Review
A review of the literature on the effects of ambient air pollution on fetal growth

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Abstract

A systematic review of the literature on the effects of air pollution on low birth weight (LBW) and its determinants, preterm delivery (PTD) and intrauterine growth restriction (IUGR), was conducted. Twelve epidemiologic investigations that addressed the impact of air pollution on four pregnancy outcomes were identified. Results were analyzed separately for each perinatal outcome because of differences in pathogenic mechanisms. Effects of air pollution were apparent on PTD and IUGR, but not on LBW. Most of the associations reported were rather small. The estimation of summary effects was not meaningful because of the heterogeneity of the effect estimates arising from differences in the measurements of outcome, exposure, and confounders and the small number of studies per outcome (four studies for PTD and six for IUGR). Current scientific knowledge on the impact of air pollution on fetal growth is still limited; thus, several issues should be examined further.

Introduction

Recent epidemiological studies have shown associations of low birth weight (LBW) and its determinants, preterm delivery (PTD) and intrauterine growth restriction (IUGR), with ambient air pollution (Bobak, 2000; Dejmek et al., 1999; Ha et al., 2001; Maisonet et al., 2001; Ritz and Yu, 1999; Ritz et al., 2000; Wang et al., 1997; Xu et al., 1995). Interpretations of these findings are difficult, in part, because LBW (birth weight <2500 g) represents a heterogeneous group of outcomes with different pathogenic mechanisms. Some infants have LBW as a result of PTD (<37 completed weeks of gestation at delivery), while others are a result of IUGR (birth weight less than that expected for a given gestational age). Some LBW infants are both preterm and growth restricted for their gestational age. Such subgroups of LBW may not necessarily arise from the same set of risk factors. Some maternal prenatal determinants may be associated with an increased risk of LBW through effects on the length of gestation alone (e.g., premature rupture of the membranes, placenta abruptio), others through effects on intrauterine fetal growth alone (e.g., maternal weight gain, hypertension), and some possibly through effects on both PTD and IUGR (e.g., maternal cigarette smoking, malformations). Whether the reported association reflects an effect of air pollution on PTD, IUGR, or both should be better understood.

Ambient air pollutants also vary in nature (e.g., CO, SO2, and particulate matter) and possible health effects. Exposure to ambient levels of CO may result in the formation of carboxyhemoglobin in the mother, which in turn could result in decreased oxygen delivery to tissues, including the fetus. Inhaled particles, in contrast, have been reported to increase blood viscosity, which may have an adverse effect on placental function, thus restricting fetal growth (Ha et al., 2001). In the absence of definite biological mechanisms for the possible associations between some ambient air pollutants and fetal growth, it is important to determine whether the associations reported are consistent and whether they reflect the effect of a specific pollutant or a mixture of pollutants. However, thus far there have been no evaluations of whether such associations are for specific air pollutants, the consistency of the reported associations, and the levels of ambient air pollution at which such associations are evident.

In this report, we review published studies on the association of ambient air pollutants with LBW, PTD, and IUGR. Because these outcome measures may have different pathogenic and etiologic mechanisms, we conducted this review by the type of outcome.

Section snippets

Literature search

The review was limited to reports published in the English language peer-reviewed scientific literature for four outcome measures: LBW, very low birth weight (VLBW), PTD (i.e., birth at <37 weeks of gestation), and IUGR. We selected reports that evaluated any of the following ambient air pollutants: CO, SO2, nitrogen oxides (NOx), particulate matter, and ozone. We identified articles through Medline searches, bibliographies of individual articles, and reviews of scientific journals from 1966

Characteristics of the studies

Twelve epidemiologic investigations that addressed the impact of air pollution on four pregnancy outcomes were identified, with some studies examining multiple outcomes (Table 1). The outcomes assessed were LBW (4 studies), VLBW (2 studies), PTD (4 studies), and IUGR (6 studies). Many of the studies shared similar design and methodologic features. The 12 investigations consisted of 9 population-based cross-sectional studies, 1 case-control study, 1 case-cohort study, and 1 ecologic study.

Most

Low birth weight

Four studies evaluated LBW as an outcome (Table 2). The contaminants assessed were TSP, SO2, CO, and NOx. One of three studies that assessed the effects of exposures to SO2 reported an increase in risk of LBW (Odds ratio (OR), 1.10 per 50 μg/m3 increase; 95% confidence interval (CI), 1.01–1.20) (Bobak and Leon, 1999). Exposures to TSP, CO, or NOx did not appear to be associated with increases in LBW.

Very low birth weight

Two studies evaluated VLBW as an outcome (birth weight <1500 g) (Table 2). The contaminants

Discussion

The studies reviewed for this analysis reported positive statistical associations between different air pollutants and LBW, VLBW, PTD, and IUGR. Each outcome was assessed separately because of differences in pathogenic mechanisms.

Overall, the number of studies that were available in the literature suggests that the scientific knowledge of the impact of air pollution on LBW and its determinants is still limited. These were but a few studies available for each outcome, which limited our potential

Summary

Increases in ambient air pollution may increase the risk of PTD and IUGR among live births. Overall, the effect of air pollution on these outcomes is smaller than the effects of other known risk factors. However, it is not uncommon to find levels of ambient air pollutants in urban areas that are similar to those observed in the studies that reported positive results. This would be a cause for concern if the prevalence of exposure to these levels is elevated, since the public health impact would

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