Air Pollution Increases Risk of Preterm Birth in Mothers with Asthma

Mothers with asthma may experience a higher risk for preterm birth after exposure to traffic-related pollutants, such as carbon monoxide and nitrous oxides, particularly with exposure just before pregnancy and in the early months of pregnancy, according to a new study.

The researchers found that an increase of 30 parts per billion in nitrogen oxide exposure in the three months prior to pregnancy increased preterm birth risk by nearly 30% for women with asthma, compared to 8% for women without asthma. Greater carbon monoxide exposure during the same period raised preterm birth risk by 12% for women with asthma, but had no effect on preterm birth risk for those who did not have asthma.

“Early environmental exposures can have significant effects on later health,” said lead author Pauline Mendola, PhD, an investigator at the National Institute of Child Health and Human Development. “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. More research will help us to better understand the potential impact of air pollution in the months surrounding conception.”

The researchers published their results online March 1, 2016 in the Journal of Allergy and Clinical Immunology.

Mendola and colleagues analyzed data from a national sample of 223,502 singleton pregnancies delivered at 19 hospitals around the country from 2002 to 2008. Electronic medical records of the women included their asthma status and date of delivery.

They estimated the odds ratio for preterm birth on the basis of the interaction of maternal asthma and particulate matter, ozone, nitrogen oxides, sulfur dioxide, and carbon monoxide, and accounted for such factors as location, age, race and ethnicity, prepregnancy weight, smoking and alcohol use, and chronic maternal health conditions.

For each gestational week 23 to 36, exposures among women who delivered were compared with those remaining pregnant. They also evaluated three-month preconception, whole pregnancy, weeks one to 28, and the last six weeks of gestation averages.

“We found that significant asthma interactions were sporadic before 30 weeks but more common during weeks 34 to 36,” the researchers stated. Higher risk among mothers with asthma was associated with exposure to nitrous oxides, carbon monoxide, and sulfur dioxide. There was also an inverse association with ozone in week 34.

The odds of preterm birth were significantly higher among women with asthma for carbon monoxide and nitrous oxides exposure prior to conception and early in pregnancy.

The last six weeks of pregnancy was another critical window for women with asthma, according to the researchers. Exposure to high levels of particulate matter also was associated with higher preterm birth risk.

“Preterm birth is a major public health problem in this country, affecting more than 1 in 10 infants born in the United States,” said Mendola. “Our study found that air pollution appears to add to the preterm birth risk faced by women with asthma. These findings set the stage for further studies designed to help prevent preterm birth in this at-risk group.”

Source: Mendola P, Wallace J, Seuk B. Preterm birth and air pollution: Critical windows of exposure for women with asthma. J Allergy Clin Immnol. Article in press. DOI: http://dx.doi.org/10.1016/j.jaci.2015.12.1309