In modern agriculture a large number of pesticides and insecticides are used.
They persist in the environment for a long time and accumulate in certain vital tissues of organisms and other items, which are used by humans.
The important categories of pesticides and their toxicity are mentioned below:
1. Organ chlorine insecticides:
These are chlorinated ethane derivations, such as DDT and Methoxychlor; cyclodienes such as endrin, aldrin, dieldrin, chlordane, heptachlor, and mirex; and hexachlorocyclohexanes (HCH) such as Lindane. Of these, methoxychlor is less toxic than DDT; endrin is extremely toxic; Lindane is also highly toxic but less cumulative; the remaining are less toxic.
They stimulate the nervous system and induce irritability, disturbed equilibrium, paresthesia, tremor, and convulsions. Some of these chemicals, such as aldrin, dieldrin and lindane affect neurotransmitter activity. DDT may exert its toxic effects in the nervous system by adversely affecting ion transport across the axon membrane (Doherty, 1979; Narahashi, 1980). Some organ chlorine insecticides including DDT are hepato toxic.
2. Organophosphorus insecticide:
These are esters of phosphoric acid (Dichlorvos) and theophosphoric acid (Parathion). Other pesticides in this group are Diazinon, Dimethoate, Malathion, Mevinphos and Dipterex (Trichlorfon). Their toxic effects vary over a wide range. They act by inhibiting acetylcholinesterase (AchE). As a result, the accumulated acetylcholine (Ach) induces tremor, incoordination and convulsion. The accumulation of Ach at the neuromuscular synapse will lead to contraction of the muscles, loss of reflexes and paralysis. Several organophosphorus compounds may cause delayed neurotoxicity.
3. Carbamate Insecticides:
Insecticides of this class include Carbaryl (Sevin), Aldicarb (Temik), Carbofliran Methomyl and Propoxur (Baygon). These are esters of N. methylcarbamic acid. They also act by inhibiting AchE. However, inhibition of AchE by a carbamate is readily reversible.
Several types of herbicides are used. Herbicides like 2, 4-D (2, 4-dichlorophenoxy- acetic acid). Paraquat and Diquat have been widely used. Some herbicides retard the growth of weeds by inhibiting photosynthesis, respiration, cell division or protein or lipid synthesis. Their toxicities in animals are relatively low. Paraquat and Diquat exert their toxicity via the formation of free radicals. Paraquat causes lung damage after inhalation and also after ingestion. It also causes hemorrhage and fibrosis.
A number of rodenticides including Warfarin, Thioureas, Sodium fluoroacetate and Sodium fluoroacetamide have been used. Warfarin is an antimetabolite to vitamin K, thus acts as an anticoagulant. Thioureas main toxicity is pulmonary edema and pleural effusion. They are highly toxic to rats but moderately toxic to humans. Sodium fluoroacetate exerts its toxic effect through blockage of the citric acid cycle.
Methyl and ethyl mercury are very effective fungicides. However, permanent neurologic damage and deaths have been reported after their use. Fungicides which have been widely used in agriculture are Dicarboximides but they are reported to have carcinogenic effects.