Each year, the U.S. spends $26.2 billion on costs associated with preterm birth — that’s birth before 37 weeks of pregnancy. Beyond the costs, babies born too early experience immediate and long-term problems, from developmental disabilities to asthma to hearing loss. For years, scientists have been studying possible environmental contributors, with many finding an association between preterm birth and air pollution. Earlier this week, a new study brought even more depth and clarity to this connection.

Published in the journal Environmental Health, the study found that exposure to high levels of fine particulate matter — particles that measure 2.5 microns or less and are commonly known as PM_2.5 — were associated with a 19 percent increased risk of preterm birth. High levels of exposure during a woman’s third trimester of pregnancy were associated with the greatest risk of preterm birth. Overall, preterm birth rates were higher among women exposed to air particle pollution levels that exceeded federal air quality standards, women 40 years old and older, black women, women with no prenatal care, and women with low educational attainment.

Study co-author Emily DeFranco, a physician-researcher at the Center for Prevention of Preterm Birth at Cincinnati Children’s and an associate professor of maternal-fetal medicine at the University of Cincinnati, described preterm birth as a complex condition with multiple contributing factors, including genetic predisposition, birth spacing and whether a woman smokes. She said the link between air pollution and preterm birth has “biological plausibility” and “makes medical sense” based on what scientists know about the health impacts of air quality as well as the growing scientific literature on pollution and birth outcomes.

“Preterm birth is a complex disease and if you have the right combination of risk factors in the same person, you can develop a perfect storm of conditions for a woman to deliver too early,” DeFranco told me.

To conduct the study, DeFranco and her research colleagues examined Ohio birth records for 2007 through 2010, which consisted of nearly 225,000 single live births, of which more than 19,000 were preterm. Those birth records were then linked with daily average measures of fine particulate matter as recorded by 57 air monitoring stations across the state. Most of the births analyzed — 97 percent — occurred in urban areas, where pollution exposure is typically highest. Fine particulate matter comes from a variety of combustion sources, such as motor vehicles and power plants. In addition, these types of air particles are highly stable, can travel over long distances and can travel deep into a person’s lungs.

Researchers found that the frequency of high exposure to fine particulate matter was greater among preterm births than among full-term births. Increases in the preterm birth rate coincided with high pollution exposures during the first and third trimesters as well as in cases with high exposure levels during the entire pregnancy. Researchers estimated that decreasing fine particulate matter levels below 15 micrograms per cubic meter of air — the U.S. Environmental Protection Agency’s air quality standard during the 2007-2010 study period — could decrease preterm birth by more than 17 percent among women with high exposure rates and by more than 2 percent among the overall population. (Since that study period, EPA has lowered its fine particulate matter standard to 12 micrograms.)

DeFranco and study co-authors William Moravec, Fan Xu, Eric Hall, Monir Hossain, Erin Haynes, Louis Muglia and Aimin Chen write: “While this study does not precisely define a safe threshold for exposure, it does support the EPA’s decision to decrease the standard exposure limit of PM_2.5 concentration for individuals in the U.S. Additional research is needed to determine individual-level exposure as measured by ambient air and internal biomarkers of exposure and effect.”

DeFranco told me that scientists aren’t entirely sure how fine particulate matter interacts with the human body to increase the risk for preterm birth. However, she said many hypothesize that because preterm birth is fueled by abnormal inflammation and because we know that air pollution is linked to inflammatory processes in the human body, inflammation could be the mechanism by which pollution adversely impacts gestational length.

DeFranco noted that as with many environmental health hazards, there’s not much one person can practically do to prevent or reduce exposure to air pollution. But from a public health perspective, this study can help underscore the need for health-protective air quality standards.

“We’re not suggesting that a woman has any control over this,” she said. “But emerging data like this can spur the desire for us — as a community — to continue to make improvements.”

To download a full copy of the study, visit Environmental Health. To learn more about fine particulate matter pollution in your community, visit EPA.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for nearly 15 years.