In recent years, scientists have discovered that exposure to air pollution can have negative impacts on a growing fetus, resulting in a lower birth weight or premature birth. But they haven’t known why — until now.

A group of scientists in Belgium has found that when pregnant women inhale black carbon pollution, the particles can travel from their lungs to the placenta, where they accumulate on the side facing the growing baby. In a study published Tuesday in the journal Nature Communications, the scientists write that black carbon particles were found on every single placenta they looked at, regardless of how much pollution the mother had been exposed to.

“Most studies that have been done are epidemiological studies—population-based studies,” said lead author Hannelore Bové, a postdoctoral researcher at the Centre for Environmental Sciences at Hasselt University. “We now have a direct measurement of the impact of black carbon particles that is not based on modeling, but on measurements on the fetal side of the placenta.”

Black carbon—or soot—is created by the incomplete combustion of fossil fuels and the burning of wood in stoves or forest fires. In addition to being a health risk, it’s also a short-lived climate pollutant. Though black carbon exists in the atmosphere for just days or weeks, one gram of it can warm the atmosphere 100 to 2,000 times more than one gram of CO2 on a 100-year timescale.

In a placenta, black carbon can cause inflammation that can lead to or exacerbate other health issues.

The scientists found that the more pollution the mothers were exposed to, the more black carbon ended up in their placentas.

“In Belgium, we have quite low concentrations in the air. And on top of that, we’re recruiting mothers in a neighborhood where there’s an especially low concentration,” said Bové. “If we can find it at low levels, it should be even worse when you’re exposed at higher concentrations.”

The scientists examined the placentas of 20 women in Belgium, performing biopsies on both the maternal and fetal sides shortly after the birth. The women were chosen based on where the live—10 lived in places where they were exposed to relatively high levels of black carbon and 10 were considered to have had low-level exposure. The scientists also looked at five placentas from miscarriages that occurred between 12 and 31 weeks of pregnancy.

Most of the women in the study were exposed to black carbon emitted from vehicles or from wood-burning stoves. Bové said one of the researchers’ next steps will be to analyze the black carbon particles to better determine the exact source of the pollutants they found.

Insight into Potential Health Effects

The placenta plays a crucial role in pregnancy. It provides a natural barrier between the mother and the fetus and provides oxygen and nutrients to the growing baby.

An earlier, unpublished study that was presented at the European Respiratory Society International Congress provided the first indication that black carbon was present in placentas. In that work, researchers from the Queen Mary University of London found black carbon in cells from five placentas. Because the cells had been removed from the placentas, though, it was unclear whether they were from the maternal or fetal side, Bové said.

Black carbon in the placenta can cause inflammation, Bové said, which can result in different adverse health effects. The women in the study are part of a birth cohort, meaning they and their babies will be returning at regular intervals for health checks. That will allow scientists to start to understand what impact the placental black carbon might be having.

“The levels of air pollution where this study took place are so low that we don’t often see adverse health outcomes, or not much,” said Joan Casey, an assistant professor in environmental health sciences at Columbia University who was not involved with the study. “But there may be a threshold where it becomes a problem. I’d love to see this done in places like Beijing or in India, where we might expect to see much higher levels.”

In the United States, on average, air pollution levels have declined in recent decades, meaning that the study’s findings likely do not provide answers about the prevalence of childhood asthma and autism diagnoses, Casey said.

Pollution and High Blood Pressure in Kids

Noel Mueller, who studies the impact of particulate matter exposure on children, said he hopes to apply the findings to his work with large cohorts in the Boston area.

In 2018, Mueller co-published a study finding that the children of women who were exposed to higher levels of particulate matter during the third trimester were significantly more likely to have high blood pressure in childhood.

“We had several hypotheses for how exposure might be affecting the fetus—one was inflammation, and the other was that particulate matter could cross the placental barrier and accumulate,” said Mueller, who is an assistant professor of epidemiology at Johns Hopkins Bloomberg School of Public Health. “This study provides proof of principle that maternal exposure to air pollution during pregnancy may be affecting not only the mother but also the fetus.”