ORIGINAL ARTICLE Residential Proximity to Major Roads and Preterm Births Takashi Yorifuji, a,b Hiroo Naruse, c Saori Kashima, a,d Shigeru Ohki, e Takeshi Murakoshi, c Soshi Takao, a Toshihide Tsuda, f and Hiroyuki Doi a Background: Preterm births cause a large public-health burden, and air pollution is considered to be a potential risk factor. We evaluated the association between proximity to major roads (as an index for air pollution) and preterm births, classified by gestational age and specific clinical manifestations. Methods: Data on parental information and birth outcomes were extracted from the database maintained by the perinatal hospital in Shizuoka, Japan. We restricted the analysis to mothers who deliv- ered liveborn single births from 1997 to 2008 (n = 14,226). Using the geocoded residential information, each birth was classified on its proximity to major roads. We estimated the multivariate-adjusted odds ratios and their 95% confidence intervals (CIs) for the associ- ation of proximity to major roads with preterm births, using logistic regression. Results: We found positive associations between proximity to major roads and preterm births at all gestational ages. Living within 200 m increased the risk of births before 37 weeks by 1.5 times (95% CI = 1.2–1.8), birth before 32 weeks by 1.6 times (1.1–2.4), and births before 28 weeks by 1.8 times (1.0 –3.2). Proximity specifically increased the risk of preterm births with preterm premature rupture of the membranes and with pregnancy hypertension. Conclusions: This study demonstrates that exposure to traffic- related air pollution increases even the risk of preterm births of less than 30 weeks’ gestational age and proposes a possible mechanism. (Epidemiology 2011;22: 74 – 80) P reterm births, defined as births before 37 gestational weeks, are associated with perinatal mortality and adverse consequences in later adulthood. 1,2 In the United States, the proportion of preterm births among all live births was about 12.7% in 2007, 3 whereas in Japan the proportion was 5.8% in the same year. 4 The proportion of preterm births is increasing in developed countries (eg, from 4.1% in 1980 to 5.8% in 2007 in Japan), 4 despite increased knowledge of the risk factors. Potential risk factors associated with preterm births have been extensively studied. 5 Air pollution is potentially an important risk factor, 6–8 because it can affect many people. Furthermore, some components of air pollutants that affect health are similar to those that result from maternal smoking. 9,10 Reviews of air pollution and preterm births published in 2004 and 2005 9 –12 concluded that the evidence was insuffi- cient to confirm this relationship. Subsequently, several stud- ies have been published. Seven studies with case-control or cohort designs, which used local air-pollution measurement difference as exposure, demonstrated a positive association between air pollution and preterm births. 13–19 Only 2 of these 7 studies reported effects of air pollution on risk of preterm birth before 30 weeks 13,19 ; the other studies did not differen- tiate the gestational age of preterm births. Preterm births can be classified by gestational age: less than 28 weeks’ gestation (extreme prematurity), 28 –31 weeks (severe prematurity), 32–33 weeks (moderate prema- turity), and 34 –36 weeks (near term). 5 The outcome of preterm births is different between extreme prematurity and near-term preterm births. 1 Therefore, evaluating whether air pollution increases preterm births at different gestational ages could be important to consider with regard to the medical outcome and public health burden of preterm births. Preterm births are also known as “preterm parturition syndromes,” reflecting the fact that preterm births can have multiple potential causes and can be induced by multiple mechanisms. 20,21 Some authors have discussed the necessity of separating preterm births to elucidate potential causes and mechanisms. 22,23 Thus, evaluating whether air pollution in- creases certain types of preterm births with specific potential causes could help to elucidate the potential mechanisms associated with this risk factor. 24 We evaluated the associations between proximity to major roads (as an index for air pollution) and preterm births classified by gestational age. We also examined the specific clinical types of preterm births (such as preterm premature Submitted 6 April 2010; accepted 16 July 2010; posted 3 November 2010. From the a Department of Epidemiology, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama, Japan; b Department of Environmental Health, Harvard School of Public Health, Boston, MA; c Department of Obstetrics, Seirei Hamamatsu General Hospital, Hamamatsu, Shizuoka, Japan; d Department of Public Health and Health Policy, Hiroshima University Graduate School of Biomedical Sciences, Hiroshima, Japan; e Department of Neonatology, Seirei Hamamatsu General Hospital, Hamamatsu, Shizuoka, Japan; and f Department Environmental Epidemiology, Okayama University Gradu- ate School of Environmental Science, Okayama, Japan. Supported by a grant for young researchers from Okayama University. Correspondence: Takashi Yorifuji, Department of Epidemiology, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, 2-5-1 Shikata-cho, Kita-ku, Okayama 700-8558, Japan. E-mail: yorichan@md.okayama-u.ac.jp. Copyright © 2010 by Lippincott Williams & Wilkins ISSN: 1044-3983/11/2201-0074 DOI: 10.1097/EDE.0b013e3181fe759f Epidemiology • Volume 22, Number 1, January 2011 74 | www.epidem.com