Air pollution leads to a variety of problems involving reproductive effects including low birth weight, miscarriage, preterm birth, and decreased sperm quality, according to a BMJ report. 

The journal BMJ Open looked at the effect of air pollution on the ratio of female to male births in Sao Paulo, Brazil. Researchers divided Sao Paulo districts into different categories based on their ambient particulate matter (PM10) concentrations over time. 

PM10 levels were used as a marker of air pollution levels. The total number of births between 2000 and 2007 was collected with the ratio of male to female births calculated during each month. A direct correlation between increased pollution and less male births was found with more than 1% fewer males and more than 1% more females during the most polluted time periods. The researchers estimated that there were nearly 31,000 less male births during the study period due to pollution.

Study authors claimed that pollution had an effect on the health of eggs and sperm. Hormone levels within the mother and father are also effected by pollution and these changes may be reflected in a difference in the gender of the offspring.