Many chemicals—like mercury, DDT, dioxins, PCBs and others—are dangerous because they’re persistent; they don’t break down easily in the environment, and living creatures cannot metabolize them.
When these chemicals are ingested or absorbed by a living organism, they accumulate in the body’s tissues and become more concentrated over time, as the creature eats more and more prey infected with the chemical.
Source: U.S. EPA
As these substances move up the food chain, they become more concentrated. The little fish eats the plankton, the big fish eats the little fish, then you or a bald eagle eats the big fish. A study in Long Island Sound in 1967 showed DDT concentrations in gulls and ospreys to be 200,000 times higher than in the water (Woodwell).
The same can happen to you, especially when you eat high-on-the-food-chain fish, or when you are surrounded by persistent chemicals in your home. When you inhale, absorb and ingest these chemicals, you often do so faster than you can metabolize them or flush them out. Over time, concentrations increase, and the risk of health problems increases.
Check out this interactive primer from the National Oceanic & Atmospheric Administration.