Are Environmental Pollutants Tied to Changes in the Human Sex Ratio?

Pollutants, or particles that damage water and the atmosphere, can have wide-ranging health consequences. Pollutants are associated with skewing human sex ratios at birth (SRBs), the ratio of newborn males to females, according to a 2021 study published in PLOS Computational Biology. In 2021, the human sex ratio was 101.68 males per 100 females. Sex ratios can vary over regions and time periods. While sex is determined at conception, hormonal factors can terminate female or male embryos, altering sex ratios.

The large-scale study performed statistical tests on country-wide data sets from half of the United States and the entire country of Sweden. The study was the first of its kind to examine environmental factors using large data sets from two continents.

The researchers examined a multitude of varying factors and their potential to affect sex ratios. The study uncovered that a stressful event, proximity to fast-food restaurants or vacant buildings and, potentially, farming could influence SRBs. However, violent crime and unemployment rates bore no influence.

How can pollutants influence sex ratios?

Researchers in the study, which was led by Andrey Rzhetsky at the University of Chicago, found mercury, chromium in water and aluminum pollution in air correlated with more boys. In contrast, lead pollution in soil increased the number of girls.

These findings were influenced by geographic regions. The prevalence of lead in soil was higher in the Northeast, Southwest and Mideast United States, but not in the South. The occurrence of mercury in water was increased in Eastern and Northeastern states.

"The dramatic increase of toxic chemicals in the air we breathe, water we drink, food we eat and products we use every day is having profound impacts on our bodies and our health. One implication of this uncontrolled chemistry experiment is the likelihood that certain pollutants are changing the ratio of boys born versus girls born," said Chris Hagerbaumer, M.P.P., the executive director of OpenAQ, a nonprofit organization in Portland, Oregon, that promotes air quality.

While the study did not draw concrete conclusions for why pollutants could be skewing sex ratios, the research includes a popular theory, the Trivers-Willard hypothesis, as a possible explanation. According to the theory, natural selection increases the number of males in favorable conditions and decreases the number of females in unfavorable environments.

"It is fair to assume that at a microdata level, the Trivers-Willard hypothesis will have ripple effects on global economics for future generations," said Urvashi Bhatnagar, D.P.T., M.P.A., a sustainability leader and healthcare executive in Washington, D.C., and co-author of "The Sustainability Scorecard: How to Implement and Profit From Unexpected Solutions."

The Trivers-Willard hypothesis, according to Rzhetsky's study, "postulates that because the cost of rearing children is much higher for females than for males, in favorable, resource-rich environments, males would have more offspring than females, and vice versa in unfavorable conditions."

Stress and sex ratios

The study found a psychologically stressful event during pregnancy can also potentially influence sex ratios. Researchers looked at two catastrophic events: the Virginia Tech campus shooting in 2007 and Hurricane Katrina in 2005. The SRB was lower than expected for 34 weeks following the shooting, however, the changes after Katrina were not significant, the study stated.

The study also examined additional factors, such as heat and different seasons, and found both were not catalysts for impacting SRBs.

Stress affects sex ratios from preconception to conception, Bhatnagar explained. He referenced a prior small-scale study that pointed to the high risks of bearing a son in men diagnosed with anxiety, hence, skewing the sex ratio while resulting in birth complications for physically stressed moms.

The research has notable implications.

"These variations are statistically significant. They might be small, but it all translates into thousands of premature deaths," Rzhetsky told NBC News in December 2021. "If something affects sex ratios, it means it affects human biology, and it probably has other health effects."

Research moving forward

The study's bottom line: Certain pollutant exposures that vary by geographic location and stressful events are associated with higher or lower SRBs. Some people, depending on location, may be more at risk than others.

"This is a complicated public health issue that requires investment, research and intentional focus," Bhatnagar said.

While the 2021 study might have the largest data sets, the issue has been circulating as more scientists link pollution with health consequences. A 2014 study in Scotland linked lower sex ratio values to areas of the country with higher industrial air pollution.

"No matter the sex at birth, what's most concerning is how pollution is impacting children's development," Hagerbaumer said. "For example, air pollution is correlated with preterm birth, low birth weight, respiratory diseases, reduced lung function, increased asthma incidence, neurodevelopmental disorders, IQ loss, pediatric cancers and increased risks for adult chronic diseases."

The 2021 study did not account for direct cause and effect between pollutants and SRBs. One limitation of the study is researchers did not have access to the sex of stillbirths, and the data set in the United States was not inclusive of the entire population.

'Air pollution is correlated with preterm birth, low birth weight, respiratory diseases, reduced lung function, increased asthma incidence, neurodevelopmental disorders, IQ loss, pediatric cancers and increased risks for adult chronic diseases.'

But for many advocates, the study sounds the alarm that pollutants are impacting health before birth. As for the impacts on pregnancy, prior research indicates that air pollution could increase miscarriage rates. A study of more than 4,000 lead-exposed male workers in the state of New York showed that exposure to lead could reduce fertility rates. Similarly, a 2017 study of Flint, Michigan's water crisis and increased lead exposure found decreased fertility rates and likely lower birth weights.

Bhatnagar recommended global action against known "forever chemicals" that have generational reproductive and systemic health impacts.

"These chemicals need to be innovated out of our everyday products that cause passive, daily health risks," Bhatnagar said.

Bhatnagar also recommended more action in banning other known carcinogens, such as Teflo, and increased public awareness.

Air pollution is accelerating, according to a 2021 report by IQAir, and moving forward, more research on the correlation between the environment and reproduction is warranted.