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What if pollution influenced the sex ratio at birth?

A recent study indicates that the proportion of newborn boys and girls would have more to do with heavy metals present in the air and water rather than with the seasons, climatic hazards and other social factors.

Huge databases

Is the sex ratio at birth related to pollution? Researchers from Karolinska Institutet (Sweden) and the University of Chicago (USA) tried to answer this question with a meta-study pre-published on the MedRxiv platform in June 2021. According to the researchers, thinking that the sex-ratio variability at birth is influenced by the seasons, the weather or certain social factors is a matter of popular belief.

The study in question is based on two important databases. The first represents about half of the population of the United States, or 150 million people and more than three million babies born between 2003 and 2011. The second concerns the population of Sweden as a whole, i.e. ten million people, including the 3.25 million births in the country between 1983 and 2013.

In addition, the researchers studied information about weather conditions and pollutants at each birth. This data comes from other national registries, including the Environmental Protection Agency (EPA) in the United States.

A study to be taken with a grain of salt

According to the results, sex-ratio variations have absolutely no relation to the seasons , temperatures, unemployment, crime or daily commuting times to work. On the other hand, researchers have established a link with many pollutants. Some are synonymous with an increase in the number of newborn males and others with a reduction. Substances with a significant correlation are generally present in the air (iron, lead, mercury, aluminum or carbon monoxide) and in water (arsenic and chromium).

The study also mentions other factors such as the road fatality rate and extreme droughts. There are also industrial permits as well as empty dwellings in a defined area. In addition, the researchers attempted to correlate certain stressful events to sex variability at birth. Thus, they found no correlation with Hurricane Katrina (2005), but did find one in the case of the Virginia Tech shooting (2007).

Are we to doubt the reliability of this meta-study? Without a doubt. First of all, it is a pre-publication and has therefore not been validated by peers . The researchers also explain that additional studies would be necessary to verify the veracity of each correlation. This study therefore cannot alone prove that pollutants have a direct link with variations in the sex ratio at birth.