Lead author Irva Hertz-Picciotto of the University of California, Davis, said PCBs, widely used in industry as cooling and insulating fluids for electrical equipment, as well as in construction and domestic products such as varnishes and caulks, are persistent organic pollutants identified worldwide as human blood and breast milk contaminants.

The study, published in the journal Environmental Health, found that among women from the San Francisco Bay Area, those exposed to higher levels of PCBs during the 1950s and 1960s, were 33 percent less likely to give birth to male children than the women least exposed.

The researchers measured the levels of PCBs in blood taken from pregnant women during a Bay Area study during the 1960s. When they compared the blood levels to the children's sex, they found that for every one microgram of PCBs per liter of serum, the chance of having a male child fell by 7 percent, the study said.

PCBs were banned in the 1970s, but the flame-retardants polybrominated diphenyl ethers currently used in plastic casings and foam products share many of the biochemical and toxicologic properties of PCBs, the researchers said.