By Alex Kirby
BBC News Online environment correspondent in Budapest, Hungary
Air pollution can damage the lungs of children even before birth, the World Health Organisation says.
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A report to be published later this year will say animal studies confirm findings that pollutants can impair lung growth in the womb.
The pollutants responsible are particulates, tiny fragments of soot emitted mainly from vehicle exhausts.
The WHO says the findings are significant, and show the need for urgent action to protect foetuses.
A WHO team carried out a systematic review of recent research on the effects of air pollution on children's health and development.
A paper circulated at a conference here of European health and environment ministers summarises the team's conclusions. The full study is expected to be published in the autumn in a peer-reviewed journal.
The paper says: "Findings of various population-based studies are supported by animal exposure studies, indicating that intrauterine as well as post-natal exposures to pollutants can lead to impaired lung growth."
Although pollution was known to cause some other forms of growth retardation in the unborn, this is believed to be the first confirmation of damage to the lungs of foetuses.
Dr Roberto Bertollini is director of the division of health determinants in WHO's Europe office.
He told BBC News Online: "This is a very important finding, and the consistency between the animal studies and those on people gives it greater strength.
"The pollutants studied in the research were particulates, 60-70% of which come from vehicles, with diesel engines particularly dangerous.
"We need to push policymakers to act: this is an overall issue of transport and mobility which now needs to be addressed urgently.
"Very drastic policies to reduce traffic or cut speed limits have produced results in some European city centres.
"I think congestion charging, which London has introduced, can be a useful tool.
"Paradoxically, introducing cleaner engines is not a solution, because the increase in traffic means the pollutant levels remain pretty stable.
"We have to experiment, we need case studies, and we have to tell people the magnitude of the problem.
"And of course this is an additional argument for pregnant women to stop smoking."
One of the most damaging forms of particulate matter is called PM10, as the particles are smaller than 10 microns in size.
They cannot be filtered out in the nose, and are able to penetrate deep into the lungs.
The WHO paper says there is now "substantial" evidence about the harmful effects of air pollution on pregnancy and infant health.
It says the evidence is enough to infer a causal relationship between particulate pollution and respiratory deaths in infants in their first year of life.
The paper says there is not enough evidence to say that outdoor air pollution levels typically found in Europe cause childhood cancer.
But it says the number of available studies is limited, and their results are not fully consistent.
It concludes: "The amount of ill-health attributable to air pollution among European children is high... current knowledge about the health effects of air pollution is sufficient for a strong recommendation to reduce children's current exposure to air pollutants, in particular to the pollutants related to traffic."