A study published Tuesday found that exposure to some air traffic pollutants could cause preterm births in women with asthma. This problem is most prevalent in women exposed to these pollutants just before conceiving, in early pregnancy and the last six weeks of pregnancy. Increased exposure within three months before birth raised preterm birth risk by 12 percent for women with asthma. According to study author Pauline Mendola, preterm birth is a problem that effects more that 1 in 10 babies in the U.S. The study looked at more than 223,000 single-child births between 2002 and 2008 and measured six types of pollutants, including nitrogen oxide and carbon monoxide exposure.