Elsevier

Annals of Epidemiology

Volume 24, Issue 12, December 2014, Pages 888-895.e4
Annals of Epidemiology

Original article
Traffic-related air pollution and risk of preterm birth in the San Joaquin Valley of California

https://doi.org/10.1016/j.annepidem.2014.10.004Get rights and content

Abstract

Purpose

To evaluate associations between traffic-related air pollution during pregnancy and preterm birth in births in four counties in California during years 2000 to 2006.

Methods

We used logistic regression to examine the association between the highest quartile of ambient air pollutants (carbon monoxide, nitrogen dioxide, particulate matter <10 and 2.5 μm) and traffic density during pregnancy and each of five levels of prematurity based on gestational age at birth (20–23, 24–27, 28–31, 32–33, and 34–36 weeks) versus term (37–42 weeks). We examined trimester averages and the last month and the last 6 weeks of pregnancy. Models were adjusted for birthweight, maternal age, race/ethnicity, education, prenatal care, and birth costs payment. Neighborhood socioeconomic status (SES) was evaluated as a potential effect modifier.

Results

There were increased odds ratios (ORs) for early preterm birth for those exposed to the highest quartile of each pollutant during the second trimester and the end of pregnancy (adjusted OR, 1.4–2.8). Associations were stronger among mothers living in low SES neighborhoods (adjusted OR, 2.1–4.3). We observed exposure–response associations for multiple pollutant exposures and early preterm birth. Inverse associations during the first trimester were observed.

Conclusions

The results confirm associations between traffic-related air pollution and prematurity, particularly among very early preterm births and low SES neighborhoods.

Introduction

Preterm birth is associated with perinatal mortality and adverse health consequences in childhood and adulthood. In the United States, 12% of all live births were preterm in 2010 [1]. Being born preterm is costly in terms of suffering of infants and their families as well as the economic burden on society. Preterm birth is a complex phenomenon and can be considered as a continuum rather than a dichotomy of birth of less than 37 completed weeks gestation (vs. ≥37 weeks) [2]. It has been argued that this classification is too simplistic for etiologic studies owing to the heterogeneity that has been observed with this outcome [3]. Indeed, more detailed phenotypic classifications have even been suggested for extremely early (<28 weeks gestation) preterm birth [4], [5].

Several studies have examined the potential association between traffic-related air pollution and preterm birth; however, many are heterogeneous with regard to exposure and outcome assessment, geography covered, and statistical methods used [6], [7]. The majority of previous studies have examined preterm birth as a binary outcome rather than a continuous or more granular ordinal set of outcomes with a few notable exceptions [8], [9], [10]. The present study examines exposures to several air pollutants (carbon monoxide, nitrogen dioxide, particulate matter <10 and 2.5 μm) and traffic during pregnancy and their associations with finer gestational designations of preterm birth in the San Joaquin Valley of California between 2000 and 2006. Additionally, we investigate neighborhood socioeconomic status (SES) and other factors as potential effect modifiers in the relationship between air pollution and preterm birth in response to earlier investigations of such interaction [11], [12], [13], [14]. Finally, we apply a multipollutant score analysis to determine the association with cumulative effects of multiple air pollutants.

Section snippets

Study population

The Study of Air Pollution, Genetics, and Early Life Events was designed to investigate the influence of exposure to traffic-related air pollution during pregnancy and birth outcomes. Birth certificates from all 2000 to 2006 births to women living in the four most populated counties in the San Joaquin Valley of California (Fresno, Kern, Stanislaus, and San Joaquin) were obtained from the California Department of Health.

Analyses were limited to singleton births between 20- and 42-week gestation

Results

The four study counties included 329,650 births in 2000 to 2006. Exclusions were multiple births (n = 8373), those missing file numbers (n = 262), those with gestational age missing or less than 20 weeks or more than 42 weeks (n = 44,699), and those with birthweight missing or less than 500 g or more than 5000 g (n = 764). Completeness of pollutant assignments was 80% for CO, 94% for NO2, 93% for PM10, 93% for PM2.5, and 96% for traffic density. The final study population included 263,204

Discussion

We observed associations between ambient air pollutants and risk of early and late occurring preterm birth. Exposure to particulate matter (PM10 and PM2.5), especially proximal to parturition, was associated with all gestational definitions of preterm birth with the strongest associations for the earliest preterm births. For PM10 and PM2.5, there was a monotonic response across the outcomes according to gestational timing, with the stronger associations for the earliest preterm births. These

Acknowledgments

Support for this study came from NIEHS (R21 ESO14891, P20 ES018173, P01ES022849, K99ES021470), CDC 6U01DD000489, and the March of Dimes Prematurity Research Center (50185) at Stanford University. This publication was made possible by United States Environmental Protection Agency (US EPA) STAR Grant RD83459601 and RD83543501. Its contents are solely the responsibility of the grantee and do not necessarily represent the official views of the US EPA. Furthermore, the US EPA does not endorse the

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    The authors have no conflicts of interest to disclose.

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