Fossil fuel soot has been found inside human placentas for the first time, showing the risk pollution poses for unborn babies.
Scientists examined the placentas of five London women, looking closely at immune cells called macrophages which they say "eat" harmful particles to protect the foetus.
"We've known for a while that air pollution affects foetal development and can continue to affect babies after birth and throughout their lives," said Lisa Miyashita, a post-doctoral researcher at Queen Mary University of London.
"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."
- New satellite images show shocking extent of Earth's air pollution
- Where is the worst air pollution in the world?
None of the five women were smokers, and all had routine pregnancies ended by planned caesarean sections.
Each placenta the researchers examined had around five square micrometres of a black substance, which turned out to be tiny carbon particles.
"Our results provide the first evidence that inhaled pollution particles can move from the lungs into the circulation and then to the placenta," said Norrice Liu, a paediatrician and clinical research fellow at Queen Mary University of London.
"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. We also know that the particles do not need to get into the baby's body to have an adverse effect, because if they have an effect on the placenta, this will have a direct impact on the foetus."
Professor Mina Gaga, president of the European Respiratory Society, said it highlights the need for cleaner air than current regulations allow for.
"The evidence suggests that an increased risk of low birthweight can happen even at levels of pollution that are lower than the European Union recommended annual limit," she said.
"We need stricter policies for cleaner air to reduce the impact of pollution on health worldwide because we are already seeing a new population of young adults with health issues."
The research was presented at the European Respiratory Society International Congress in Paris. It has yet to undergo peer review.