Pregnant women who have been exposed to higher levels of some types of air pollution are slightly more likely to give birth to underweight babies, a large international study has found. The results are published online today in Environmental Health Perspectives.
Low birth weight — defined as a newborn baby weighing less than 2.5 kilograms — increases the risk of infant mortality and childhood diseases, and has been associated with developmental and health problems later in life, including diabetes and cardiovascular disease.
Previous studies have looked at whether exposure to tiny airborne particles during pregnancy leads to more low-birth-weight babies. Although many studies have found links, others have failed to establish a connection.
“The thorn in the side of many studies of air-pollution exposure and impact on fetal growth has been the variability in study design and in exposure assessment,” says Leonardo Trasande, a children’s environmental-health researcher at New York University in New York City. “This one does a tremendous service by making them very comparable.”
The observational study pooled data from 3 million births at 14 research centers in 9 countries, including the United States, South Korea and Brazil. It focused on two classes of hazardous air pollutants: inhalable particulate matter (PM) with a diameter of less than 2.5 micrometers (PM2.5) and of less than 10 micrometers (PM10). These particles are produced from the combustion of fossil fuels by industry and transportation and from burning firewood, but can also include particles of dust or sea salt.
“Those centers that have higher levels of air pollution report higher risks of low birth weights compared with those centers that have lower levels of pollution,” says Payam Dadvand, an epidemiologist at the center for Research in Environmental Epidemiology in Barcelona, Spain, who is first author on the study.
By the authors’ calculations, each increase in PM10 by 10 micrograms per cubic meter (μg m–3) was associated with a 3% higher chance of an infant being underweight and with an overall average weight reduced by 3 grams. That reduction in average weight tripled to 9 g when the authors adjusted for local variables such as maternal age or tobacco use. The calculations took socioeconomic status into account.
The median PM10 value varied across the 14 sites, from 12.5 μg m–3 in Vancouver to 66.5 μg m–3 in Seoul. For a subset of centers that included information on PM2.5 exposure, the odds of lower birth weight increased by 10% for each increase in exposure.
As Trasande explains, the risks are small at the individual level, but “on a population basis, a shift can produce large increases in the percentage of low-birth-weight infants”, he says. Smoking, alcohol and drug use and poor maternal health are also linked to low birth weights.
In a study of more than 220,000 US births published last month, Trasande and his colleagues found that outdoor air pollution was associated with longer hospital stays and greater health-care costs. In 2010, 8.2% of infants born in the United States were of low birth weight.
The authors of the latest study made efforts to control for socioeconomic and lifestyle factors that might skew the results. However, some of those adjustments had limitations. Not all centers included information about whether the mother smoked during pregnancy; maternal education and address were used as proxy measurements to give an idea of socioeconomic status; and the mothers' exposure to air pollution during pregnancy was estimated rather than measured directly.