Live in a polluted city? You may be 1% more likely to have a girl.

A study in Sao Paulo, Brazil found that the birth ratio of females in the city was slightly higher in the most polluted areas of the city compared to the cleanest areas. But if you're hoping for a daughter, don't start packing for Los Angeles (the most polluted city in the US) just yet. The female birth ratio increased by only 1%, and boy births still outnumbered girls.

The margin is very slight, but over the large populations living in the world's polluted cities, it could make a difference. The researchers calculated that in Sao Paulo, 1,180 more boys would have been born if the birth ratio had been the same in the polluted areas as in the clean areas.

Pollution was found to cause larger discrepency in the birth ratio of mice. Mice raised in filtered air had a higher sperm count and higher male birth ratio than mice raised in unfiltered air.

These findings seem to support the idea that Y-bearing (male producing) sperm is more fragile than X-sperm, and under conditions of environmental stress, X-sperm is more likely to survive.

• Pollution makes for more girls
  The stress of dirty air skews sex ratios in Sao Paulo
  Nature
• City pollution bad enough to skew male birth rate
  The pollution in Sao Paulo is severe enough to change the natural ratio of male and female birth rates and the problem is likely affecting cities all over the world.
  EDIE News