Recent research from top institutions reveals that heavy metals in air and water may influence newborn sex ratios more than seasons, weather, or social factors.
Can pollution alter the sex ratio at birth? Scientists from Karolinska Institutet in Sweden and the University of Chicago in the US tackled this question in a meta-study pre-published on MedRxiv in June 2021. They challenge the common notion that sex-ratio variability at birth stems from seasons, weather, or social influences.
Drawing on two vast datasets, the study covers roughly half of the US population—over 150 million people and more than three million babies born from 2003 to 2011. It also includes Sweden's entire population of 10 million, with 3.25 million births from 1983 to 2013.
The researchers cross-referenced this with detailed weather conditions and pollutant levels from national registries, including the US Environmental Protection Agency (EPA).
Findings show no links between sex-ratio variations and seasons, temperatures, unemployment, crime, or commute times. However, clear correlations emerged with pollutants: some, like iron, lead, mercury, aluminum, and carbon monoxide in the air, or arsenic and chromium in water, showed significant associations—either boosting or reducing male births.
Other factors included road fatality rates, extreme droughts, industrial permits, and vacant housing in local areas. Stressful events yielded mixed results: no tie to Hurricane Katrina (2005), but a correlation with the Virginia Tech shooting (2007).
That said, approach these results cautiously. As a pre-publication, the study awaits peer review. The authors stress that more research is needed to confirm each link, and it cannot conclusively prove pollution's direct causal role in sex-ratio shifts.
