by uncoupling oxidative
phosphorylation and decreasing oxygen consumption in
tissue.
• Large intentional overdoses with chlorophenoxy acids have
resulted in symptoms of coma, metabolic acidosis, myotonia,
mucous membrane irritation, and myalgias

28. Cont…
Bipyridyl Compounds-Paraquat and Diquat
 Paraquat (1,1′-dimethyl-4,4′-dipyridylium) and diquat (1,1′-
ethylene-2,2′- bipyridylium) are bipyridylium herbicides.
 Paraquat poisoning can lead to multiorgan toxicity.
 Sufficient dermal exposure to paraquat can also cause dermal
irritation, blistering, and ulceration.
 Its proposed mechanism of action leading to pulmonary
fibrosis is that by which free radicals are generated leading
to lipid peroxidation.

29. Cont…
• Diquat causes less dermal irritation and injury than does
paraquat.
• diquat has an effect on the central nervous system, whereas
paraquat does not.
• The mechanism of action of diquat is thought to be similar to
that of paraquat, involving the production of superoxide
radicals that cause lipid membrane destruction.

30. Cont…
Glyphosate
• Glyphosate is a widely used herbicide that interferes with
amino acid metabolism in plants.
• In animals it is thought to act as a weak uncoupler of
oxidative phosphorylation.
• Glyphosate is moderately absorbed through the
gastrointestinal tract.
• Various signs and symptoms include gastro-intestinal
irritation and damage, as well as dysfunction in several organ
systems (e.g., lung, liver, kidney, CNS, and cardiovascular
system).

31. FUNGICIDES
Pentachlorophenol
• Pentachlorophenol is readily absorbed via the skin, lung, and
gastrointestinal tract.
• Pentachlorophenol and its biotransformation products are
excreted primarily via the kidneys.
• The biochemical mechanism of action of pentachlorophenol
is through an increase in oxidative metabolism from the
uncoupling of oxidative phosphorylation.
• In fatal cases of poisoning from pentachlorophenol, body
temperatures as high as (almost) 41.8 °C (107.4 °F) have
been reported.

32. Cont…
Dithiocarbamates
• Dithiocarbamates are a group of fungicides that have been
widely used since the 1940s to control about 400 fungal
pathogens in a variety of crops.
• The dithiocarbamates have low acute toxicity by the oral,
dermal, and respiratory route.
• These compounds are metabolized to a common metabolite,
ethylenethiourea (ETU), which is responsible for the effects
of dithiocarbamates on the thyroid.

33. Cont…
• ETU produced thyroid tumors in rats and mice.
• That result from the inhibition of the synthesis of the thyroid
hormones thyroxine (T4) and triiodothyronine (T3) .
• This leads to elevated serum levels of thyroid stimulating
hormone (TSH).
• There is also some evidence that dithiocarbamates may cause
neurotoxicity by mechanisms not involving ETU.

34. RODENTICIDES
Anticoagulants
 This class of rodenticides works by depression of the vitamin
K synthesis of the blood clotting factors II (prothrombin),
VII, IX, and X.
 Warfarin is a commonly used coumarin rodenticide that
causes its toxic effects by inhibiting the formation of
prothrombin and the inhibition of vitamin K–dependent
factors in the body.
 Other anticoagulant rodenticides include coumafuryl,
brodifacoum, difenacoum, and prolin.

35. Cont…
Signs and symptoms of intoxication with warfarin include:
• epistaxis,
• hemoptysis
• bleeding gums
• gastrointestinal tract and
• genitourinary tract hemorrhage

36. Cont…
Sodium Fluoroacetate
• This compound is easily absorbed via ingestion as well as
through inhalation and dermal routes.
• The toxicity of sodium fluoroacetate is due to the reaction of
three molecules of fluoroacetate which form fluorocitrate in
the liver.
• Fluorocitrate adversely affects cellular respiration through
disruption of the tricarboxylic acid cycle (inhibiting the
enzyme cis-aconitase).
• It is thought that the accumulation of citrate in tissues also
accounts for some of the acute toxicity associated with this
compound

37. Cont…
• The target organs of sodium fluoroacetate are the heart (seen
as arrhythmias leading to ventricular fibrillation) and the
brain (manifested as convulsions and spasms)
• A specific antidote to sodium fluoroacetate intoxication does
not exist.
• Treatment consists of decontamination and supportive
therapy, including gastric lavage and catharsis.

38. Classification based on sources of origin
• Pesticide is a chemical or biological substance that aims to
destroy the pests or prevent the damage caused by pests.
• Based on sources of origin, pesticide may be classified into
chemical pesticide and bio-pesticides.
• The main benefits of using biological pesticides are host
specificity.
• Whereas chemical pesticides are usually of wide range
which affects large group of non-target organisms.

39. Cont…
Bio-pesticides
• Bio-pesticides group of pesticides derived from natural
materials such as animal, plant and microorganism (bacteria,
viruses, fungi, and nematodes). They are classified into three
groups.
• Microbial pesticides
• Plant incorporated protectants
• Biochemical pesticides

40. Cont…
Microbial pesticides
• The active ingredient in microbial pesticides is
microorganism such as bacterium, fungus or protozoan.
• These pesticides kill insects either by toxins released by
microbial organisms, or by infection by the organisms.
• Two most common pesticides that fit within this group
include:
• The bacterial toxin produced by Bacillus thuringiensis (Bti)
• the live bacteria, Bacillus sphaericus (Bs).

41. Cont…
Plant incorporated protectants
• These groups of pesticides are produced by plants naturally.
• the gene necessary for production of pesticide is introduced
into the plant through genetic engineering.
• Hence, the pesticide then produced by such plant and the
genetic material introduced are together defined as plant
incorporated protectants (PIPs).

42. Cont…
Biochemical pesticides
• Biochemical pesticides which include natural materials that
have nontoxic mechanisms to control pests.
• Examples of Biochemical pesticides are insect sex
pheromones (act by interfering in mating), a range of
aromatic plant extracts (work by attracting insect pests into
traps).

43. Based on toxicity of pesticides
• Depending on the health risk associated with pesticides and
toxic behavior of pesticides.
• The World Health Organization (WHO) classified them into
four categories.
• WHO conducted an experiment on rats and other laboratory
animals by administering a dose of pesticide orally and
dermally? They then estimated the median lethal dose
(LD50) that produces death in 50% of exposed animals to
reach this conclusion.

44. Table: WHO classifications of pesticides based on toxicity
WHO class Toxicity level
LD50 for the rat
(mg/kg body weight)
Examples
Oral Dermal
Class Ia
Extremely
hazardous <5 <50
Parathion, Dieldrin
Class Ib
Highly hazardous
5–50 50–200
Eldrin, Dichlorvos
Class II
Moderately
hazardous 50–2000
200–2000 DDT, Chlordane
Class III
Slightly hazardous
>2000 >2000 Malathion
Class IV
Unlikely to
present acute
hazard in normal
use
>5000
Carbetamide,
Cycloprothrin

45. SUMMARY
• Several groups of pesticides have been classified based on
numerous criteria.
• Most popular basis of pesticides classification are the nature
of target pests kill and,
• chemical composition and characteristics of pesticides, mode
of entry and mode of action or the way a pesticide destroys
or controls the target pest.
• Pesticides with similar structures have similar characteristics
and usually have a similar mode of action.
• Most pesticide active ingredients are either inorganic or
organic pesticides

46. Cont…
• Pesticides have also been classified according to how or
when they work.
• It is seen that the in appropriate application of pesticide may
adversely affect all levels of biological organization and
every component of the environment.
• The effects can be global or local, temporary or permanent,
or short-lived (acute) or long-term (chronic).
• The most serious effects involve destruction of non-target
pest organism

47. References
1. Ernest H. Modern Toxicology. Fourth edition, John
Wiley & Sons, Inc.,2010:55-66.
2. Casarett and Doull´s Toxicology, The basic science of
poisons, seventh edition. McGraw-Hill,2008:883-922.
3.William PL., James Rc., Roberts Sm. Principles of
Toxicology, Environmental and Industrial
Applications, Second edition. John Wiley & Sons,
Inc.,2000:345-363.
4.Yadav, I.C., Devi, N.L. Pesticides Classification and Its
Impact on Human and Environment. Environmental
Science and Engineering.2017,6(2):140-158.
5.Giliomee, J.H. Pesticides: Environmental Management
in South Africa,2014:746-764