Now a study, carried out after women gave birth, has found soot particles in the placenta. Picture: Pixabay

London - Tiny particles of soot breathed in by pregnant women have been found in the womb, where they can harm the growing baby, research shows.

Air pollution has long been linked to a lower birth weight, higher infant mortality and childhood respiratory problems such as asthma.

Now a study, carried out after women gave birth, has found soot particles in the placenta.

This raises the possibility that the poisonous black particles could reach the foetus, the authors said. The research, by a team from Queen Mary University of London, was presented to the European Respiratory Society International Congress in Paris.

Dr Lisa Miyashita, a post-doctoral researcher, said: "We’ve known for a while that air pollution affects foetal development and can continue to affect babies after birth and throughout their lives. We were interested to see if these effects could be due to pollution particles moving from the mother’s lungs to the placenta. Until now, there has been very little evidence that inhaled particles get into the blood from the lung."

Scientists worked with five pregnant women who lived in London and were due to have caesarean deliveries at the Royal London Hospital. All were non-smokers with an uncomplicated pregnancy and each one gave birth to a healthy baby. Experts who studied the placentas after delivery were interested in cells called placental macrophages – part of the body’s immune system which engulf harmful particles such as bacteria and pollution to protect the foetus.

The team studied a total of 3 500 placental macrophage cells and examined them under a high-powered microscope.

They found 60 cells that between them contained 72 small black areas. Researchers believe these were carbon particles. On average, each placenta contained around five square micrometres of this substance.

The scientists then studied the placental macrophages from two placentas in greater detail using an electron microscope, and again found material that they believe was made up of tiny carbon particles.

Researcher Dr Norrice Liu said: "Our results provide the first evidence that inhaled pollution particles can move from the lungs into the circulation and then to the placenta.

"We do not know whether the particles we found could also move across into the foetus, but our evidence suggests that this is indeed possible."