Pregnant women who are exposed to even low levels of air pollution are at an increased risk of giving birth at term to low birthweight babies, according to a large-scale study.
Air pollutants - in particular fine particulates found in traffic fumes and industrial air pollutants - along with traffic density increased the risk of low birthweight and reduced average head circumference of babies born at term, research has shown.
The study, drawn from data on 74,000 pregnant women in 12 European countries gathered between 1994 and 2011 and published in the Lancet Respiratory Medicine, estimated concentrations in the air of nitrogen oxides and fine particulates at home addresses.
Traffic density on the nearest road and total traffic load on all major roads within 100 metres of the residence was also recorded.
Researchers estimated that for every increase of five micrograms per cubic metre in exposure to fine particulate matter - emitted by sources including diesel engines and coal-fired power stations - during pregnancy, the risk of low birthweight at term rose by 18%.
This increased risk remained at levels below the existing European Union annual air quality limit of 25 micrograms per cubic metre.
The average exposure to fine particulate matter during pregnancy in those studied ranged from less than 10 micrograms to nearly 30 micrograms per cubic metre.
The study authors estimated that if levels of fine particulates were reduced to 10 micrograms per cubic metre - the World Health Organisation annual average air quality guideline value - 22% of cases of low birthweight among term deliveries could be prevented.
Low birthweight for a baby born at term was classified as less than 2.5kg. The study took into account other factors such as maternal smoking, age, weight and education.
Lead author Dr Marie Pedersen, from the Centre for Research in Environmental Epidemiology in Barcelona, said: " Our findings suggest that a substantial proportion of cases of low birthweight at term could be prevented in Europe if urban air pollution, particularly fine particulate matter, was reduced.
"The widespread exposure of pregnant women worldwide to urban ambient air pollution at similar or even higher concentrations than those assessed in our study provides a clear message to policy makers to improve the quality of the air we all share."
Professor John Wright, of the Bradford Institute for Health Research and a co-author of the report, called for "greener transport" including electric cars to reduce traffic pollution.
"On the plus side, our air is getting better," he said.
"But our study has shown that at levels half of what the European Union are suggesting, air pollution does harm. We need greener transport," he said.
The findings are the latest in a long series of studies documenting the negative effects of traffic pollution on health and well-being.
Low level exposure to traffic fumes can increase the risk of lung cancer and exposure to traffic fumes and industrial pollution can dramatically increase a mother's chances of having a child with autism.
Defective k idneys may be a sign that traffic pollution is harming the arteries and t raffic fumes may also increase the risk of stroke by narrowing the arteries that carry blood to the brain, research has suggested.
Traffic pollution has also been found in research to cause as much childhood asthma as passive smoking.
Writing in a linked comment piece to the research, Professor Jonathan Grigg, from Queen Mary University of London, said UK policy makers had so far shied away from radical solutions to air pollution.
"Overall, maternal exposure to traffic-derived particulate matter probably increases vulnerability of their offspring to a wide range of respiratory disorders in both infancy and later life...," he wrote.
"Dissemination of [these] results to the wider public could therefore further increase the pressure on policy makers to reduce exposure of urban populations to particulate matter.
"Difficult decisions still need to be made.
"The introduction of the low emission zone in London has had little effect on concentration of particulate matter, although the vehicle mix has been altered.
"UK policy makers have shied away from radical solutions to the issue, such as changing diesel-powered black cabs - which contribute 20% of London's locally generated particulate matter - to cleaner petrol-powered alternatives."
Dr Patrick O'Brien, spokesman for The Royal College of Obstetricians and Gynaecologists, said: "The results of this study suggest that maternal exposure to common air pollutants and traffic is associated with restricted fetal growth, resulting in reductions in birth weight and head circumference.
"Exposure to some level of air pollution is unavoidable in day-to-day life and the risk still remains fairly low.
"Other factors, such as smoking, high blood pressure or excessive alcohol consumption, may contribute more to the risk of having a low birthweight baby.
"This research, using data from several large population-based studies, is very helpful in providing further evidence on the potential health impacts of air pollution.
"More research in this area could further our knowledge on the impact of air pollution on women and their babies."