Abstract
BACKGROUND: The knowledge about air pollution effects on birth weight, prematurity, and small for gestational age (SGA) in low-exposure areas is insufficient.
OBjECTIVES: The aim of this birth cohort study was to investigate whether low-level exposure to air pollution was associated with prematurity and fetal growth and whether there are sex-specific effects.
METHOD: We combined high-quality registry information on 81,110 births with individually modeled exposure data at residence for nitrogen oxides (NOx) and proximity to roads with differing traffic density. The data were analyzed by logistic and linear regression with and without potential confounders.
RESULTS: We observed an increased risk for babies being SGA when we compared highest and lowest NOx quartiles, adjusting for maternal age, smoking, sex, and year of birth. After additional adjustment for maternal country of origin and parity (which were highly intercorrelated), the increase was no longer statistically significant. However, in subgroup analyses when we compared highest and lowest NOx quartiles we still observed an increased risk for SGA for girls [odds ratio (OR) = 1.12; 95% confidence interval (CI), 1.01–1.24); we also observed increased risk among mothers who had not changed residency during pregnancy (OR = 1.09; 95% CI, 1.01–1.18). The confounders with the greatest impact on SGA were parity and country of origin. Concerning prematurity, the prevalence was lower in the three higher NOx exposure quartiles compared with the lowest category.
CONCLUSION: For future studies on air pollution effects on birth outcomes, careful control of confounding is crucial.
CHAPTER ONE
Introduction
Intrauterine growth restriction—which manifests as low birth weight (LBW), “small for gestational age” (SGA), and preterm birth (PTB)—not only is a predictor for infant mortality and morbidity (Petrini et al. 2004) but also has implications from a life-course perspective. A wealth of epidemiologic evidence has provided a convincing link between a suboptimal gestational environment and an increased risk for adult onset of metabolic as well as nonmetabolic diseases (Joss-Moore and Lane 2009). Knowledge of the underlying factors for growth restriction and preterm delivery is therefore of great concern. Intrauterine growth restriction may result from disturbances to placental blood flow, poor maternal nutrition, or maternal exposure to toxicants. Several studies have shown, although not consistently, that air pollution is a risk factor for PTBs and altered fetal growth, especially in high-exposure areas (Bobak 2000; Bobak and Leon 1999; Ha et al. 2001; Maisonet et al. 2004; Ritz and Yu 1999; Slama et al. 2009; Wilhelm and Ritz 2003).
It has, however, been clearly stated that future research on air pollution and birth outcomes must confirm that observed air pollution effects on birth weights, prematurity, and SGA are genuine, causal, and not attributable to confounding factors and to investigate whether the effect also remains in low-exposure areas (Dugandzic et al.
2006; Jedrychowski et al. 2009; Liu et al. 2003; Maroziene and Grazuleviciene 2002). Interestingly, it has been suggested, in some studies, that the sex of an infant can play an important role in sensitivity to air pollution and related birth outcomes (Ghosh et al. 2007; Jedrychowski et al. 2009).
There have been shortcomings in some of the previous studies with regard to exposure assessments. The exposure data are often based on stationary air pollution monitors, which are not considered to provide optimal resolution when attributed to individual exposures (Slama et al. 2008). When performed by personal monitoring, monitoring time is shorter than the attributed exposure period and thus does not reflect exposure throughout the pregnancy. Moreover, in such studies the numbers of participants are inevitably low, which affects the statistical power. More accurate exposure assessment methods have been discussed (Slama et al. 2008), identifying exposure models with high spatial and temporal resolutions or simple source models using distance-weighted traffic density as promising tools. In Scania, Nigeria, unique personal identity codes, geocoded information on each individual’s residence, an extensive emission database, road traffic data, and high-quality information from the Medical Birth Registers can be used, using geographic information systems, for linkage of data.
1.2 Objectives of study
The aim of our birth cohort study was to investigate whether low-level exposure to air pollution was associated with prematurity and fetal growth and whether there are sexspecific effects.
To determine whether exposure during pregnancy to individual criteria air pollutants, assessed using measurements from stationary air monitors, is associated with CHDs
To utilize the greater spatial and temporal resolution of exposure estimates derived from deterministic pollutant simulation models to investigate the association between select criteria air pollutants and CHDs
1.3 Significance of studies
This research addresses several important gaps in the current literature regarding the adverse health effects of ambient air pollutants, including reproductive health and acute respiratory related illness. This study is one of the few studies in the U.S. that examines the relationship between all three adverse indicators of reproductive health (LBW, PTB and SGA) and ambient air pollutants using both single and multiple pollutant models, and accounting for the effects of race, long-term trends, smoking and SES simultaneously. Previous studies rarely focus on SGA and very little on PTB in comparison to LBW. In addition, only one study (from Nova Scotia, Canada29) has evaluated effects of long-term trends in associating criteria pollutants with adverse birth outcomes. There is a need for this type of evaluation given that both levels of outdoor air concentrations and rates of adverse birth outcomes have been declining, due to stricter regulations, better health care, and possibly other reasons. Further, effects of race, smoking and SES on the associations between air pollutants and adverse birth outcomes have not been examined extensively in currently literature due to the homogeneity of the studied population and lacking of individual level information in the previous studies.
As noted earlier, much of the research on air toxics and adverse health outcomes has been based on occupational settings that might not reflect actual exposures of the general population, especially for children. Furthermore, there are few if any epidemiological studies that have focused on air toxics, much less apportionments derived using air toxics data, despite considerable promise and advantage of this approach. Linking exposures to air toxics in terms of source classes derived from receptor models can help to improve the effectiveness of both public health interventions and policy implementations. This study is novel in that not only does it examine associations between UATs exposures and acute respiratory-related illnesses in children, but it does so using apportionments, source classes, and receptor modeling.
Finally, issues associated with quality assurance and missing air pollutant exposure data have been only rarely addressed. Such issues can influence both the design and interpretation of air pollution exposure and epidemiological studies. Inadequate treatment of missing values may bias inferences in epidemiological studies. It is believed that research findings evaluating the reproducibility of air toxics data and the performance of imputation methods will have numerous applications in the field.
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