The relationship between air pollutants and maternal socioeconomic factors on preterm birth in California urban counties

Abstract

Background: Preterm birth is the leading cause of perinatal morbidity and mortality in the U.S. and disparities among racial and ethnic groups persist. While etiologies of preterm birth have not been fully elucidated, it is probable that environmental and social factors play a role.

Objective: We hypothesized that there is an interactive association between exposure to fine particulate matter (PM_2.5) or ozone (O₃) and neighborhood socioeconomic factors that increase the risk of preterm birth.

Methods: We conducted a retrospective study using geocoded birth certificate data between 2007 and 2011, daily ambient air quality data on PM_2.5 and O₃, and American Community Survey (2007-2011 5-year estimates) data to assess census tract-level socioeconomic factors in California urban counties.

Results: Our study found a small positive association between maternal exposures to PM_2.5 and O₃ and preterm birth that varied by gestational exposure period. In mixed-effects models, we found an increase in the risk of preterm birth for a one-unit change in PM_2.5 averaged across the entire pregnancy (AOR = 1.02, 95% CI: 1.01, 1.02) and O₃ during 3-months pre-pregnancy (AOR = 1.03, 95% CI: 1.02, 1.04). Interaction between census tract-level factors and air pollutants showed an increase in the risk of preterm birth among mothers living in higher socioeconomic areas, though, a fixed cohort bias sensitivity analysis showed these associations were not significant.

Significance: These findings substantiate previous studies that showed associations between air pollution and preterm birth, even as pollution levels have decreased. This study has important implications for policy decisions and may help inform research on potential mechanisms of preterm birth.

Keywords: Air pollution; Ozone; PM2.5; Preterm birth; Socioeconomic status.

Fig. 1. Associations between air pollutants, PM_2.5 and O₃, and preterm birth by maternal exposure period.

Adjusted odds ratio (aOR) included a season of conception, cigarette use, age, race/ethnicity, payment, prenatal visits that began in the first trimester. A For preterm birth (<37 weeks), the aOR for all exposure periods was greater than one. For O₃ exposure, the aOR was highest in the 3-month pre-pregnancy period for every 1-unit (ppb) increase in O₃ (aOR = 1.10, 95% CI: 1.08, 1.12), while for PM_2.5, the highest aOR of preterm birth was associated during the whole pregnancy exposure period (aOR = 1.10, 95% CI: 1.09, 1.12). B For early preterm birth (<32 weeks), an increased risk was associated with O₃ during the whole pregnancy period (aOR = 1.07, 95% CI: 1.03, 1.11) and PM_2.5 during the third trimester (aOR = 1.09, 95% CI: 1.04, 1.14).