Pesticides

Pesticides are hazardous compounds which are used to control or eliminate unwanted species of insects (insecticides), acarides (acaridicides), fungi (fungicides), higher plants (herbicides), rodents (rodenticides), or nematods (nematodicides).

The biocidal action of pesticides includes a variety of disturbances of physiological processes, such as inhibition of acetylcholinesterase by insecticidal organophosphates, blockade of neurotransmission by chlorinated hydrocarbons, and inhibition of oxidative phosphorylation by herbicidal dinitrophenols.

The insecticidal organophosphates and carbamates inhibit acetylcholinesterase, the enzyme that regulates neurotransmission by hydrolyzing acetylcholine. Another group of insecticides, the chlorinated hydrocarbons, cause blockade of neurotransmission by interaction with the sodium/potassium channels, resulting in inhibition of nerve membrane depolarization. A third group of pesticides, the herbicidal dinitrophenols, are uncouplers of the oxidative phosphorylation. Most of these biological targets also exist in man. Therefore, it is not surprising that accidental massive poisoning following inaccurate use of pesticides regularly occurs all over the world.

Parathion, an insecticidal organophosphate

Parathion, an insecticidal organophosphate

Carbaryl, an insecticidal carbamate

Cl

Aldrin, an insecticidal chlorinated hydrocarbon

Aldrin, an insecticidal chlorinated hydrocarbon

Dinitro - o - cresol, a herbicidal dinitrophenol;

Dinitro - o - cresol, a herbicidal dinitrophenol;

Organochlorine and carbamate pesticides can also induce long-term effects: cancer and malformations. The former pesticides are highly lipid-soluble and are only slowly broken down. Therefore, they persist in the environment for a long time and accumulate in food chains.

Table 10.4 NOAELs and ADIs of some pesticides

Max. Residue' ADI Safety in meat, fish

Pesticide

Mutagenic and carcinogenic aldrin/dieldrin DDT

50 (rat) 100 (human) 1.25 x 104 (monkey) 1 x 105 (rat)

3-10

1-86

methyl parathion captan

26-40

Mutagenic and non-carcinogenic dichlorvos malathion

4 20

8 10

4-96

* High residues are found particularly in meat, fish, and poultry.

** In spite, of its carcinogenic potential for humans, the applicable safety factor for DDT is only tenfold. The ADI

is conditional; only permission for application when no available subtitutes can be used. Source: Concon, 1988.

The carcinogenic organochlorine pesticides include aldrin and dieldrin. They need metabolic activation to become carcinogenic. The main targets in rats and mice are liver and lung. An example of the carcinogenic insecticidal carbamates is carbaryl. The members of this group become carcinogenic on conversion to nitroso compounds in the reaction with nitrite.

Further, pesticides have been reported to induce malformations when given to mammals during pregnancy, e.g., aldrin, dieldrin, and carbaryl. However, carcinogenicity and teratogenicity have not yet been confirmed in valid epidemiological studies. As a result, the ADIs of most pesticides are based on animal data. This calls for continuous attention to potential hazards to humans due to the presence of pesticides in food. Therefore, safety factors of 100 or higher are applied (Table 10.4). This means that the ADI is usually 1% of the no-adverse-effect level observed in the most sensitive species. In the few cases where toxicity data in humans were available, a safety factor of 10 has been applied, accounting for intraspecies variation in man.

The organochlorine pesticides have been largely replaced by carbamates. The latter are less persistent in the environment. Further, their carcinogenic potential is lower than that of the organochlorine pesticides.

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