Polar bears are the apex predator in Arctic marine ecosystems and are exposed to high levels of pollutants that are magnified with each step higher in the food web. Most of the pollution in the Arctic is transported northward by the large rivers draining into the Arctic and on wind and ocean currents that bring pollutants from southern latitudes. The pollutants of most concern are organochlorines that are, or were, used in industry or as pesticides. A key characteristic of the pollutants is that they are persistent in the environment and resist degradation. Some pollutants such as polychlorinated biphenyls (PCBs) were used widely in industrial applications precisely because they were extremely stable. Other pollutants such as dieldrin, DDT, toxaphene, and chlordanes were used as pesticides but they are also stable enough to be transported long distances to the Arctic. Many of the pollutants are now banned from use in most countries but they are so persistent that they will likely remain in the environment for decades to come. Unfortunately, many of the organochlorine pollutants are lipophilic or "fat loving" and bond tightly to fat molecules. Because the Arctic marine ecosystem is highly dependent on fat for insulation, buoyancy and energy storage, these pollutants are accumulated in higher and higher levels up the food chain. Simple organisms have limited capacity to metabolise and excrete these chemicals so they bioaccumulate higher in the food web. Polar bears are particularly vulnerable to organochlorines because they eat a fat rich diet. Ringed, bearded, and harp seals comprise the main food of polar bears and the blubber layer is preferentially eaten by the bears and subsequently, the intake of pollutant is high.
The most polluted polar bears live in NE Greenland, the Barents Sea and the Kara Sea (see figure). The reasons for this are related to global transport and deposition patterns of pollutants. Based on studies in other species, it is reasonable to believe that the pollutants load of polar bears in some areas are negatively affecting the immune system, hormone regulation, growth patterns, reproduction, and survival rates of polar bears. Recent studies have suggested that the immune system is weaker in polar bears with higher levels of PCBs. A weakened immune system may mean that these polar bears are more susceptible to succumbing to disease or parasites. Additionally, there is evidence that the hormone system of polar bears is affected by pollution and this may interfere with reproduction and growth. A major concern with polar bears pertains to their reproductive system. Breeding occurs in spring but the development of the embryo is halted at a multi-celled staged called the blastocyst. In the autumn, the blastocyst implants and continues development. There are suggestions that species with such delayed implantation are more vulnerable to the effects of pollution through endocrine (hormone) disruption. Further, because females polar bears are fasting during gestation their pollution loads increase because they are using their fat stores for energy and the pollution is retained in the blubber. Cubs are born in an altricial state (very immature) and weigh only ca. 600 g (1.5 pounds). Because the cubs are nursed on fat rich milk, the cubs are exposed to very high pollution loads from their mother. Data is lacking to determine the effects of these pollutants on cubs but there are suggestions that cubs of more polluted females have higher mortality rates.
Given that a polar bear likely contains several hundred chemicals that originated from humans, it is very difficult to determine which of these chemicals are causing effects and which are not. Polar bears are very efficient at metabolising (breaking down) some pollutants but the problem is that many of these metabolites (break down products) are very active in the body before they are excreted.
It is likely that only the most polluted polar bear populations are severely affected by pollution but it is also possible that effects occur at much lower levels. Given that no polar bear in the world is free from pollution, there is cause for concern. New pollutants are also being found in polar bears. Recently, brominated flame retardants have been detected in polar bears. It is also possible that many other compounds will be identified. On the bright side, some pollutants like PCBs, now banned in most countries, are being to show signs of decrease in the Arctic and in polar bears.