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Re: "Preterm birth: the interaction of traffic-related air pollution with economic hardship in Los Angeles neighborhoods".

We read with great interest the article by Ponce et al. (1), which described well the interaction effects of traffic-related air pollution and economic hardship on preterm birth. However, Ponce et al. discarded some birth weight data (<500 g and >5,000 g) as extreme values. Moreover, they excluded patients who had undergone preterm cesarean delivery from the subjects. Obstetric studies on preterm delivery should not discard lower extreme values, because extremely (or very) low birth weight is closely related to very preterm birth (2). The rate of cesarean section in preterm delivery is larger than that of term delivery (3). The study by Ponce et al. would have been better researched and more useful if the lower extreme values of birth weight data had been included in their study.