Babies are exposed directly to air pollution while still in the womb, scientists have discovered for the first time, after finding particles of soot buried deep in the placenta.

Black carbon was found in the placenta of five pre-term, and 23 full-term, births in Belgium with the number of particles rising if a woman lived in a highly polluted area.

The placenta provides oxygen and nutrients to the baby during development, and gets rid of waste products, but particles were found on both the mother's side and the baby's, suggesting the pollutants were passing through.

Researchers at the University of Belgium said the finding may provide a reason why pollution during pregnancy is linked to detrimental health effects in children, such as low-birth weight and asthma.

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In a linked article in the journal Nature Communications, the team said that the particles may carry free radicals on their surface which cause cell damage, and also harm the mitochondria — the cell batteries — and telomeres, which protect the tips of chromosomes and prevent DNA damage.

Writing in the journal, Professor Tim Nawrot, of Hasselt University in Belgium, said: "Children are at higher risk of adverse health effects caused by air pollution, even at low levels, because their immune system and lungs are not fully developed, especially during in utero and early life.

"Our results demonstrate that the human placental barrier is not impenetrable for particles.

"Developmental vulnerability should be a priority for environmental public health policies and practices to protect the most susceptible period of human life due to the long-term consequences."

Black carbon particles are released every day into the air, largely from the combustion of fossil fuels.

Researchers found that 10 mothers who had been exposed to high levels of residential black carbon particles — 2.42 micrograms per cubic metre — during pregnancy had higher levels of particles in the placenta than 10 mothers exposed to low levels of residential black carbon — 0.63 micrograms per cubic metre.

Commenting on the research, Andrew Shennan, Professor of Obstetrics, at King's College London (KCL), said: "Small particles, such as through smoking, can cause considerable disease related to the placenta, and these findings of particles in the placenta are a concern.

"Their possible effects on the baby and mother warrant further investigation. The placenta is the interface between mother and baby and is key to nourishing and supporting all the needs of the baby.

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"Both the function and structure of the placenta is important, not only to the baby's growth and wellbeing, but also to that of the mother."

- Telegraph Group