If you’re pregnant, dirty air might do more than trigger an asthma attack.
New research from the University of Cincinnati suggests that high levels of particle pollution raise the risk of preterm birth. The main source of particle pollution, also called particulate air pollution, is the burning of fuels, such as gasoline, diesel and wood.
After the scientists accounted for other factors that raise the risk of preterm birth, such as older age, smoking and no prenatal care, they concluded that the increased risk of preterm birth related to exposure to high levels of particle pollution was a modest 19%. But because all pregnant women have to breathe the air, the potential impact on preterm birth rates could be significant, the study authors say.
About 10% of U.S. babies are born preterm, defined as before the 37th week of pregnancy. Preterm birth is the main contributing factor to infant deaths, which mainly occur in babies born very preterm, or before the 32nd week of pregnancy, according to the U.S. Centers for Disease Control and Prevention. Preterm birth is also a leading cause of long-term neurological disabilities, the CDC says.
This is how the Cincinnati scientists connected women’s exposure to air pollution and their risk of preterm birth: They linked records of about 225,000 live Ohio births from 2007-2010 with average daily levels of particle pollution measured at the air quality monitoring station closest to the mothers’ address. The Environmental Protection Agency (EPA) operates 57 air quality monitoring stations around the state.
About 11% of the babies’ mothers had been exposed while pregnant to levels of particle pollution above what EPA at the time said was safe, the scientists write in the journal Environmental Health. And 8.5% of the births in the study were preterm. The scientists estimated that roughly one in six preterm births among women exposed to high air pollution levels were due to poor air quality.
How could breathing dirty air cause a woman to deliver her baby too early? The scientists had a few theories: After she inhales toxic substances, they could enter her bloodstream and be carried through her body. The substances could trigger inflammation in the lungs or elsewhere in the body. Lastly, the substances could disrupt the autonomic nervous system, which controls such functions as labor.
Previous studies into the connection between air pollution and preterm births had mixed results, the authors write. One reason might be because they didn’t take into account other medical or socioeconomic factors that could increase women’s risk of preterm birth.
“The connections between maternal toxin exposures and adverse birth outcomes is an emerging field of study,” the authors of the new study note.
No study is perfect, and the authors of this one pointed out some of its potential problems, which, they said, are common in studies looking at the relationship between air pollution and pregnancy:
- They didn’t know the air concentrations of specific pollutants, only their totals, so they can’t say whether any particular pollutants are of more concern than others.
- They know only the level of particle pollution measured by the EPA station nearest each woman’s address—not her personal level of exposure.
- They don’t know when exposure to air pollution has the biggest effect in pregnancy, although the third trimester appears to be the worst time.
“Likely the best way to truly determine the connection between air pollution and preterm birth would be to have subjects (pregnant women) carry continuous pollutant monitoring systems,” although that would be tough for a study as large as theirs, the scientists write.
While their study didn’t precisely define a safe threshold for exposure to particle pollution, the scientists wrote, the findings do support EPA’s 2012 decision to lower it.