Why DDT is banned in western countries?

Answer: (a) It remains in environment for a long time

DDT is persistent: it lasts for years in soil and body fat, and biomagnifies up food chains (thin eggshells in birds of prey).

The idea

A good pesticide should do its work and then break down. DDT breaks down very slowly: it stays in soil for years and collects in the body fat of animals. So even small, repeated use keeps adding to a growing store in the environment.

Sprayed on fields → Stays in soil and water for years → Enters small animals and settles in their fat → Concentration rises up the food chain → Fish-eating birds lay thin-shelled eggs

Why the others are wrong

(b) Insect resistance is real, but it is why DDT stopped working, not why it was banned.

(c) Its acute toxicity to humans is actually low. The problem is persistence and biomagnification.

The trap

(c) sounds obvious, but DDT is not especially poisonous in one dose. It builds up over time.

Also know

Paul Müller won the Nobel Prize (1948) for discovering its power against insects, and it saved many lives from malaria and typhus. The USA banned it in 1972. India banned its use in farming in 1989 but still permits limited use against malaria mosquitoes.