Polychlorinated biphenyls (PCBs) are industrial chemicals that were manufactured from 1929 until 1979 when they were banned. PCBs have been shown to cause adverse health effects, including potential cancers, and negative effects on the immune, nervous and endocrine systems.

PCBs can pose serious health risks to people who frequently eat contaminated fish. They can be transferred from a mother to her unborn baby, increasing the risk of preterm delivery and low birth weight. They may also be transferred from mother to baby through breast milk, and exposure has been associated with learning defects.

PCB remnants still cycle between air, water and soil, and traces are found all over the world. They settle into water and sediment, where they are taken up by small organisms, and increasingly accumulate in fat and organs such as liver in fish and animals (including humans) that eat fish. Small amounts are found in meat, dairy products and drinking water. Fish are the major dietary sources of PCBs, especially fish caught in contaminated lakes or rivers.

Level of PCBs found in fish will vary with region and the type of fish native to that region. So if you eat fish caught by family or friends, check for local advisories. In general, bottom-feeding fish (striped bass, bluefish, American eel, sea trout) and larger predator fish (bass, lake trout, walleye) caught in contaminated waters contain higher levels of PCBs.

Farmed salmon that are fed ground-up fish have been found to be higher in PCBs, compared with wild-caught salmon.

The Food and Drug Administration sets PCB residue limits for foods, while the Environmental Protection Agency sets limits for lakes, streams and drinking water. They recommend the following measures to reduce exposure to contaminants such as PCB:

• Trim away fatty areas (belly, top back and dark meat along the side)
• Remove skin before cooking to allow fat to drain off
• Grill, bake or broil fish and allow fat to drain off
• Do not fry or deep-fry fish because frying seals in any chemical pollutants in the fish's fat

Two other groups, Physicians for Social Responsibility and the Association of Reproductive Health Professionals, suggest limiting your intake of fish containing high levels of PCBs, such as salmon and bluefish. With respect to salmon, their guidelines say:

• Canned Pacific salmon can be eaten twice a week
• Fresh or frozen wild Pacific salmon can be eaten up to twice a month
• Fresh or frozen farmed Atlantic salmon can be eaten once every two months

The bottom line: Eat a variety of fish twice a week and keep the portion size at 4 ounces (113 grams). When you eat fatty fish such as salmon, make sure it is prepared using the above guidelines. When you catch your own fish, follow local and state fish advisories for restrictions.