Specks of soot coughed out from car engines and fossil fuel plants can be found in the fetal side of the placenta, according to a study published today in the journal Nature Communications.

A team led by scientists from the Hasselt University in Belgium studied the post-birth placentas of 28 women who had been exposed to air pollution. Using high-resolution imaging, they revealed the presence of microscopic black carbon particles, the sooty black material pumped out during fossil fuel combustion, in the fetal-side of the placenta.

"The key finding is that soot particles can enter the fetal part of the placenta, which means that during one of the most vulnerable stages of life, [when] organ systems are under full development, translocation of particles from the mother's lungs to the fetus is possible," Tim Nawrot, study author and professor of environmental epidemiology at Hasselt University's Centre for Environmental Sciences, told IFLScience.

"These particles cause inflammatory responses and might also react with the DNA," added Nawrot.

"We need to develop air pollution standards which protect the most vulnerable of the population," they concluded.

Mothers who lived in areas choked with high air pollution during pregnancy had higher levels of residential black carbon particles (over 2.4 micrograms per m³) compared to the 10 mothers who had been exposed to lower levels of residential black carbon (0.63 micrograms per m³).

The placenta is an organ that develops in the uterus during pregnancy to provide oxygen and nutrients to the growing fetus, as well as to remove waste products from the baby's blood. It can act as a barrier for some infections, but it’s unable to protect against viruses. Drugs, such as alcohol and nicotine, can cross the placenta and cause damage to the unborn baby. It’s believed that air pollution particles may also translocate into and cross the placental barrier.

While the idea of air pollution passing across the placenta has been pointed to before in smaller studies, the research includes evidence from the largest number of human cases found to have soot in the fetal-side of the placenta. In an accompanying review article in the journal Clinical Epigenetics, researchers from the same team present evidence that suggests how air pollution might cause changes to the placenta. For example, they suggest air pollution could spark oxidative stress, inflammation, changes in energy metabolism, and epigenetic alterations within the placenta. 

Further studies are needed before we can reach any conclusions about the health implications of black carbon particles crossing to the fetal-side of the placenta. That said, a mounting body of evidence has linked exposure to air pollution during pregnancy to a number of negative health effects, including everything from low birth weight and premature birth to stunted growth and autism.