The ill effects of air pollution on respiratory health are well known. Now a study from the National Institutes of Health NIH says that when pregnant women with asthma are exposed to high levels of certain traffic related air pollutants they face a greater risk of preterm birth. The increased risk is associated with both ongoing and short term exposure to nitrogen oxides and carbon monoxide particularly when women were exposed to those pollutants just before conception and in early pregnancy. 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 percent for women with asthma compared to 8 percent for women without asthma. Greater carbon monoxide exposure during the same period raised preterm birth risk by 12 percent for asthmatic women but had no effect on preterm birth risk for non asthmatics. The last six weeks of pregnancy was another critical window for women with asthma. Exposure to high levels of particulate matter very small particles of substances like acids metals and dust in the air also was associated with higher preterm birth risk. The study was published online in the Journal of Allergy and Clinical Immunology. People with asthma who are concerned about exposures to air pollution may want to limit their outdoor activity during periods when the air quality is forecast to be unhealthy for sensitive groups.