Preterm birth is a problem that, in the United States, affects more than 10% of newborns. According to a new study from the National Institutes of Health, women with asthma may be at a greater than normal risk of preterm birth if they are exposed to higher than average levels of some air pollutants.

The study looked at the records of 223,502 women who gave birth to a single child between 2002 and 2008. They matched those records to daily air quality measurements at various points during those pregnancies, as well as other factors. What they found was that women with asthma who were exposed to air pollution shortly before conception or in the last six weeks of pregnancy were more likely to have preterm deliveries.

air pollution

Exposure to pollutants such as nitrogen oxide in the three months prior to conception, increased the chances of preterm birth in women with asthma by 30%, but only 8% in other women. Carbon monoxide increased the chances by 12% for women with asthma, but had no affect on other women.

While other studies have looked at how air pollutants impact preterm birth, this is the first study to investigate impacts due to contact before conception. They aren’t sure why this happens yet, though the researchers have suggested that air pollution might cause inflammation or other stresses that impact embryo implantation or placental development. They are calling for more research along these lines to develop a greater understanding of the problem and the mechanics behind it, and in order to help women with asthma.

In the mean time, the researchers suggest that anyone with asthma, who is concerned about the potential effects of air pollution on them, take some simple precautions. They suggest checking daily air quality reports, available from the EPA at and avoid extended outdoor activity during times when that forecast indicates higher levels of pollutants.