Some of these chemicals were banned decades ago. Yet they're still building up in our waters and fish. Here's why...

• What makes a pollutant a "legacy" pollutant?
• What are some examples of legacy pollutants?
• If legacy pollutants have been banned, why are they still a risk?
• What are the health risks of PCBs?

What makes a pollutant a "legacy" pollutant?

Legacy pollutants are chemicals, often used or produced by industry, which remain in the environment long after they were first introduced. Oftentimes, they weren't recognized as harmful when they were being used. "What was acceptable in the '40s and '50s, we would find very objectionable today," explains Steve Tochko, an environmental remediation manager for Boeing. "People did not know the damage that some of these materials caused at the time. They did not know the long-term effects of them that we do today."

Environmental scientists distinguish between legacy pollutants and "emerging" contaminants found in everyday products like pharmaceuticals, herbicides, pesticides and personal care products. Emerging contaminants pose a threat to drinking water because older filtering systems are not designed to catch all of them.

What are some examples of legacy pollutants?

Legacy pollutants include heavy metals like lead and mercury, but the major legacy pollutants at sites like Seattle's Duwamish River are PCBs. Short for polychlorinated biphenyls, PCBs are a group of more than 200 manmade chemicals that were produced in the United States from 1929 until the late 1970s, when they were banned.

PCBs are very stable and were used as coolants and insulators, particularly in transformers and other electrical equipment. Motor and hydraulic oils, old fluorescent lights, thermal insulations, caulking, paints and plastics may also contain PCBs. (For a longer list, see this Environmental Protection Agency Web page.)

If legacy pollutants have been banned, why are they still a risk?

Even if they're no longer used, they may have entered the environment when they were manufactured as the result of an accident or spill, or through the improper disposal of hazardous materials. PCBs in particular are very stable and tend to linger in the environment. "PCBs are probably the number one persistent contaminant of concern anywhere in the Northern Hemisphere," says Peter Ross, a marine biologist with Canada's department of Fisheries and Oceans.

PCBs also "bioaccumulate," meaning that they build up in animal tissues as they progress up the food web. PCBs generally settle in the sediment on the bottom of rivers or other bodies of water. "The mud at the bottom of the river is where all your little microorganisms and little bugs on the bottom live," explains B.J. Cummings of the Duwamish River Cleanup Coalition. "Those get eaten by fish that are there -- herring, for example -- which then get eaten by salmon, which then get eaten by sea lions and by orca whales and by the people who are fishing here."

PCB levels increase with each step up the chain, which is why orcas are at particular risk and why the EPA has announced guidelines on how often people should eat fish caught in the Duwamish.

Persistence and bioaccumulation are two traits common to persistent organic pollutants, a group of chemicals that includes PCBs, the pesticide DDT and dioxin, a toxic byproduct that was discovered in Seattle's South Park neighborhood in 2008. These pollutants are among the "dirty dozen" regulated by an international treaty, the Stockholm Convention on Persistent Organic Pollutants.

What are the health risks of PCBs?

Based on animal testing and studies of workers who were exposed to PCBs on the job, the EPA considers the chemicals "probable human carcinogens." Animal tests have also revealed toxic effects on the immune, reproductive, nervous and endocrine systems. A 2008 study found that women exposed to PCBs were more likely to give birth to girls, and 2009 research found the chemicals alter the development of brain cells.

For more on the health risks of PCBs and how to protect yourself from exposure, read this FAQ from the U.S. Department of Health and Human Services.