**3. Classification of pesticides**

The word "pesticide" is an umbrella term for all insecticides, fungicides, herbicides, rodenti‐ cides, garden chemicals, wood preservatives, and household disinfectants that may be used to kill some pests. Pesticides have different identities and physical and chemical properties. Synthetic pesticides are classified based on various ways. In general, there are three main ways to classify them: classification based on the (i) mode of action, (ii) targeted pest species, and (iii) chemical composition of pesticides [35].

#### **3.1. Classification based on the mode of action**

Pesticides are classified based on the way they act to bring about the desired effect in this classification. Under this type of classification, pesticides are classified as nonsystemic and systemic pesticides. Nonsystemic pesticides are those that do not appreciably penetrate plant tissues and consequently not transported within the plant vascular system. On the contrary, systemic pesticides are those that effectively penetrate plant tissues and transported within the plant vascular system to bring about the desired effect [36].

#### **3.2. Classification based on the targeted pest species**

Classification by target pest is perhaps the most familiar. For example, insecticides are pesticides that target insects, and herbicides target plants. The others are rodenticides, fungicides, acaricides and miticides, molluscicides, bactericides, avicides, and virucides.

#### **3.3. Classification based on the chemical composition**

In this type of classification, pesticides are characterized regarding their chemical nature and active ingredients. This is the most useful one for researchers studying the field of pesticides and the environment, because it is this kind of classification that gives the clue of the efficacy and physical and chemical properties of the respective pesticides and precautions that need to be taken during application and the application rates, the knowledge of which is important in the mode of application [37].

According to chemical properties, pesticides can be generally divided into about seven types, including organochlorines, organophosphorus, carbamates, pyrethroids, amides, anilins, and azotic heterocyclic compounds. Organochlorine chemicals are organic compounds with five or more chlorine atoms. Organochlorines were the first synthetic organic pesticides to be used in public health and in agriculture. These pesticides generally have a steady chemical structure and often accumulate and persist in the environment. Most of them are widely used as insecticides for the control of a wide range of insects. Organochlorine insecticides act as nervous system disruptors leading to convulsions and paralysis of the insect and its eventual death. They can cause serious endocrine disorders in mammals, fish, and birds, so most of them have been banned in agriculture worldwide [36,38]. Organophosphates are another type of highly toxic pesticides that contain a phosphate group and occupied up to 48.6% of all pesticides in 1997 [39]. The importance of synthetic organophosphates increased considerably during World War II with their use as warfare materials. Since then, these pesticides have been used in agriculture, industry, cosmetics, medicine, and many other areas [40,41]. These chemical compounds inhibit the acetylcholinesterase enzyme, which hydrolyses acetylcholine in the nervous system of a number of species, including humans [42]. Although they are easier to be degraded than organochlorines, organophosphate pesticide residues is one of the biggest threats to the ecosystem and food industry because their acute toxicities are irreversible [43].

Many people are exposed to pesticides occupationally, and pesticide self-poisoning is a major public health problem [44]. Annually, 3 million cases of acute poisoning have been reported from pesticide exposure, resulting in the deaths of 250 to 370,000 people every year [45,46]. Therefore, the usage of organophosphates has been restricted or banned all over the world [43].

Carbamates are organic pesticides, reversibly inactivating the enzyme acetylcholinesterase; these pesticides are derived from carbamic acid. The cholinesterase inhibition of carbamates differs from that of organophosphates in that it is species specific and is reversible [35,47]. Organochlorines, organophosphates, and carbamates are three generations of traditional highly toxic pesticides, and the later developed pyrethroids, anilines, amides, and azotic heterocyclic compounds are generally less toxic [48].

Pyrethroids are synthetic analogues of the naturally occurring pyrethrins, a product of flowers from pyrethrum plant (*Chrysanthemum cinerariaefolium*), and were detected in the 1980s to mimic the insecticidal activity of the natural pyrethrum. Pyrethroids are acknowledged for their fast knocking down effect against insect pests, facile biodegradation, and low mammalian toxicity [37]. These pesticides are nonpersistent sodium channel modulators and are much less toxic than carbamates and organophosphates to mammals. Therefore, the usage of pyrethroids has been increased greatly in the last 30 years. Unfortunately, pyrethroids are highly toxic to aquatic organisms such as mollusks, fish, and arthropods [49,50].

Amide herbicides, such as acetochlor, butachlor, and metolachlor, are widely used in recent years. However, butachlor can persist in the environment for up to 10 weeks, and what's even worse is that butachlor and metolachlor have been identified as mutagens. Another type of pesticides is aniline and dinitroaniline. Trifluralin and pendimethalin are widely used in this group of pesticides. These pesticides show high toxicity to aquatic organisms and they can impair the thyroid gland and liver. Hence, these two aniline herbicides have been banned in many European countries. Nitrogen-containing heterocyclic compounds, especially for imidazole and triazole heterocyclic chemicals, have become the hotspot for new pesticide development. In the last 10 years, they occupied no less than 70% of all the newly developed chemical pesticides [48].

Except for these classifications, pesticides are classified according to the mode of formulation, activity spectrum, and toxicity level. According to the mode of formulation, pesticides are classified into six groups as wettable powders, emulsifiable concentrates, baits, granules, dusts, and fumigants. In active spectrum, pesticides are classified into two groups as broadspectrum pesticides and selective pesticides. Broad-spectrum pesticides are designed to kill a wide range of pests and other nontarget organisms. On the contrary, selective pesticides are designed to kill only specific pests. In toxicity level, the World Health Organization (WHO) has developed a classification system that group pesticides according to the potential risks to human health and they are grouped into the following classes: class Ia=extremely hazardous, class Ib=highly hazardous, class II=moderately hazardous, class III=slightly hazardous, and class IV=products unlikely to present acute hazards in normal use [37].
