The study is "the first to show that air pollution particles can reach the fetal side of the placenta," said study author Hannelore Bove, a postdoctoral researcher with the Centre for Environmental Sciences and Biomedical Research Institute at Hasselt University in Diepenbeek, Belgium.
The finding follows an analysis of high-resolution images taken of placenta tissue retrieved from 28 women who had just given birth at East-Limburg Hospital in Genk, Belgium.
Five gave birth prematurely, while the remainder carried to term. In each case, placenta tissue was retrieved within 10 minutes after delivery.
Bove and the research team reported detecting black carbon particles - most commonly produced by fossil fuel combustion -- in the placenta of all the participating mothers.
But the tissue of those residing in relatively polluted areas had collected notably higher levels of carbon particles in their placenta than those from less polluted locations.
The study is published in the Sept. 17 issue of Nature Communications.
Whether those particles definitely harm fetal development remains an open question. The researchers only set out to see whether pollution was present in the placenta. "This is the subject for another study," said Bove.
Still, the new findings underscore a basic fact: that a mother's environment is also the fetus' environment. And the placenta cannot offer foolproof protection from the outside world.
"The placenta's principal function is to deliver nutrition and oxygen to the fetus," explained study corresponding author Tim Nawrot, a professor of environmental epidemiology at Hasselt University. "But to some extent, it also forms a barrier against toxicants."
In fact, "decades ago it was thought that nothing can cross the placenta," said Bove. "And that the fetus is completely safe inside the womb."
But in the 1960s, the thalidomide crisis shattered that belief, she noted. The infamous sleeping pill's ingredients were found to have crossed the placenta in pregnant women, triggering massive birth defects.
Since then, it's become clear that all kinds of substances - including alcohol, drugs and now fossil fuel pollution - can and do cross the placenta, said Bove.
Once inside the placenta, fetal exposure to pollution rings a number of alarm bells.
For one, "black carbon particles are thought to be especially toxic, since they can absorb toxic compounds like, for example, heavy metals and benzene," said Bove. Such cancer-causing agents can mix with the oxygen and nutrients that feed a developing fetus.
Ingestion of combustion pollution can also boost infection risk, trigger problematic alterations at the molecular level, and provoke placental inflammation, according to Dr. Mitchell Kramer.
Kramer, who was not part of the study team, is chairman of the department of obstetrics and gynecology at Northwell Health's Huntington Hospital, in Huntington, N.Y. He warned that such inflammation "is known to be associated with low birth weight and preterm birth, which lead to adverse developmental and cognitive impairment."
The bottom line, Kramer suggested, is that it's "indisputable" that inhalation of smog pollution poses a real danger to fetal health.
What can expectant parents do? Both study authors said there is no easy answer.
"Individuals cannot do much to avoid pollution," said Nawrot.
And Bove said that "moving homes is not realistic" for most people living in highly polluted areas.
"But small changes in daily routines may already reduce the exposure levels," she added. Using candles, cooking with gas or wood, barbecues, and walking near busy roads are avoidable, she said, and women should "limit their time in the neighborhood of these sources."