About 140 million babies were born globally last year—the equivalent of adding an entire new Russia to the world’s population. Not counted among those typically blessed events are the number of families whose pregnancies end tragically. According to the United Nations Interagency Group for Child Mortality Estimation, about 2 million pregnancies around the world end in stillbirth each year. The causes of natal death are numerous—from fetal abnormalities to labor complications to maternal hypertension to infections to placental malformation.
Now, according to a new study in Nature Communications, there is another, particularly pernicious cause that may account for up to 39.7% of stillbirths in low- and middle-income countries: air pollution, specifically in the form of fine particles measuring 2.5 microns—or millionths of a meter—or less. The particles, about 3% of the width of a human hair, typically come from vehicle exhaust, the burning of coal and heating oil, and natural sources like wildfires.
According to the new research, led by environmental scientist and public health specialist Tao Xue at Peking University Health Science Center in Beijing, every 10 micrograms—or millionth of a gram—of so-called PM2.5 particles per cubic meter of air increases the risk of stillbirth by 11%, with the toll greater on older mothers than younger ones.
“The United Nations calls the global burden of stillbirths a neglected tragedy,” the paper’s authors write. “Preventing stillbirths depends on a comprehensive understanding of the underlying risk factors.”
The World Health Organization (WHO) has established a so-called reference level—or maximum safe exposure—of PM2.5 at 5 micrograms per cubic meter of air. Above that level, the particles can contribute to heart disease, asthma, decreased lung function, and premature death in people with pre-existing heart or lung disease. Researchers have long drawn a potential link between PM2.5 exposure and stillbirth, but what they hadn’t done, until Tao and his colleagues took the topic on, is to study how the burden falls disproportionately on people in poorer countries.
According to the World Bank, the average global concentration of PM2.5 is an alarming 46 micrograms per cubic meter—or nine times the WHO’s reference level. But the burden is not spread equally. In Australia, it’s a more tolerable 9 micrograms; in Canada, it’s just six. The Bahamas clocks in at just 5.58. It’s elsewhere, in the less developed world, that the pollution problem hits the hardest.
To conduct their study, the investigators selected 137 low- and middle-income countries in which data from the Department of Health Surveys (DHS), a division of the U.S. Agency for International Development (USAID), show are home to 98% of the world’s incidence of stillbirths. They cross-indexed those mortality figures with other data from the WHO’s Air Quality Guidelines detailing the severity of PM2.5 pollution in each of those countries. The results drew a bright line between the particulate emissions and the incidence of stillbirths.
India, the hardest-hit country, with an annual average of 217,000 stillbirths (out of 25 million live births), had a PM2.5 concentration of 60.15 micrograms per cubic meter of air—or 12 times the WHO’s reference level. Pakistan, the second most severely affected country, with 110,000 stillbirths per year (compared to 6.075 million live births), weighed in at 63.16 micrograms of pollution. Following them were Nigeria (93,000 stillbirths, 7.8 million live births, and 69.66 micrograms); China (64,000 stillbirths, 10,6 million live births, and 51.11 micrograms), and Bangladesh (49,000 stillbirths, 2.8 million live births, and 69.58 micrograms). In addition to PM2.5 air pollution, the study also cites the possibility of naturally occurring high concentrations of dust—particularly in sub-Saharan Africa and the Arabian Desert region—as another source of dangerous particles.
Maternal age played a significant role in the mortality risk. Relying on DHS data, as well as two existing meta-analyses of stillbirth incidence, the researchers calculated that mothers who lost their children at birth were, on average, 3.81 years older than those whose babies were born successfully. The greatest risk of stillbirth in high PM2.5 countries was among mothers ages 34 years or older.
Just what the mechanism is that links PM2.5 concentration to stillbirth is uncertain. “Although potential biological mechanisms for the association of PM2.5 exposure and pregnancy loss are not clear yet,” the researchers wrote, “some pathways can explain it to some extent.” The researchers propose three possibilities.
First, when a pregnant person inhales PM2.5 particles, they enter the bloodstream and may directly cross the placental barrier where they flow into the fetus, leading to low oxygen levels or immune problems in the baby, both of which can be associated with fetal death. There is also the possibility that exposure to PM2.5 can lead to the development of methemoglobin in the parent—a form of hemoglobin that does not bind with oxygen and thus does not deliver sufficient quantities of oxygen to the fetus, also leading to immune problems and potentially death in the womb. Finally, PM2.5 can lead to abnormalities or malformations in the placenta itself, preventing it from sustaining a fetus throughout pregnancy.
The outlook was not entirely bleak. The study notes that the global stillbirth rate actually decreased by 1.95% from 2000 to 2009, and fell again by 2.05% from 2009 to 2019. It attributes the trend to improved air quality in multiple countries‚ notably China, that are taking slow steps away from coal-fired power plants and cleaning up their power grid with renewable sources of energy.
In the meantime, pregnant people can take steps to reduce their exposure to PM2.5 pollution and better monitor the potential impact of the particles. Wearing N95 or KN95 masks outdoors on heavily polluted days, avoiding the outdoors when air quality is at its worst, and installing air purifiers in the home can all help. So too can improvements in prenatal care and timely intervention—including cesarean sections—in the case of premature or complicated labor.
Pregnancy and childbirth have never been easy. But more often than they do now, they should result in a happy outcome. In 137 countries at least, our own poor stewardship of the health of the planet is too often denying families that joy.
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