But mothers have something else to worry about. Scientists and environmentalists are calling for a ban on a chemical flame retardant that has been shown to accumulate in breast milk. The chemical, polybrominated diphenyl ether, or PBDE, is commonly used in foam furniture and plastics to reduce risk of fire by up to 45 percent, according to manufacturers. But PBDE, like PCBs and DDT, is a persistent organic pollutant (POP) that bioaccumulates in the environment and in body fat. In 1998, Swedish scientists found that levels of PBDE in Swedish woman’s breast milk had increased 40-fold since 1972; in December, a study found that North American mothers had levels of PBDE 40 times that of the Swedes — an amount researchers called “humongously high.” The exact health effects are unknown. The most dangerous variety of the chemical will be banned in Europe beginning next year; whether the U.S. will follow suit is uncertain.