A Global Issue
Persistent organic pollutants (POPs) are toxic chemicals that adversely affect human health and the environment around the world. Because they can be transported by wind and water, most POPs generated in one country can and do affect people and wildlife far from where they are used and released. They persist for long periods of time in the environment and can accumulate and pass from one species to the next through the food chain. To address this global concern, the United States joined forces with 90 other countries and the European Community to sign a groundbreaking United Nations treaty Exit in Stockholm, Sweden, in May 2001. Under the treaty, known as the Stockholm Convention, countries agreed to reduce or eliminate the production, use, and/or release of 12 key POPs (see box), and specified under the Convention a scientific review process that has led to the addition of other POPs chemicals of global concern.
What Are POPs?
Many POPs were widely used during the boom in industrial production after World War II, when thousands of synthetic chemicals were introduced into commercial use. Many of these chemicals proved beneficial in pest and disease control, crop production, and industry. These same chemicals, however, have had unforeseen effects on human health and the environment.
Many people are familiar with some of the most well-known POPs, such as PCBs, DDT, and dioxins. POPs include a range of substances that include:
- Intentionally produced chemicals currently or once used in agriculture, disease control, manufacturing, or industrial processes. Examples include PCBs, which have been useful in a variety of industrial applications (e.g., in electrical transformers and large capacitors, as hydraulic and heat exchange fluids, and as additives to paints and lubricants) and DDT, which is still used to control mosquitoes that carry malaria in some parts of the world.
- Unintentionally produced chemicals, such as dioxins, that result from some industrial processes and from combustion (for example, municipal and medical waste incineration and backyard burning of trash).
The DDT Dilemma
DDT is likely one of the most famous and controversial pesticides ever made. An estimated 4 billion pounds of this inexpensive and historically effective chemical have been produced and applied worldwide since 1940. In the United States, DDT was used extensively on agricultural crops, particularly cotton, from 1945 to 1972. DDT was also used to protect soldiers from insect-borne diseases such as malaria and typhus during World War II, and it remains a valuable public health tool in parts of the tropics. The heavy use of this highly persistent chemical, however, led to widespread environmental contamination and the accumulation of DDT in humans and wildlife – a phenomenon brought to public attention by Rachel Carson in her 1962 book, Silent Spring. A wealth of scientific laboratory and field data have now confirmed research from the 1960s that suggested, among other effects, that high levels of DDE (a metabolite of DDT) in certain birds of prey caused their eggshells to thin so dramatically they could not produce live offspring.
One bird species especially sensitive to DDE was the bald eagle. Public concern about the eagles’ decline and the possibility of other long-term harmful effects of DDT exposure to both humans and wildlife prompted the Environmental Protection Agency (EPA) to cancel the registration of DDT in 1972. The bald eagle has since experienced one of the most dramatic species recoveries in our history.
What Domestic Actions Have Been Taken to Control POPs?
The United States has taken strong domestic action to reduce emissions of POPs. For example, none of the original POPs pesticides listed in the Stockholm Convention is registered for sale and distribution in the United States today and in 1978, Congress prohibited the manufacture of PCBs and severely restricted the use of remaining PCB stocks. In addition, since 1987, EPA and the states have effectively reduced environmental releases of dioxins and furans to land, air, and water from U.S. sources. These regulatory actions, along with voluntary efforts by U.S. industry, resulted in a greater than 85 percent decline in total dioxin and furan releases after 1987 from known industrial sources. To better understand the risks associated with dioxin releases, EPA has been conducting a comprehensive reassessment of dioxin science and will be evaluating additional actions that might further protect human health and the environment.
How Do POPs Affect People and Wildlife?
Studies have linked POPs exposures to declines, diseases, or abnormalities in a number of wildlife species, including certain kinds of fish, birds, and mammals. Wildlife also can act as sentinels for human health: abnormalities or declines detected in wildlife populations can sound an early warning bell for people. Behavioral abnormalities and birth defects in fish, birds, and mammals in and around the Great Lakes, for example, led scientists to investigate POPs exposures in human populations (see below for more information on the Great Lakes).
In people, reproductive, developmental, behavioral, neurologic, endocrine, and immunologic adverse health effects have been linked to POPs. People are mainly exposed to POPs through contaminated foods. Less common exposure routes include drinking contaminated water and direct contact with the chemicals. In people and other mammals alike, POPs can be transferred through the placenta and breast milk to developing offspring. It should be noted, however, that despite this potential exposure, the known benefits of breast-feeding far outweigh the suspected risks.
A number of populations are at particular risk of POPs exposure, including people whose diets include large amounts of fish, shellfish, or wild foods that are high in fat and locally obtained. For example, indigenous peoples may be particularly at risk because they observe cultural and spiritual traditions related to their diet. To them, fishing and hunting are not sport or recreation, but are part of a traditional, subsistence way of life, in which no useful part of the catch is wasted. In remote areas of Alaska and elsewhere, locally obtained subsistence food may be the only readily available option for nutrition (see below for more information on the Arctic).
In addition, sensitive populations, such as children, the elderly, and those with suppressed immune systems, are typically more susceptible to many kinds of pollutants, including POPs. Because POPs have been linked to reproductive impairments, men and women of child-bearing age may also be at risk.
POPS and the Food Chain
POPs work their way through the food chain by accumulating in the body fat of living organisms and becoming more concentrated as they move from one creature to another. This process is known as “biomagnification.” When contaminants found in small amounts at the bottom of the food chain biomagnify, they can pose a significant hazard to predators that feed at the top of the food chain. This means that even small releases of POPs can have significant impacts.
Biomagnification in Action: A 1997 study by the Arctic Monitoring and Assessment Programme, Exit called Arctic Pollution Issues: A State of the Arctic Environment Report, found that caribou in Canada’s Northwest Territories had as much as 10 times the levels of PCBs as the lichen on which they grazed; PCB levels in the wolves that fed on the caribou were magnified nearly 60 times as much as the lichen.
