DDT is environmentally harmless when used as directed.

DDT had real advantages. It was highly effective against mosquitoes, lice, and agricultural pests, helped control diseases such as malaria and typhus, and was less acutely toxic to humans than many older insecticides.

Those benefits made widespread postwar use seem reasonable. The environmental problem appeared more slowly.

DDT is persistent, meaning it breaks down slowly, and it dissolves readily in fat. An animal can therefore accumulate more DDT over time than is present in any single meal. This buildup inside an organism is called bioaccumulation.

Concentrations can rise further as one animal eats many contaminated organisms and is then eaten by another predator. That increase up a food chain is called biomagnification.

The breakdown product DDE became especially important in birds. In sensitive species, it interferes with eggshell formation, producing shells that are thinner and more likely to break. Eagles, falcons, pelicans, and other birds suffered reproductive losses where exposure was high.

Field observations, residue measurements, controlled experiments, and later population recoveries after restrictions tied the evidence together.

DDT still has recognized public-health value, and international rules allow restricted use against disease-carrying insects in some circumstances. What failed was the broader claim that its environmental effects were harmless.