del Rio Gomez, I, T Marshall, P Tsai, Y-S Shao and YL Guo. 2002. Number of boys born to men exposed to polychlorinated biphenyls. Lancet 360: 143–44.

In the latest publication tracking the health impacts of cooking oil contamination by a mixture of PCBs, dioxins and furans in Taiwan in 1979, del Rio Gomez et al. report that men exposed while under the age of 20 by the contamination are significantly less likely to father male off-spring. For men exposed at an older age, there appears to be no effect.

What did they do? del Rio Gomez et al. analyzed the birth records from 1980 to 1999 of parents exposed to contaminated cooking oil, calculating the sex ratio of their offspring separately for men and women. The scientists further divided parents by age and considered separately those people who had been exposed before they reached the age of 20 vs. those who were 20-yrs old or older in 1979.

What did they find? Overall, exposed men had fewer male children than expected. Once del Rio Gomez et al. separated men by age, it was apparent that this effect was significant only in men who were less than 20-yrs old at the time of exposure. No effect of exposure was seen in the sex ratio of exposed women.

While overall, exposed men fathered fewer male offspring (p < 0.037), the effect of PCBs appears restricted to men exposed under the age of 20, for whom the pattern was highly significant (p < 0.020). Older men did not show an effect (p > 0.60).

What does it mean? This is the latest and largest study demonstrating an effect of dioxin-like compounds on the sex ratio of children born to exposed parents. Mocarelli et al. reported a similar pattern in children of parents exposed in the 1976 Seveso, Italy, chemical plant explosion. As in this paper, the effect was in men exposed before they reached maturity.

The consistency of these results gives additional credence to concerns that contamination by dioxin-like compounds may be having a broad effect on human sex ratio. Reviews of trends in the ratio of boys:girls born in several countries, including Denmark, the US, Canada and the Netherlands, indicate that the ratio is shifting downward, with fewer boys born than expected.

As del Rio Gomez et al. point out, the mechanism of action by which the sex ratio is altered is not yet known. They focus attention on the fact that dioxin-like compounds are active hormonally, with estrogenic, anti-estrogenic, and anti-androgenic characteristics.

They hypothesize that "dioxin-like compounds might have a negative effect on the viability of the Y-chromosome carrying sperm or on the development of XY-fertilised eggs. Furthermore, a relation might exist between reproductive cells and the aryl hydrocarbon receptors of dioxin-like compounds, inducing a mutation in proteins that could result in either a gain or a loss of receptor function, altering the paths of sex determination."