In recent years, a number of studies have found that pregnant women exposed to heavy air pollution have a slightly higher risk of delivering prematurely than those who breathe cleaner air.
Experts said the new study, published March 29 in Environmental Health Perspectives, is the first to put a price tag on the issue.
"Studies like this can put us in a stronger position to advocate for cleaner air," said Dr. Edward McCabe, chief medical officer for the non-profit March of Dimes. "The effects on the U.S. economy are not trivial."
Most Americans are not in a position to simply move away from sources of pollution, such as high-traffic roads and power plants, said McCabe, who was not involved in the study.
"So we need to reduce the sources around them," he explained.
According to McCabe, numerous studies have found that pregnant women exposed to air pollution face increased risks of either preterm delivery or having an underweight newborn.
Those studies cannot prove that air pollution is the culprit, McCabe said: They can only point to an association between dirty air and pregnancy complications.
But, McCabe added, when multiple studies are coming to similar conclusions, that builds the case that "it's more than an association."
In a study published earlier this year, researchers found that women's preterm delivery risk rose when they lived in areas where fine-particle pollution surpassed standards set by the U.S. Environment Protection Agency.
Pregnant women exposed to that level of pollution were 19 percent more likely to deliver prematurely, versus women living in areas that met EPA standards.
That study was conducted in Ohio, which, in the new study, was in the heart of the U.S. region with the most preterm births linked to air pollution.
Many factors influence preterm birth risk, and for any one woman, the impact of air pollution would be "modest," according to Dr. Leonardo Trasande, the lead researcher on the new study.
But across the large U.S. population, that small individual risk translates into a major public health issue, said Trasande, an associate professor at NYU Langone Medical Center, in New York City.
One argument against tighter air pollution regulation is that it creates an economic burden, Trasande said. "But that's a one-sided view," he said.
And it has to be countered with the costs of inaction, Trasande added.
To arrive at their estimates, the study authors pulled together several sources. First, they used past research to calculate the average risk of preterm birth at different levels of air pollution exposure. Then they used federal databases to look at preterm birth rates and air quality in most U.S. counties over one year.
Nationwide, the researchers estimated, about 3.3 percent of all preterm births in 2010 could be attributed to heavy air pollution.
And that, they said, comes with a total yearly cost of over $4.3 billion.
Trasande said the figure includes medical costs -- immediate ones, and the long-term costs of caring for disabilities caused by preterm birth. It also includes the lifelong "lost productivity" connected to those disabilities and to lower IQ. (On average, Trasande noted, preterm babies go on to have a lower IQ, compared with full-term babies.)
Still, the dollar figure does not include all potential costs, such as effects on mothers' health. "So $4.3 billion is probably an underestimate," Trasande said.
As for why air pollution would affect preterm birth risk, Trasande said lab research has given some clues: Breathing in toxins may cause inflammation and immune system stress that can weaken the placenta and possibly trigger earlier labor.
In the new study, many of the states with the highest percentages of pollution-related preterm births were in the Ohio Valley -- including Ohio, Illinois, Indiana and West Virginia. Some other states with relatively high rates included Alabama, California, Georgia, Maryland and North Carolina, and well as Washington, D.C.
For now, Trasande suggested that pregnant women in high-pollution areas limit their exposure by using air filters and staying inside at times when local traffic is heavy, or on days when air quality is poor.
He pointed to the EPA website airnow.gov, which allows people to check their local air quality conditions.More information
The March of Dimes has more on air pollution and pregnancy.
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