A large study spanning four continents finds that babies carried by mothers exposed to outdoor air pollution caused by tiny particles in fumes from traffic, heating systems, and coal-fired power stations, are more likely to be of low birth weight.

A report on the work, led jointly by Tracey J. Woodruff, professor of obstetrics and gynecology and reproductive sciences at University of California – San Francisco (UCSF), and Jennifer Parker, of the National Center for Health Statistics at the US Centers for Disease Control and Prevention (CDC), is published this week in an advance online issue of Environmental Health Perspectives.

Low birth weight babies are at higher risk for health problems and death. Although most survive, they also have a higher chance of developing chronic health problems like diabetes and heart disease later in life.

The study is the largest of its kind, and examines data on more than three million births, collected from 14 sites in 9 countries in North America, South America, Europe, Asia and Australia.

The sites are research centers involved in the International Collaboration on Air Pollution and Pregnancy Outcomes, an international project set up in 2007. Most of the data used in the study was gathered during the mid-1990s to the late 2000s; the rest dates from before that period.

The reason for the study, note the researchers in their background information, is that while a growing body of evidence has linked maternal exposure to outdoor air pollution with adverse effects on fetal growth, it is inconsistent.

Their analysis shows that for all the sites worldwide, the higher the concentration of microscopic particles or particulates in the air, the more low birth weight babies there were.

Air pollution is measured according to the concentration, in micrograms per cubic meter (µg/m3), of microscopic particles whose size is measured in microns (millionths of a meter).

In a statement, Woodruff says what is significant about the findings is that “these are air pollution levels to which practically everyone in the world is commonly exposed.”

“These microscopic particles, which are smaller than the width of a human hair, are in the air that we all breathe,” she explains.

She points out that countries that tightly regulate air pollution have lower levels of these particles, and the US has seen considerable benefits to health and wellbeing in past decades from reducing levels of air pollution, benefits that far exceed the costs.

Federal regulation in the US requires that the average concentration over a year should not exceed 12 µg/m3 of particles smaller than 2.5 microns.

In the European Union the limit is much higher at 25 µg/m3, although it is under review.

One of the researchers, Mark Nieuwenhuijsen, of the Centre for Research in Environmental Epidemiology (CREAL) in Barcelona, Spain, says:

“This study comes at the right time to bring the issue to the attention of policy makers.”

Nieuwenhuijsen explains that in Beijing, China, levels higher than 700 µg/m3 have been observed recently.

“From the perspective of world health, levels like this are obviously completely unsustainable,” he urges.

Funds from the Environmental Protection Agency and the National Institute for Environmental Sciences in the US, the Wellcome Fund in the UK, and the Ministry of Science and Innovation in Spain, helped finance the study.

Two studies published early in 2012 in the Archives of Internal Medicine suggest that air pollution at levels experienced by most Americans or considered safe by the Environmental Protection Agency is linked to higher risk of cognitive decline and stroke.

Some of the children involved in this study will be followed into adulthood, as a result of which researchers hope to be able to see if mothers’ exposure to air pollution affects health of their children in adulthood, regardless of birth weight.

Written by Catharine Paddock PhD