Air pollution puts mothers with asthma at higher risk for preterm birth

Pregnant women with asthma have a higher risk for preterm birth when exposed to traffic-related air pollutants, particularly when exposure to pollution occurs in the weeks before conception and in the early weeks of pregnancy, according to a study published online March 1, 2016 in the Journal of Allergy and Clinical Immunology.

“Preterm birth is a major public health problem in this country, affecting more than 1 in 10 infants born in the United States,” said the study’s lead author Pauline Mendola, PhD, an investigator at the Eunice Kennedy Shriver National Institute of Child Health and Human Development, in Bethesda, MD.

“Our study found that air pollution appears to add to the preterm birth risk faced by women with asthma,” Dr. Mendola added. “These findings set the stage for further studies designed to help prevent preterm birth in this at-risk group.”

For this study, the researchers analyzed electronic medical records from 223,502 singleton pregnancies delivered at 19 US hospitals from 2002 to 2008. The researchers matched that data with daily air quality measures from the regions surrounding each of the hospitals to assess the potential effects of air pollution, week by week, on preterm birth risk.

They included 6 pollutants in their study: ozone (O₃), nitrogen oxides (NO_x), sulfur dioxide (SO₂), carbon monoxide (CO), and two types of particulate matter (those less than 10 microns in diameter and those less than 2.5 microns in diameter).

After the researchers accounted for factors such as location, age, race, ethnicity, pre-pregnancy weight, smoking, alcohol use, and chronic maternal health conditions, the results showed an increased risk for preterm birth associated with both ongoing and short-term exposure to NO_x and CO, particularly when exposure to those pollutants occurred shortly before conception and in early pregnancy.

For example, an increase of 30 parts per billion in NO_x exposure in the 3 months before conception increased preterm birth risk by nearly 30% for women with asthma, compared with 8% for women without asthma. Similarly, greater CO exposure during the same period raised preterm birth risk by 12% for women with asthma, but had no effect on preterm birth risk for women without asthma.

The last 6 weeks of pregnancy was another critical period for women with asthma, the researchers found. Higher preterm birth risk occurred in women with asthma who were exposed to particulate matter less than 10 microns in aerodynamic diameter.

Prior studies have examined the effects of various air pollutants on preterm birth risk, but this is the first study to find that air pollution exposure very early in pregnancy is associated with preterm birth risks later in pregnancy.

“Early environmental exposures can have significant effects on later health,” Dr. Mendola said. “In this case, it may be that early exposure to air pollution sets off inflammation or other internal stresses that interfere with embryo implantation or placental development. Those disruptions could lead to preterm delivery down the road.”

She added, “More research will help us to better understand the potential impact of air pollution in the months surrounding conception.”