Babies born in highly polluted areas are more likely to be girls.© Alamy

Toxic fumes favour the fairer sex, a group of researchers in Brazil has found.

Jorge Hallak and his team at the University of Sao Paulo turned up the surprising result by studying babies born in their city. They divided the metropolis of 17 million people into areas of low, medium and high air pollution, using test results from air-quality monitoring stations. They then studied birth registries of children born from 2001 to 2003.

The team found that 48.3% of babies were female in the least polluted areas, but 49.3% were female in the dirtiest parts of town. After measuring the ratio of boys to girls born in all the areas, they calculated that 1,180 more babies would have been boys in the polluted areas if they had the same sex ratios as the cleaner areas. The team reported their findings on 17 October at the American Society for Reproductive Medicine meeting in Montreal.

It has been known for the past 60 years that, for humans, the ratio of males to females in newborns usually tips towards sons. Scientists are not really sure why this occurs, but certain conditions, such as those after the Second World War, have been found to alter this balance.

Researchers who were at the meeting say Hallak's study raises intriguing questions about the health effects of air pollution, but caution that more rigorous, large studies will be needed to confirm the finding. "I think it's a fascinating and serious problem," says Anthony Thomas, a urologist who heads the male infertility section of the Cleveland Clinic in Ohio. "But this is just a beginning, and now they need to do more work to examine this carefully."

Safe strategy

Hallak believes his findings suggest that pollution is a reproductive stress similar to others that skew sex ratios. Research shows that natural disasters and crises such as terrorist attacks can increase the probability that a newborn is female. This is thought to be the safer reproductive bet, as girls are likely to grow up and have a few children of their own. Boys are a more risky venture: they could father dozens of children, or none at all.

"It looks as if the human race is trying to repopulate itself, and of course females are important for that," Hallak says.

Thomas points out that the Sao Paulo researchers did not identify which components of the polluted air were skewing the sex ratio, so they cannot say for sure that the pollution itself caused the effect. It is possible, for example, that more polluted areas were also poorer, and that economic differences were the actual causal factor.

But Hallak says that his team has found preliminary evidence that pollution exerts its effect by targeting sperm, altering the proportion that carry an X or Y chromosome. The researchers found that if they exposed male mice to pollution, then the males' mates gave birth to more females than expected. Pollution also reduced total sperm counts in the mice, Hallak says.

The weaker sex

It is still not clear why pollution would skew the sex ratio. Other researchers have found that chemicals, such as soil disinfectants, can have a short-term effect on sex ratios in children born to workers who handle the chemicals. Chemicals can also hurt sperm quality and sperm count.

Such findings have led some scientists to speculate that Y-chromosome sperm, which will produce boys, are weaker than X-chromosome sperm, and therefore more susceptible to environmental stresses. But that has not actually been proven, Thomas says.

Nevertheless, the study adds another concern to the list of pollution's adverse effects on health. If the finding is solid, it may have implications for sex ratios in huge cities such as Jakarta and Beijing, where air quality is notoriously poor (see 'Satellite view alerts China to soaring pollution').

"It's an initial study that has interest, and I think the city and state of Sao Paulo need to look at this more carefully," says Thomas. 

University of Sao Paulo