Air Pollution Tied to Preterm Birth Risk for Women With Asthma

Troy Brown, RN

March 10, 2016

Air pollution exposure before conception, in early pregnancy, and in late pregnancy may increase the risk for preterm birth in women with asthma, according to a new study from the National Institutes of Health.

Specifically, the researchers found that an increased nitrogen oxide exposure of 30 parts per billion in the 3 months before conception was tied to nearly a 30% increase in the risk for preterm birth among women with asthma vs 8% for women without asthma. Similarly, increased carbon monoxide exposure before conception was tied to a 12% increased risk of preterm birth among women with asthma, but showed no effect on the risk among women without asthma.

"Early environmental exposures can have significant effects on later health," Pauline Mendola, PhD, lead author and an investigator at the Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland, said in a National Institutes of Health news release. "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 findings online March 1 in the Journal of Allergy and Clinical Immunology.

Approximately 9% of reproductive-age women in the United States suffer from asthma, a lung disease associated with an increased risk for pregnancy complications and health problems for infants.

"Preterm birth is a major public health problem in this country, affecting more than 1 in 10 infants born in the United States," Dr Mendola explained in the news release. "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."

The researchers collected data on gestational age; maternal demographics; medical, reproductive, and prenatal history; and a summary of labor and delivery information from the Consortium on Safe Labor (2002 - 2008). The Consortium on Safe Labor is a retrospective cohort of births at 23 weeks' gestation or more with data gathered from electronic records from 19 hospitals across the United States.

The cohort included 233,736 newborns, 87% of whom were born between 2005 and 2007. After excluding multifetal pregnancies and those missing air quality data, the analytic sample included 223,502 newborns among 204,175 women. Most of the women in the study contributed only one pregnancy to the cohort.

The researchers matched those data with measures of six air pollutants (carbon monoxide, nitrogen oxides, ozone, particulate matter with an aerodynamic diameter of less than 10 microns, particulate matter with an aerodynamic diameter of less than 2.5 microns, and sulfur dioxide from the areas around each of the hospitals) and assessed the potential effects of air pollution on the risk for preterm birth at each week of gestational age. They adjusted for location, age, race and ethnicity, prepregnancy weight, smoking and alcohol use, and chronic maternal health conditions.

Some 11.7% of singleton births were preterm, and one third (33.6%) of those were early preterm or at less than 34 weeks' gestational age. Preterm birth was more common among black women, those at the extremes of maternal age, single women, those who had public insurance, those who smoked or drank alcohol while pregnant, and women with chronic comorbid conditions before pregnancy. Women with asthma had a higher risk of delivering preterm, both early and overall.

When the researchers analyzed the data for each week of gestation, they found a significantly increased risk for women with asthma when exposed to the pollutants throughout their pregnancies, but certain critical windows were especially risky.

"Women with asthma have higher risks for preterm birth after acute exposure to most criteria air pollutants both early (weeks 23, 26, and 29) and later in gestation (weeks 34-36)," the authors explain. "Preconception and the first weeks of pregnancy appear to be important windows of susceptibility for preterm birth risk with respect to [carbon monoxide, nitrogen oxides, particulate matter with an aerodynamic diameter of less than 10 microns, and particulate matter with an aerodynamic diameter of less than 2.5 microns] exposure. Risks associated with [ozone] appear higher later in gestation and are increased 16% for early preterm and 8% for total preterm in relation to whole pregnancy exposure among women without asthma."

The researchers found an increased risk associated with continued and short-term exposure to nitrogen oxides and carbon monoxide, especially when the women were exposed to them just before conception and during early pregnancy.

"Small increases in preterm birth risk associated with air pollution merit attention given that asthma is common in obstetric population and these exposures are difficult to manage at the individual level," the researchers conclude.

This research was supported by the Intramural Research Program of the National Institutes of Health, Eunice Kennedy Shriver National Institute of Child Health and Human Development. One coauthor has received grants from Laboratoriolääketieteen edistämissäätiö and Paulo Foundation.

J Allergy Clin Immunol. Published online March 1, 2016. Abstract

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