Are persistent organic pollutants a threat to Arctic marine mammals?

Persistent Organic Pollutants in the Arctic

Extracted from "Arctic Pollution Issues: A State of the Arctic Environment Report", prepared by the Arctic Monitoring and Assessment Program (AMAP) of the intergovernmental Arctic Council by Dr. John Calder, Director of NOAA's Arctic Research Office.

Courtesy of Polar Bears Alive, Copyright © 1997, 1998, 1999, Polar Bears Alive. Photographs are © Dan Guravich. For more information, please contact Polar Bears Alive.

Stiff and lean after six months curled in a den, a female polar bear squeezes herself out of her winter home. Two small cubs emerge tentatively at her heel for their first view of the world beyond a snow cave. Entirely dependent on their mother, the cubs follow obediently. Having used up most of her fat stores, the female scans the sea ice below and ponders a meal of seal blubber. But her cubs are not yet ready to travel, and her milk will have to sustain them for some time to come. The milk is rich and nourishing but today it also harbors a threat. Chemicals from lands far beyond her sea-ice domain taint the seals that the mother has feasted on in the past, and will need again soon. The chemicals that bind to the fat of the seals have accumulated in her own fat stores. Unwittingly, the mother passes the toxins to her young in her fat-rich milk, with effects that are still unclear.

Persistent Organic Pollutants (POPs): A Background

In 1945, a booming chemical industry launched a new, effective tool for dealing with insect pests: DDT. It held great promise, including the hope of saving crops and eradicating disease-carrying insects. Twenty years later, DDT and other similar chemicals had indeed benefited agriculture and relieved some of the problems associated with insects in many areas of the world. However, these gains came at a price, as DDT is toxic to many more organisms than those it was intended to kill. In particular, birds of prey had trouble reproducing, and their populations declined in many polluted parts of the world.

As early as 1970, when it was detected in the blubber of ringed seals, it was evident that DDT was present in the Arctic. By the mid-1970s, DDT and other pesticides had been detected in beluga whale, polar bear, and fish. Moreover, birds of prey declined in northern areas that were thought to be uncontaminated.

In addition to pesticides, most tests of animals also found traces of industrial oil made of chemicals known as PCBs. By 1980, there was evidence that these chemicals had reached the Arctic via long-range transport. In the late 1980s, there was evidence that human mother's milk at a location in the Northwest Territories of Canada contained enough PCBs to cause concern about effects on human health. The most likely source was the food the woman had eaten.

Long-range transport via the atmosphere is the most likely source of these persistent organic pollutants in the Arctic. However, efforts to quantify the amount of POPs transported in this way and to determine source regions, are quite limited.

The Biological Effects of POPs

Organic contaminants in the Arctic environment share many characteristics that make them especially insidious for people and wildlife.

POPs are stored in fat and are persistent

A common characteristic of most synthetic chemicals found in Arctic animals is that they break down very slowly. This persistence in the environment allows them to accumulate in animals, and to pass up the food web. Most of the organic pollutants are fat-soluble and accumulate in the fatty tissues of animals. Arctic animals store energy as fat for survival in the cold, and therefore fat is an important part of the diet for both animals and people. Along with the fat in their diet, animals and people take in the organic contaminants. As predators take on the energy (fat) from their prey, the contaminants too work their way up the food chain (from fish, to seals, to polar bears, to people), often becoming more concentrated in the top predators (bears, people).

A broad attack on reproduction

Most of the visible effects of POPs on animals are related to the ability to conceive and raise young. For example, thinning of their eggshells, which made it impossible for the birds to hatch their chicks successfully, caused the early declines in birds of prey. The effects of POPs on mammals are well documented in polluted areas such as the Baltic Sea. Malformations in reproductive organs, fewer young or even complete failures to reproduce are some of the detrimental signs of high contaminant levels.

One of the underlying causes of failure to reproduce is that some of the chemicals interfere with sex hormones. Such hormone disrupters can mimic or block hormones because they are similar enough in structure to fit into the body's biochemical receptors. Contaminants that block the estrogen receptor can inhibit the growth of the reproductive tract and the mammary glands. In fish the same receptor stimulates the production of a precursor to egg yolk.

Sex hormones are important to the normal sexual development of young animals. In polluted temperate environments, high levels of hormone disrupters have been connected to malformations in the reproductive organs, change of sex in some species, and abnormal mating behavior.

The immune system is very sensitive

One of the most sensitive targets for organic contaminants may be the immune system, the body's primary defense against disease. The thymus, which normally produces antibodies to fight infectious agents, can waste away and cease to function. There are signs that animals with a high load of organic contaminants are more susceptible to infections. POPs also limit cell-mediated immunity, the branch of the immune system that fights cancer cells and parasites.

Liver enzymes are telltale signs of intoxication

In the body, many toxic chemicals are converted into less toxic substances that can be excreted. The liver is the site of most of this detoxification, and many organic contaminants stimulate the production of specific liver detoxification enzymes. Measurements of these enzymes are now used as biological indicators of the load of contaminants in an animal. Unfortunately, these same enzymes are also capable of breaking down hormones. This side effect of detoxifying high levels of contaminants can increase the breakdown of hormones and impair critical hormone-dependent functions, such as reproduction.

Increased risk of tumors

Several POPs are suspected of being responsible for increased rates of tumors in wildlife in polluted areas. There are two ways by which a contaminant can increase the risk of cancer. The fist is a mutation of hereditary material in the cells, the DNA, which makes the cell lose control of it growth. The second allows a cell damaged in this way to turn into a tumor. Contaminants implicated in the latter process are called promoters, and this group includes most POPs. They do not cause cancer by themselves, but can act together with DNA-damaging chemicals.


Some POPs disturb production of the pigment in red blood cells, which in severe cases leads to the disease porphyria. Symptoms include skin damage after exposure to sunlight as well as damage to the nervous system. The biochemical changes associated with porphyria, which are measurable long before symptoms appear, are used as sensitive biological markers of POPs in the environment.

Effect assessments include many uncertainties

Most of our knowledge of the toxicology of organic pollutants comes from studies of laboratory animals, studies with a few species of wild animals or studies of the association between contaminant levels and effects in wild animals. There are many uncertainties associated with these studies. Thus, when POP levels in Arctic biota reach biological effects thresholds determined from studies such as those mentioned, it should be interpreted as a warning signal rather than as evidence that such effects actually do occur in the Arctic.

Priorities of the Arctic Monitoring and Assessment Program

The most recent AMAP recommendations for POPs give priority to:

  1. Monitoring spatial distribution, contaminant levels and biological effects in Arctic species having body burdens of POPs at or above levels of concern;
  2. Improving our understanding of the adverse effects of POPs on human populations, especially on child development; and
  3. Filling data gaps, particularly in the U.S. and Russia.