Women exposed to lead are more likely to have boys

Demography has its vocabulary. Among the terms specific to this scientific field, there is the sex ratio: it designates the number of men per 100 women at birth. For several years, this figure has been decreasing. Researchers from Japan’s Tohoku University wanted to figure out why. In the specialist journal Science of the Total Environmentthey explain that these variations in the rate could be linked to the environment, and more particularly to the presence of lead in the air.

Lead, harmful to fertility

“Generally, for every 100 women, 104 to 107 men are bornexplains Nozomi Tatsuta, associate professor of medicine and lead author of this study. In recent years, the sex ratio has declined worldwide and the number of male births has fallen. “However, this ratio can be influenced by various factors, including toxins present in the environment. “Previous studies have reported that the sex ratio is affected by exposure to chemicals such as dioxins, as well as heavy metals such as methylmercury“, adds study co-author Kunihiko Nakai, a professor of environmental medicine. As lead may reduce female fertility as well as male sperm quality, researchers have speculated that it may in the evolution of the sex ratio.

The higher the blood lead level, the more male births there are

More than 85,000 pregnant women took part in this research, carried out in Japan. They were divided into different groups according to their level of lead in the blood, measured during pregnancy. Family income and smoking during pregnancy were taken into account in the data analysis. Scientists have used different methods to observe the effects of lead on the sex ratio. All show an increase in the proportion of male births when blood lead concentrations are higher. “The correlation between maternal lead exposure and sex ratio was consistent even after adjusting for other potential variables“, say the authors.

Reduced lead exposure

According to them, these results may explain why the percentage of male births in Japan has decreased: they point out that several measures have been put in place to reduce exposure to lead, particularly in paints or gasoline. As a result, lead levels in the air have dropped, which could explain why there are fewer male births. Furthermore, these results could have an impact on public health recommendations. Japanese scientists observe that a maternal blood level of less than 1 nanogram per gram already has an impact on pregnancy and can affect the sex ratio of the offspring, but current recommendations require that blood lead levels do not exceed 50 nanograms per gram of blood.