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Through biological magnification.
Harmful chemicals get concentrated at every tropic level as they are nonbiodegradable and ultimately reach the topmost level- humans.

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Pesticides may enter the food chain by lot of ways The pesticides may get washed away from a field due to excessive rain to nearby water bodies or into soil. From soil,plants absorb water and nutrients and along with it these pesticides also.from water bodies,animals drink the water contaminated by pesticides.Thus they keep on accumulating in our food chain because they are non biodegradable Human beings are the top tropic level of any food chain.Thus amount of pesticides is more at this level .This is known as biological magnification