Exposure to common air pollutants and traffic during pregnancy significantly increases the risk of restricted fetal growth, even at levels well below those stipulated in current European Union air-quality directives, according to one of the largest studies of its kind, published in The Lancet Respiratory Medicine.
The researchers estimate that for every increase of 5 micrograms per cubic metre (5µg/m³) in exposure to fine particulate matter during pregnancy, found in for example traffic fumes and industrial air pollutants, the risk of low birthweight at term rises by 18%. Importantly, this increased risk persists at levels below the existing EU annual air quality limit of 25µg/m³.
"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", explains lead author Dr Marie Pedersen from the Centre for Research in Environmental Epidemiology in Barcelona, Spain.
Pedersen was part of a team of European researchers coordinated by CREAL (Barcelona) and INSERM (Grenoble) to assess the impact of exposure to low levels of air pollution during pregnancy on low birthweight at term (<2500g, after 37 weeks of gestation) which has been linked with respiratory problems in childhood, as well as other diseases later in life. The study also looked at the impact on head circumference because of the potential effect on neurodevelopment.
Using data from the European Study of Cohorts for Air Pollution Effects (ESCAPE, coordinated by the University of Utrecht, the Netherlands), the investigators pooled data from 14 cohort studies in 12 European countries involving over 74 000 women who had singleton babies between Feb, 1994 and June, 2011.
Air pollution concentrations of nitrogen oxides and particulate matter were estimated at the home addresses using land-use regression models. Traffic density on the nearest road and total traffic load on all major roads within 100m of the residence were also recorded.
All air pollutants, particularly fine particulate matter (PM 2.5; with a diameter of 2.5 micrometers or less), and traffic density increased the risk of term low birthweight and reduced average head circumference at birth, after accounting for other factors like maternal smoking, age, weight, and education.
Average exposure levels of PM2.5 during pregnancy in the study population ranged from less than 10μg/m³ to nearly 30μg/m³.
The researchers estimated that if levels of PM 2.5 were reduced to 10µg/m³ (the WHO annual average air quality guideline value), 22% of cases of low birthweight among term deliveries could be prevented.
According to Dr Pedersen, "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."
Writing in a linked Comment, Professor Jonathan Grigg from Queen Mary, University of London, UK says "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…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, UK, 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."