Based on data from 183 countries, a research team from the Stockholm Environment Institute at the University of York extrapolated the impact of maternal exposure to different levels of outdoor pollution on preterm birthrates. The researchers concluded that PM2.5 was a “potentially substantial global risk factor” for a baby being born earlier than 37 weeks of gestation — a point in pregnancy that increases the risk of infant mortality and physical and neurological problems.

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The team calculated that in 2010, exposure to PM2.5 was strongly associated with 18 percent of preterm births globally, or about 2.7 million premature births. The majority of those births were in South and East Asia, the Middle East and North Africa and West sub-Saharan Africa. (Poverty, inadequate prenatal care and a mother’s age are among the other factors that also can raise the likelihood of preterm births.)

“This study highlights that air pollution may contribute to a health effect, which contributes substantially to infant mortality as well as lifelong health effects in survivors,” institute researcher Christopher Malley, the study's lead author, said via email.

It also underscores that comprehensive action is required to reduce levels of particulate matter, said the institute's policy director, Johan Kuylenstierna, another study author. Although natural sources of PM2.5 are implicated in preterm births, the study found human-produced sources play a much bigger role.

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In South and East Asia, for example, such sources were responsible for more than 80 percent of that pollution, and those two regions had the largest proportion of PM2.5-associated preterm births, according to the study. Pregnant women there are frequently exposed to indoor air pollution because of the wood and other biomass fuels they use in cooking.

Fine particulate matter from natural sources, such as dust storms, contributed more to preterm birthrates in the Middle East, North Africa and West sub-Saharan Africa.

While maternal exposure is more acute in developing countries, PM2.5 in developed countries also affects preterm birthrates, the study noted. The United States had around 50,000 PM2.5-associated preterm births in 2010; each cost about $51,600 in medical care, special education services for the infant as well as lost household and work productivity associated with the child's neurological or physical disabilities, according to a report referenced in the study.

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The study builds on previous research showing the significant harm that air pollution causes among all ages. The World Health Organization has estimated that there were 7 million premature deaths globally in 2012 because of exposure to air pollution. The WHO also said the data showed “a stronger link” between exposure, both outdoors and indoors, and heart disease, stroke, cancer and respiratory diseases.

“Estimates of impact from this analysis indicate the importance of also considering pregnant women as a vulnerable population and further support the need to rapidly address air pollution as a global health threat,” said Michael Brauer, a professor in the School of Population and Public Health at the University of British Columbia, who was not part of the study.