Elsevier

Environmental Research

Volume 110, Issue 8, November 2010, Pages 778-785
Environmental Research

Preterm birth and exposure to air pollutants during pregnancy

https://doi.org/10.1016/j.envres.2010.09.009Get rights and content

Abstract

Background

Research has shown that prenatal exposure to air pollutants may have a detrimental effect on fetal development, with the strength of the relationship depending on the effect being studied. The evidence to date, however, is insufficient to establish a direct causal link between such exposure and preterm delivery. This study evaluates the specific effect of prenatal exposure to NO2 and benzene on preterm births.

Methods

The population under study comprised 785 pregnant women who formed part of the INMA cohort in Valencia, Spain (2003–2005). Multiple regression models were used for mapping outdoor nitrogen dioxide (NO2) and benzene levels throughout the area. Individual exposure was assigned as the estimated outdoor levels at each woman’s home measured during each trimester as well as throughout the entire pregnancy. Odds ratios (OR) and 95% confidence intervals (95% CI) were calculated in order to assess the association between preterm birth and exposure to NO2 and benzene. The shape of the exposure–response curve between air pollution and the risk of preterm birth was analyzed with a flexible approach, introducing a natural cubic spline for air pollution levels into the model.

Results

Pregnant women exposed to NO2 and benzene have an increased risk of preterm birth. This risk was shown to be significant when women were exposed to NO2 levels >46.2 μg/m3 during the second and third trimesters as well as throughout the entire pregnancy and to benzene levels >2.7 μg/m3 throughout the entire pregnancy.

Conclusion

These results suggest that maternal exposure to traffic-related air pollution is associated with preterm birth.

Introduction

In the past decade, the study of alterations in fetal health with relation to exposure to environmental toxins has attracted increasing interest. Fetuses are especially vulnerable to such exposure due to their high rate of cell proliferation and differentiation, their greater absorption and retention of xenobiotics, and their lower DNA reparation efficiency (Bocskay et al., 2005). Research on the effects of this exposure at birth has thus become a field of great importance for environmental epidemiology since these effects are good indicators of health in newborns and children (Perera et al., 2002, Landrigan et al., 2004).

Various literature reviews of studies on the relation between prenatal exposure to air pollutants and fetal development have been published in the past few years (Glinianaia et al., 2004, Maisonet et al., 2004, Lacasaña et al., 2005, Sram et al., 2005, Wang and Pinkerton, 2007, Ritz and Wilhelm, 2008, Slama et al., 2008). The effects most frequently reported in these studies are low birth weight, preterm birth, delayed intrauterine growth, congenital defects, stillbirths, and infant mortality.

One general conclusion of such studies is that although there is evidence that prenatal exposure to air pollutants has a detrimental effect on fetal development, the strength of this relationship depends on the effect being studied (Sram et al., 2005). Specifically, in studies that have examined the relationship between exposure to pollutants and preterm birth, the evidence is insufficient to suggest causality. Further research on this subject is thus necessary (Hackley et al., 2007).

A crucial aspect of the study of prenatal exposure to air pollutants is the identification of temporal windows of vulnerability to the detrimental effects of this exposure during pregnancy (Woodruff et al., 2009). The results of the studies published to date are inconsistent (Hackley et al., 2007), although the evidence seems to indicate that the first and third trimesters are the most vulnerable periods for low birth weight and preterm births. For birth defects, in contrast, the specific developmental periods for each organ must be taken into account (Ritz and Wilhelm, 2008, Slama et al., 2008).

Geographic information systems (GIS), which are capable of providing, managing, and displaying spatial data, have been shown to be a powerful tool for evaluating exposure to air pollution. When GIS data is available, Land Use Regression (LUR) is an extremely useful analytical technique that uses regression models to construct air pollution maps with geographic variables such as land use, traffic intensity, and topography as predictors for individual exposure to air pollution. LUR has thus become an increasingly useful analytical tool in epidemiological studies on the reproductive effects arising from prenatal exposure to air pollution (Slama et al., 2007, Brauer et al., 2008, Aguilera et al., 2008).

NO2 is considered to be a good proxy of other outdoor air pollutants that originate from the same sources (i.e. traffic) and which have a proven detrimental effect (e.g. particulate matter or PM) (Brunekreef, 2007). Benzene, in addition to being an indicator for traffic pollutants, is also a marker for the presence of other air pollution sources, such as industry or gasoline stations (World Health Organization (WHO) 2000).

The INMA (Infancia y Medio Ambiente or Childhood and the Environment) Project is a multicenter cohort study with the aim of analyzing the effects of environmental exposures and diet during pregnancy on fetal and child development (Ribas-Fitó et al., 2006). The present research has been carried out within the framework of the INMA project with the goal of evaluating the specific effect of prenatal exposure to NO2 and benzene on preterm births.

Section snippets

Study population

The sample population in this study consisted of pregnant women who agreed to participate in the INMA cohort in Valencia, Spain, between February 2004 and June 2005. In their first trimester, women attending the prenatal population-based screening program at their referring hospital who met the inclusion criteria (subjects had to reside in the study area, be at least 16 years old, have a singleton pregnancy, have their first prenatal visit in the main public hospital or health center of the

Results

The prevalence of preterm births among women of the cohort was 6.4%. The average age of the study population was 30 years (sd: 4.6). Unemployed women had a higher risk for preterm births than working women, both at the 12th and the 32nd week (age-adjusted OR: 2.33; 95% CI: 1.27–4.25 and OR: 3.51; 95% CI: 1.53–8.06, respectively) (Table 2). Women with only a primary school education had an OR of 2.05 (95% CI: 1.03–4.07) for preterm births with respect to women with a secondary school education

Discussion

The results of this study suggest that women exposed to elevated levels of NO2 and benzene throughout their pregnancies have a higher risk for preterm birth. The form of this relationship suggests the existence of a certain threshold level, after which the risk of preterm birth greatly increases.

In the past five years, six studies analyzing the relationship between NO2 exposure during pregnancy and the risk of preterm birth have been published (Hansen et al., 2006, Leem et al., 2006, Jalaludin

Conclusions

The results of this study suggest that prenatal exposure to levels of NO2 above 46.2 μg/m3 and to levels of benzene above 2.7 μg/m3 is related to the risk of preterm birth.

While the results of this study already contribute important information to the body of scientific knowledge concerning the reproductive effects of exposure to air pollution, it would be highly beneficial to conduct further studies of a similar nature both to carry out a prospective follow-up of the study population and to

Acknowledgments

The researchers of the INMA study wish to thank the women of the cohort and their families for their collaboration. We are also grateful to Rosalía Fernandez-Patier and her group at the laboratories of the Environmental Health Department of the Instituto Carlos III for their help in determining the levels of the air pollutants studied.

Funding: The INMA Network has been set up thanks to the “Instituto de Salud Carlos III” (G03/176). This study has also been supported by the Fondo de

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