title: A meta-analysis of exposure to particulate matter and adverse birth outcomes.
abstract: OBJECTIVE
  The objective of this study was to conduct a systematic review to provide summarized evidence on the association between maternal exposure to particulate air pollution and birth weight (BW) and preterm birth (PTB) after taking into consideration the potential confounding effect of maternal smoking.
  
  
  METHODS
  We systematically searched all published cohort and case-control studies examining BW and PTB association with particulate matter (PM, less than or equal to 2.5μm and 10.0 μm in diameter, PM2.5 and PM10, respectively) from PubMed and Web of Science, from January 1980 to April 2015. We extracted coefficients for continuous BW and odds ratio (OR) for PTB from each individual study, and meta-analysis was used to combine the coefficient and OR of individual studies. The methodological quality of individual study was assessed using a standard protocol proposed by Downs and Black. Forty-four studies met the inclusion criteria.
  
  
  RESULTS
  In random effects meta-analyses, BW as a continuous outcome was negativelyassociated with 10 μg/m(3) increase in PM10 (-10.31 g; 95% confidence interval [CI], -13.57 to -3.13 g; I-squared=0%, p=0.947) and PM2.5 (-22.17 g; 95% CI, -37.93 to -6.41 g; I-squared=92.3%, p <0.001) exposure during entire pregnancy, adjusted for maternal smoking. A significantly increased risk of PTB per 10 μg/m(3) increase in PM10 (OR, 1.23; 95% CI, 1.04 to 1.41; I-squared=0%, p =0.977) and PM2.5 (OR, 1.14; 95% CI, 1.06 to 1.22; I-squared=92.5%, p <0.001) exposure during entire pregnancy was observed. Effect size of change in BW per 10 μg/m(3) increase in PM tended to report stronger associations after adjustment for maternal smoking.
  
  
  CONCLUSIONS
  While this systematic review supports an adverse impact of maternal exposure to particulate air pollution on birth outcomes, variation in effects by exposure period and sources of heterogeneity between studies should be further explored.
authors.affiliation: Department of Social and Preventive Medicine, Inha University School of Medicine, Incheon, Korea.
authors.forename: Dirga Kumar
authors.identifier: 0000-0002-9263-3707
authors.initials: DK
authors.lastname: Lamichhane
authors.affiliation: Department of Occupational and Environmental Medicine, Inha University School of Medicine, Incheon, Korea.
authors.forename: Jong-Han
authors.identifier: 0000-0003-3292-6492
authors.initials: JH
authors.lastname: Leem
authors.affiliation: Department of Social and Preventive Medicine, Inha University School of Medicine, Incheon, Korea.
authors.forename: Ji-Young
authors.identifier: 0000-0002-0067-182X
authors.initials: JY
authors.lastname: Lee
authors.affiliation: Department of Occupational and Environmental Medicine, Inha University School of Medicine, Incheon, Korea.
authors.forename: Hwan-Cheol
authors.identifier: 0000-0002-3635-1297
authors.initials: HC
authors.lastname: Kim
keywords: Birth weight; Heterogeneity; Maternal smoking; Meta-analysis; Particulate matter; Preterm birth