That's the startling finding of a new study that reviewed data on pregnant women in San Francisco who were exposed to high levels of polychlorinated biphenyls (PCBs) - a group of banned environmental pollutants - in the 1950s and 1960s.
The women most exposed to PCBs were 33 percent less likely to give birth to male children than the women least exposed
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Women exposed to high levels of certain kinds of pollutants are less likely to give birth to male children.
That's the startling finding of a new study that reviewed data on pregnant women in San Francisco who were exposed to high levels of polychlorinated biphenyls (PCBs) - a group of banned environmental pollutants - in the 1950s and 1960s.
And it's not an isolated finding either, reports ScienceDaily.
Similar exposure is thought to have occurred in Wales, after a quarry on the edge of Groesfaen village near Cardiff was used as a toxic dumping ground from 1965 to 1972.
PCBs are persistent organic pollutants identified worldwide as human blood and breast milk contaminants. They were 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.
PCBs were banned in the 1970s because of their general toxicity and persistence. They were associated with effects on immune, reproductive, nervous, and endocrine systems.
Given the high quality measurements, the new research provides the strongest evidence to date that PCBs affect sex ratio in human children.
Irva Hertz-Picciotto, lead author of the study, explained how marked the effect was.
"The women most exposed to PCBs were 33 percent less likely to give birth to male children than the women least exposed," she said.
The researchers measured the levels of PCBs in blood taken from pregnant women during a Bay Area study in the 1960s. When they compared these levels to the children's sex, they found that for every one microgram of PCBs per litre of serum, the chance of having a male child fell by 7 percent.
"These findings suggest that high maternal PCB concentrations may either favour fertilisation by female sperm or result in greater male embryonic or foetal losses. The association could be due to contaminants, PCB metabolites or the PCBs themselves," said Hertz-Picciotto.
