Every day, people all across the world breathe in poison. Outdoor air pollution kills an estimated 7 million people annually, according to the World Health Organization. Most of that toxic, deadly pollution is fine particulate matter, known as PM2.5, which comes from industry, agriculture, burning fossil fuels, dust storms, and other sources.
Inhaling PM2.5 is known to have big health consequences, and research has linked long-term exposure to lung problems, heart disease, cancer, premature death, and mental health disorders. Studies have also demonstrated that exposure to air pollution can negatively impact pregnancies, contributing to things like preterm births and birth defects.
Now, for the first time, an analysis has estimated the total proportion of stillbirths around the world caused by particulate pollution. In a re-assessment of previously collected data and existing studies on stillbirths and air pollution from 137 countries, the researchers determined that about 830,000 stillbirths every year (out of a total 2.09 million) in those countries were caused by exposure to fine particulate pollution—nearly 40%.
In the U.S., most of the impacts of air pollution disproportionately burden people of color and lower-income communities. And worldwide, that same trend generally holds. Stillbirths, too, are largely a tragedy experienced by people in poorer countries. The study authors did not look at data from Australia, New Zealand, the U.S., Canada, and Europe, because 98% of stillbirths occur in Central and South America, Asia, and Africa.
To come to their conclusions, the researchers analyzed Demographic and Health Surveys information on 46,391 pregnancies from 13,870 people from 1998 to 2016 across Asia and Africa. They divided the pregnancies into two groups: a control group and a stillbirth group. Then, they estimated air pollution exposure based on where each subject lived and controlled for environmental and demographic factors.
They found that the control group had significantly less exposure to fine particulate air pollution compared with the stillbirth group. Specifically, for every 10 microgram per cubic meter increase in fine particulate pollution, there was an associated 11% increase in risk of stillbirth—and the older the mother, the bigger that risk got—particularly for those older than 34 years.
“Among all subtypes of pregnancy loss, stillbirth was most strongly associated with PM2.5 exposure. In addition, advanced maternal age significantly enhanced the PM2.5–stillbirth association,” wrote the study authors.
The study authors also assessed fine particulate pollution exposure among pregnant people in their larger 137 country study set. They found that almost all pregnancies (99.96%) in these countries were exposed to particulate pollution levels above the recently halved 5 µg/m3 air quality guideline recommended by the World Health Organization.
Because of the ubiquitous level of exposure at 5µg/m3 among their focal countries, the study authors used the pre-2021, higher 10 µg/m3 threshold level for most of their analysis. They found that, using the higher threshold, at least a quarter of all stillbirths worldwide were attributable to air pollution exposure during pregnancy.
The researchers determined that the countries with the largest number of air pollution-related stillbirths were India, Pakistan, Nigeria, China, and Bangladesh. The countries with the highest proportion of stillbirths related to air pollution were Qatar, Saudi Arabia, Kuwait, Niger, and the U.A.E..
The exact mechanism by which fine particulate pollution causes stillbirths is unknown. However, at least one 2019 study demonstrated that pollution particles can cross the placental barrier and directly impact embryos and fetuses. Other research has suggested that air pollution harms maternal health, inhibiting oxygen transport to embryos and fetuses. The study authors also suggest it’s possible that exposure to air pollution causes placental abnormalities, which in turn lead to stillbirth.
The new study has multiple limitations. For instance, the authors didn’t track each subject’s location over the course of pregnancy, meaning that the data could have been impacted by peoples’ unreported movements. They also relied on largely self-reported survey data of pregnancy outcomes, and peoples’ recall can be flawed. Further, the researchers didn’t include every possible confounding heath risk for stillbirth. Finally, because they were comparing and pooling data from many countries, inconsistencies in reporting and data presentation likely impacted the study findings, noted the authors.
However, the study is a valuable contribution to the ever-growing body of evidence on how air pollution harms us, Gregory Wellenius, an environmental health researcher at Boston University who wasn’t involved in the new study, told The Guardian. “Although the fraction of stillbirths that might be prevented through meaningful reductions in PM2.5 is uncertain, the study adds to the abundance of scientific evidence showing that reducing air pollution levels would improve the health of people around the world, particularly among the most vulnerable individuals,” Wellenius said.
And, in good news, it seems that reduction and accompanying improvement are already happening. Within the 137 countries assessed, the rate of stillbirth declined over time—about 2% each year between 2000 and 2019. And part of that decline is likely due to reductions in air pollution in countries like China. Tackling air pollution saves lives and “meeting the WHO air quality targets could thus prevent a considerable number of stillbirths,” wrote the scientists.