High exposure to certain now-banned industrial chemicals may lead to fewer female births, a new study suggests.

The findings, reported in the journal Environmental Health, add to evidence that the two groups of related chemicals -- polybrominated biphenyls (PBBs) and polychlorinated biphenyls (PCBs) -- may affect human reproduction.

PBBs were once widely used as flame retardants in plastics, electronic and textiles, while PCBs were used in everything from appliances and fluorescent lighting to insulation and insecticides.

While the chemicals were banned in the 1970s as potential health hazards, they remain a public-health concern because they linger in the environment and accumulate in the fat of fish, mammals and birds.

For the current study, researchers used data from a group of Michigan residents who, in the early 1970s, had been inadvertently exposed to high levels of PBBs; the chemicals had been accidentally mixed into animal feed, leading to human exposure through contaminated meat, eggs and milk.

The researchers observed that, from 1975 to 1988, women in the study group had a higher-than-average rate of male births, relative to the national average.

There was also a suggestion of increased odds of a male birth when both parents' combined PBB exposure was particularly high -- above the midpoint for the study group -- compared with couples whose PBB exposure was lower.

Similarly, couples with high PCB levels had a higher rate of male births.

What this all means for the public at large, however, is unknown, according to lead researcher Metrecia Terrell, of Emory University in Atlanta.

This was a unique situation, so it's very difficult to extend the findings to people with everyday exposures, she said in an interview.

Exposure in the general population would be much lower, Terrell pointed out, and we just don't know if there are effects on sex ratio.

Male births have always outnumbered female ones, but some research suggests that the male-to-female birth ratio is declining in the U.S. and elsewhere. One recent study found that in the U.S. in 2001, there were 104.6 boys born for every 100 girls; that compared with a ratio of 105.5 male births for every 100 female ones in 1970.

The researchers speculate that environmental toxins might be playing a role. Certain chemicals may, for example, affect the viability of sperm that bear the Y chromosome -- which determines male sex -- or the viability of male fetuses.

In this study, however, high exposure to PBBs and PCBs was linked to an increase in male births. Exactly why is unclear, according to Terrell.

Certain PBBs and PCBs have been shown to alter levels of male and female sex hormones, she and her colleagues note. But whether they promote the survival of Y-bearing over X-bearing sperm, or affect the survival of female fetuses is unknown.

Terrell said that continuing research on the chemicals' potential reproductive effects is needed.

SOURCE: Environmental Health, online August 15, 2009.

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