A study has shown that unborn babies are directly exposed to the unfiltered air pollution particles breathed in by the mother.
The research is the first study to show the placental barrier can be penetrated by the black carbon produced by motor traffic and fuel burning.
It found thousands of the tiny particles per cubic millimetre of tissue in every placenta analysed.
According to the Guardian, the link between exposure to dirty air and increased miscarriages, premature births and low birth weights is well established.
The research – which also examined placentas from miscarriages and found the particles were present even in 12-week-old foetuses – suggests the particles themselves may be the cause, not solely the inflammatory response the pollution produces in mothers.
“This is the most vulnerable period of life," said Prof Tim Nawrot at Hasselt University in Belgium, who led the study.
"All the organ systems are in development. For the protection of future generations, we have to reduce exposure.”
The detection of the particles on the foetal side of the placental barrier means it was very likely the foetuses were exposed, Nawrot continued.
Work to analyse foetal blood for particles is now underway, as is research to see if the particles cause DNA damage.
A comprehensive global review recently concluded that air pollution may be damaging every organ and virtually every cell in the human body.
“It is really difficult to give people practical advice because everyone has to breathe,” he said.
"But what people can do is avoid busy roads as much as possible. There can be very high levels next to busy roads, but just a few metres away can be lower.”