Ambient Air Pollution and Pregnancy Outcomes: A Review of the Literature

Radim J. Srám; Blanka Binková; Jan Dejmek; Martin Bobak

Disclosures

Environ Health Perspect. 2005;113(4):378-382. 

In This Article

Abstract and Introduction

Over the last decade or so, a large number of studies have investigated the possible adverse effects of ambient air pollution on birth outcomes. We reviewed these studies, which were identified by a systematic search of the main scientific databases. Virtually all reviewed studies were population based, with information on exposure to air pollution derived from routine monitoring sources. Overall, there is evidence implicating air pollution in adverse effects on different birth outcomes, but the strength of the evidence differs between outcomes. The evidence is sufficient to infer a causal relationship between particulate air pollution and respiratory deaths in the postneonatal period. For air pollution and birth weight the evidence suggests causality, but further studies are needed to confirm an effect and its size and to clarify the most vulnerable period of pregnancy and the role of different pollutants. For preterm births and intrauterine growth retardation (IUGR) the evidence as yet is insufficient to infer causality, but the available evidence justifies further studies. Molecular epidemiologic studies suggest possible biologic mechanisms for the effect on birth weight, premature birth, and IUGR and support the view that the relation between pollution and these birth outcomes is genuine. For birth defects, the evidence base so far is insufficient to draw conclusions. In terms of exposure to specific pollutants, particulates seem the most important for infant deaths, and the effect on IUGR seems linked to polycyclic aromatic hydrocarbons, but the existing evidence does not allow precise identification of the different pollutants or the timing of exposure that can result in adverse pregnancy outcomes.

There is extensive evidence that ambient air pollution affects human health (e.g., Brunekreef and Holgate 2002; Künzli et al. 2000; Pope et al. 2002). Most studies have focused on the effects of air pollution on adult mortality and respiratory morbidity (Dockery et al. 1993; Schwartz and Marcus 1990). However, some age groups appear to be more susceptible than others. For example, it has been shown that the effects are larger in the elderly than in the general adult population (Saldiva et al. 1995). Studies on childhood health risks, such as respiratory symptoms or hospital admissions for asthma, suggest that the opposite end of the age spectrum is also more vulnerable to air pollution than is the general population (Dockery and Pope 1994; Heinrich et al. 1999; Schwartz et al. 1994). In addition to these "traditional" end points in children, there is now emerging evidence that air pollution is also associated with elevated risk of adverse pregnancy outcomes (Glinianaia et al. 2004; Maisonet et al. 2004).

The study of birth outcomes is an important emerging field of environmental epidemiology. Birth outcomes are important in their own right because they are important indicators of the health of the newborns and infants. In addition, low birth weight (LBW), intrauterine growth retardation (IUGR), and impaired growth in the first years of life are known to influence the subsequent health status of individuals, including increased mortality and morbidity in childhood and an elevated risk of hypertension, coronary heart disease, and non-insulin-dependent diabetes in adulthood (Barker 1995; Osmond and Baker 2000).

It is increasingly apparent that there is a critical period of development when the timing of exposure and the dose absorption rate can be even more important for the biologic effects than is the overall dose (Axelrod et al. 2001). Fetuses, in particular, are considered to be highly susceptible to a variety of toxicants because of their exposure pattern and physiologic immaturity (Perera et al. 1999; Sˇrám 1999). Their developing organ systems can be more vulnerable to environmental toxicants during critical windows (sensitive periods of development) because of higher rates of cell proliferation or changing metabolic capabilities (Calabrese 1986). Therefore, prenatal exposure to environmental pollution can result in some adverse reproductive outcomes, similar to the association between maternal active and passive smoking and impaired reproductive outcomes (Misra and Nguyen 1999; Salihu et al. 2004). The specific mechanisms that may account for the link between ambient air pollution and adverse reproductive outcomes are also reviewed in this article.

The objective of this review is to examine the evidence linking adverse birth outcomes with ambient air pollution. For the purpose of this review, birth outcomes have been divided into five groups: a ) mortality of fetuses and infants, b ) LBW, c ) premature (preterm) births, d ) IUGR, and e ) birth defects. In this article we review the evidence on each of these separately. For each of the outcomes, we assess the three critical issues in interpreting epidemiologic studies (random error, selection or measurement bias, and confounding); issues related to all reviewed outcomes (e.g., publication bias or biologic plausibility) are considered together at the end of the article. By weighting the evidence, we attempt to draw balanced conclusions about the relations between air pollution and birth outcomes.

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