A typical adult takes around 20,000 breaths per day. If you live in a megacity like Beijing, with many of those lungfuls you're likely to inhale a noxious mixture of chemicals and pollutants.
According to BreatheLife, the air in the Chinese capital is 7.2 times above safe pollution levels, based on guidelines set out by the World Health Organization.
For expectant mothers, that means an increase in the risk of "silent" miscarriages, suggests a new large-scale study published in Nature Sustainability, Deutsche Welle reports.
Air pollution is already known to raise the risk of premature birth, low birth weight and life-threatening health complications for pregnant women, like preeclampsia, which is marked by high blood pressure, or gestational hypertension.
But its impact on "silent" or missed miscarriages — where the fetus died or did not develop, but has not been physically miscarried — has been more difficult to establish.
A collaborative effort from 16 different authors at several universities in China, the study retrospectively examined the records of more than a quarter million pregnant women in Beijing from 2009 to 2017 in light of the womens' exposure to air pollution.
Among the women whose clinical records were studied, 17,497 — which is 6.8% — were found to have experienced these types of miscarriages.
The researchers found the probability of missed miscarriages increased with higher concentrations of air pollutants, taking into consideration different ages, occupations and air temperature.
Women who were older than 39 when they became pregnant, as well as farmers and blue-collar workers, had a higher risk of a missed miscarriage associated with air pollution, the authors said.
The authors defined "air pollution" as consisting of four pollutants: fine particulate matter (referred to as particulate matter 2.5 or PM 2.5), sulfur dioxide, ozone and carbon monoxide, and calculated the levels based on historic data from Beijing's network of air monitoring systems.