Polar bears are at the top of the food chain in the Arctic, and are therefore the ones that are in the most danger of becoming extinct. They eat the bearded seal, which eats the polar cod, which feeds on Pelagic invertebrates. When the organism at the very bottom of the food chain gets contaminated by pollution then gets eaten, the animal that ate that organism is also contaminated by pollution.
Before, they might even have already had pollution in their bodies, so now they have more than ever. By the time that the polar bear eats a seal, for example, that seal would have been so contaminated by that time that the polar bear would probably get terribly sick and die soon after.