High levels of air pollution are reducing the number of boys born and could be linked to increased rates of miscarriage, research suggests.
Boys are more vulnerable in the womb
A team from Sao Paulo University in Brazil found fewer boys were born in the most polluted areas.
Experiments on mice exposed to high levels of air pollution produced a similar gender imbalance in the pups.
Details were presented to the American Society for Reproductive Medicine annual meeting in Montreal.
It is known that female foetuses tend to be more robust, and able to survive in harsh conditions.
Boy foetuses are more likely to perish in the womb or suffer obstetric complications.
The Sao Paulo team examined birth registries for numbers of babies born between January 2001 and December 2003.
In the least polluted areas 51.7% of the babies born were male - but in the most polluted areas the percentage of males born decreased to 50.7%.
Next, the researchers compared the offspring of male mice exposed for the first four months of their lives to either filtered air, or unfiltered ambient air.
After four months both groups were mated with female mice that had not been exposed to pollution.
Males from the filtered air environment produced offspring with a 1.34 male/female ratio, while males that had been exposed to polluted air produced offspring with a 0.86 male/female ratio.
Tests also revealed that mice exposed to air pollution showed greater signs of problems producing sperm.
Separate research found that pregnant mice exposed to air pollution were more likely to miscarry than those breathing filtered air.
Professor Jorge Hallack, head of the Sao Paolo team, said it was possible that pollution triggers genetic defects that make the foetus non-viable.
"Another hypothesis is that the pollution affects the viability of the placenta for the mice," he said.
Dr Allan Pacey, a fertility expert at the University of Sheffield and secretary of the British Fertility Society, told the BBC News website:
"We have known for some time that the proportion of boys to girls born can change markedly in response to both natural phenomena such as earthquakes as well as events like wars or famine.
"But we do not really understand the biology that underpins such changes.
"It is of some concern that man made chemicals can dramatically affect the ratio of boys to girls being born.
"This is something that needs to be monitored very closely."