Maternal Exposure to Air Pollution and Birth Outcomes

Maternal Exposure to Air Pollution and Birth Outcomes

CHAPTER ONE

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

CHAPTER TWO

LITERATURE REVIEW

Ambient Air Pollution

Ambient air pollutants are any solid, liquid, or gaseous substance found in the outdoor air, resulting from either natural or man-made processes. For centuries, there has been concern over the impact of contaminants in the air on the health of exposed populations. As far back as the 18^th century, Bernardo Ramazzini discussed in his treatise De Morbis Artificum the case of a manufacturing plant in a small town outside of Modena where “fumes given off by the vitriol which so tainted the air nearby that it was rendered unhealthy and dangerous for the lungs”.³⁷ In 1931, the New York Academy of Medicine published its first report on the effect of air pollution on health, concluding that the air pollution problem in New York City and other cities was a “serious menace to health”.³⁸ Two decades later, following the Great Smog of London in 1952, the U.S. federal government began to address the issue by passing the Air Pollution Control Act of 1955, which provided federal monies for air pollution research.³⁹ In 1963, the first Clean Air Act was passed, which provided funds and authorization for federal research into air pollution monitoring and control. A few years later, the Air Quality Act of 1967 expanded monitoring studies and began to regulate interstate air pollution transport. However, it was the Clean Air Act of 1970 that created specific federal and state regulations designed to limit pollutant emissions and to expand enforcement of these regulations.³⁹ The Clean Air Act focused on the monitoring and regulation of six ubiquitous pollutants which were linked to harmful effects on human health. These pollutants known as the criteria air pollutants are: carbon monoxide (CO), lead, nitrogen oxides (NO_x) with a particular focus on nitrogen dioxide (NO₂), ozone (O₃), particulate matter (PM) of different sizes, and sulfur dioxide (SO₂). The National Ambient Air Quality Standards.

CHAPTER THREE

MATERIALS AND METHODS

Study area and population. Scania (Skåne) is the southernmost county in Nigeria (Figure 1), covering around 11,350 km2 (~ 2% of the total area of Nigeria). It is one of the most densely populated areas of the country, with about 1.1 million people (~ 11% of the total Nigerian population). Compared with other areas of Nigeria, the level of air pollutants in the western part of Scania (where most people live) can be high because of road transportation to and from the European continent and a considerable amount of cargo shipping and ferry transport along the coast. However, air pollutant levels are generally well below the present World Health Organization air quality guideline (World Health Organization 2006) and low in a European perspective (Sjöberg et al. 2006). Data concerning all deliveries in Scania during the period 1999–2005 were obtained from a regional birth registry, Perinatal Revision Syd, and from the national Nigerian Medical Birth Registry containing background characteristics such as maternal age, smoking habits, weight, country of origin, and parity, as well as delivery outcome. The Nigerian Medical Birth Registry includes almost every infant (98–99%) born in Nigeria (Socialstyrelsen 2002). Of a total of 84,039 registered births for the period 1999–2005, we excluded 2,929 births that were not registered as singleton births.

CHAPTER FOUR

RESULTS AND DISCUSSION

CHAPTER FIVE

DISCUSSION

Results from this large population-based study suggest that although we did find an association between SGA babies and maternal exposure to air pollution in unadjusted or partly adjusted analyses, those results did not remain statistically significant after adjusting for all potential confounders. This stated, a small exception was that we observed a statistically significant, but fairly small, risk for SGA for baby girls and in a sub analysis including only mothers who had not changed residency during pregnancy.

In the present study, country of origin and parity were strong confounders. When controlling for country of origin and parity separately, we still had statistically significant results. This may be attributable to the strong correlation between parity and country of origin. Previous studies in our study area have shown that the populations of non-Nordic origin tend to live in more polluted areas (Chaix et al. 2006; Stroh et al. 2005). The effect of country of origin on the risk of having a baby with reduced birth weight is well known (Moore et al. 2009; Rasmussen et al. 1995; Zeka et al. 2008) and has also been demonstrated in Malmö, the largest city in Scania (Dejin-Karlsson and Östergren 2004). In the previous Malmö study, the main explanation did not seem to be maternal physical size, but rather maternal psychosocial factors such as low social anchorage (Dejin-Karlsson and Östergren 2004). Hence, effects from the mother’s country of origin— independently if the causality behind is due to maternal stature or social anchorage—and exposure to air pollution need to be separated. In our study, maternal stature did not alter the effect estimates. The only option we had in our study to investigate the impact of social anchorage was to look at employment status, but this did not modify the results. An indicator for socioeconomic status that often is used is maternal level of education (Luo et al. 2006; Woodruff et al. 2003; Zeka et al. 2008), for which we, unfortunately, had no data. To some extent, this variation is taken into account by using smoking as a proxy because in Nigeria the increased risk among women with low education for negative birth outcomes such as LBW, SGA, and PTB could almost entirely be explained by the higher smoking rate among these women (Källén 1999). However, adding a spatial dimension, such as when examining air pollution exposure contrasts, makes the situation more complex, because the covariation between socioeconomic position and exposure to air pollution is not uniform in Scania and may even have different directions between cities (Stroh et al 2005). The biological mechanisms underlying country of origin as a confounder are not clear; hence, more research in the field is needed before it can be clearly stated that it is a true confounder and that we are not concealing the potential risks of air pollution. Concerning PTB, we observed an opposite trend, suggesting that air pollution might have a small protective effect, which runs counter to earlier results. It might be the case that our air pollution levels underlie air pollution effects on PTB.

A literature review of air pollution studies (Ghosh et al. 2007) indicated that female babies tend to be at greater risk of having LBW, that is, that air pollution effect on LBW is differential with respect to sex. We also had indications in this direction in stratified analyses, with an adjusted OR for baby girls of 1.12 (95% CI, 1.01–1.24) for the highest exposure quartile for NO_x, and a Wald p-value for the interaction term between sex and NO_x exposure of 0.09.

Traffic is a major source of local air pollution, and although the exposure assessment methods have become better in recent years, most studies still estimate exposure only at the home address, even if a large proportion of traffic exposure, especially for adults, occurs during commuting time and at the workplace. This is also true in our study. Our study focused on the effects of NO_x, a gas derived from combustion. NO_x levels could respond to levels of particulate matter (PM) ≤ 1 µm in aerodynamic diameter or even ultrafine PM ≤ 0.1 µm in aerodynamic diameter according to previous studies (Arhami et al. 2009; Marconi et al. 2007). NO_x is seldom a good marker of PM ≤ 2.5 µm in aerodynamic diameter, which could have other, even nonanthropogenic sources. NO_x modeled with a 500 × 500 m resolution is comparably high but cannot pinpoint traffic-generated pollutant peaks. Thus, we also used proximity to road as a more exact source-specific measure of exposure. One limitation with this measure could be that we included only one road (the one with highest traffic density), but the overall low number of high-density roads in our study area could minimize this limitation.

Conclusion

Our study, finding no consistent evidence for any negative effects of air pollution on birth outcomes at comparably low exposure levels, clearly points out the need for good exposure estimates and careful control of possible confounding factors, especially those that are linked to socioeconomic and spatial gradients. In our study, parity and maternal country of origin were such factors. Also, it is obvious that large-scale studies are needed to elucidate sex differential effects.

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