Dec. 2 (UPI) -- Air pollution may impact the gender of babies born in a given region, a study published Thursday by the journal PLOS Computational Biology found.
The analysis of more than 6 million births in the United States and Sweden found that areas with high levels of polychlorinated biphenyls, or PCBs, in the air and water saw more babies born male than those with lower amounts of these pollutants, the data showed.
PCBs were once used as coolants in electrical appliances before research revealed their impacts on human health, though they are still often found in the environment, studies suggest.
Similarly, regions with high levels of aluminum in the air, typically those where the metal was processed, also had increased birth rates for males versus females, as did areas with more chromium, arsenic and mercury water contamination, the researchers said.
However, high levels of lead and iron in soil, found in many areas with a history of heavy industry, had the opposite effect, causing a rise in female births.
Meanwhile, climate trends had no impact on birth rates for either gender, according to the researchers.
The findings could encourage policymakers to "to make steps toward reducing environmental pollution," study co-author Andrey Rzhetsky, a professor of medicine and human genetics at the University of Chicago, said in a press release.
Historically, the ratio of baby boys to baby girls, or the sex ratio at birth, has fluctuated over time, research indicates.
At a biological level, the sex ratio at birth can be affected by hormonal factors that specifically terminate female or male embryos during pregnancy, according to Rzhetsky and his colleagues.
Past studies have suggested that pollutants, changes in weather and psychological stress experienced by pregnant people as possibly changing the sex ratio at birth, but the results have been inconclusive.
For this study, the researchers analyzed records from the IBM Health MarketScan insurance claim dataset for more than 3 million births in the United States between 2003 and 2011, as well as records on more than 3 million births in the Swedish National Patient Registry from 1983 to 2013.
Additional information on weather and pollutants at the time of each birth was available from other national databases, the researchers said.
Seasons, ambient temperature, violent crime rates, unemployment rates and parent commute times had no influence on the sex ratio at birth, the data showed.
There were, however, numerous pollutants that were associated with changes to the sex ratio at birth, with some the ratio of boys and others decreasing it.
These pollutants included PCBs, mercury, carbon monoxide and aluminum in the air and chromium and arsenic in water, all of which increased the rate of babies born male, they said.
Extreme droughts also rose the rate of male babies born, but stressful events such as Hurricane Katrina and the Virginia Tech shooting appeared to have no effect, according to the researchers.
The study could not determine whether or not the pollutants actually caused the changes in the ratio of boys to girls, Rzhetsky said.
"Ideally, each sex ratio at birth-pollutant association could now be followed up with experimental work using human cell lines to dissect the underlying mechanism," he said.
