Pesticides and why they linger

Pesticides protect crops and stored grain from insects, rodents, weeds, bacteria and fungi. The main types:

• Organochlorines: DDT, lindane, endosulfan, aldrin.
• Organophosphates: parathion, malathion.
• Carbamates: carbaryl, carbofuran.
• Synthetic pyrethroids: cypermethrin.
• Organometallics: methylmercury.
• Inorganic pesticides: compounds of arsenic and mercury.

Agrochemicals helped the green revolution of the late 1960s. DDT, an organochlorine and the first pesticide made in a laboratory, controlled the mosquitoes that spread malaria. But insects developed resistance, and it harmed many species it was never meant to.

Why some linger. Organochlorines do not mix with water (they are hydrophobic) but dissolve in fat. So they are non-biodegradable and very persistent, staying in soil and bodies for years. Organometallics also persist. Carbamates and pyrethroids, by contrast, are broken down easily: pyrethroids by sunlight within days.

How some act. Organophosphates and carbamates block the enzyme cholinesterase, so the nerve chemical acetylcholine builds up at the junctions between nerves. Some of the substances they break down into can be more harmful than the original.

Biomagnification. Pesticide residues pass from soil and water into plants, and then up the food chain to animals and finally to humans. Because fat-soluble residues are stored rather than excreted, their concentration rises at each step. That is why removing them, even when present in small amounts, is crucial.