Just over half of the world’s population lives in urban areas, with cities expected to absorb virtually all of the world’s future population growth.1 The way that we design our cities and interact with this “built environment” can profoundly affect our health, but empirical evidence tracing influences through specific mechanisms remains rare. In recent years, a substantial literature has developed that focuses on associations between within-city variability in air pollution concentrations and a diverse array of health effects, including birth outcomes.2 This evolution from between-city comparisons now fully integrates air pollution research into the realm of urban form, of which there are multiple (potentially spatially clustered) environmental exposures. For example, traffic and airport noise also vary over space and have been linked to health effects.3 A multidimensional approach to urban environmental quality and health is therefore needed and beginning to emerge in the literature.
It has also become clear that urban form is not limited to environmental “hazards” but that influences also arise through other pathways, such as the modification of individual behaviors. Physical inactivity and obesity are at the forefront of today’s public health challenges, and changes in the design of neighborhoods and urban form can improve individual physical activity levels and reduce prevalence of obesity.4 The influences of urban form on individual behaviors and environment hazards are not mutually exclusive. For example, urban heat islands can exacerbate heat-related mortality, and a component of climate change mitigation focuses on minimizing these at-risk areas.5 Linked to this is an emerging area of environmental epidemiology that associates proximity to natural (water and green) spaces within cities to improved health.6,7 While the mechanisms of this association are not clear, natural spaces may be refuges from harmful exposures (polluted air, noise, heat), may provide a space for healthy physical activity or social interactions, or may relieve stress through psychological mechanisms (a pathway that has been extensively studied by environmental psychologists).8
It matters what specific pathways link urban design to health, as these pathways can inform the most effective interventions, allowing us to design and retrofit cities for health. For example, mixed land use is thought to make cities more livable—decreasing the distance between home, work, and amenities. However, it is not known how much of the associated health benefit might be due to housing quality, access to healthy or unhealthy amenities, environmental exposures, or the modification of individual risk behaviors. An important question is whether we can continue to address each of these factors in isolation.
In this issue of Epidemiology, Dadvand and colleagues9 extend previous analyses, suggesting that proximity to major roads is a risk for term low birth weight. They consider the mediating roles of air pollution, noise, heat, and road-adjacent trees in a cohort of 6400 term births in Barcelona. Their analysis suggests that air pollution and heat jointly account for one-third of the measured association between road proximity and low birth weight. More than in prior analyses, these authors consider multiple potential exposures related to urban form, making this analysis unique. This article takes an important step toward more fully capturing the potential spatially clustered environmental health influences related to the design of cities.
However, this is just an initial step. More comprehensive and hypothesis-driven research is needed. For example, given the known relationships among urban form, transportation options, and physical activity, and between physical activity and pregnancy outcomes,10 this pathway would seem to be important to examine. Furthermore, because this cohort was restricted to one city, there may be urban form issues unique to this setting. The fact that 62% of participants lived within 200 m of a major road and that this group was more affluent than those who lived further away suggests that these findings cannot be directly extrapolated to settings where such relationships differ.
Perhaps also related to the setting is the observation that the association between road proximity and pregnancy outcomes was better explained by air pollutant measures less related to traffic (PM2.5 rather than NOx). This observation may reflect confounding of the road proximity measure by other spatial scales of air pollution variation (eg, distance from the coast). Similar problems may be present with analysis of the heat exposure variable, which suggested a novel (but perhaps unrealistically large) association of very small changes in temperature with low birth weight. Furthermore, each of the exposure measures (road proximity, air pollution, noise, and tree buffering variables) incorporates varying degrees of exposure measurement error, as each was measured at a different spatial and temporal scale. Most exposure models used similar predictive variables, which highlights the need for new exposure methods to describe the spatial patterns of urban environmental exposures.
The results of this analysis suggest that exposures related to road proximity other than those considered by the authors are responsible for the majority of the association between road proximity and low birth weight. These could be due to residual confounding from the exposures examined in this study, from other environmental exposures not evaluated, or from behavioral, physiological, or psychosocial factors related to living near a major road. The unexplained components of the association between road proximity and birth weight remain unclear. An alternative to the approach used by Dadvand and colleagues9 (assessing the mediating factors for a biologically implausible association between proximity to major roads and birth outcomes) is to examine the joint affects from specific hypothesized exposures. However, if exposures are highly correlated over space (and their effects therefore not separable), latent constructs representing healthy urban form may need to be developed. Informing specific design decisions from such studies will be difficult, but the approaches are complementary and both will be required to guide policy.
We applaud the authors for extending prior analyses to investigate multiple exposures related to urban form that may be linked to health outcomes. In the quest to better understand the diverse ways in which urban design can affect health, it will be necessary to refine exposure measures to address specific hypotheses, to apply advanced analytical approaches for evaluating interactions and possible nonlinear relationships, and ultimately to evaluate interventions.
ABOUT THE AUTHORS
MICHAEL BRAUER is a professor in the School of Population and Public Health at the University of British Columbia. He studies the health effects of traffic-related air pollution and pathways through which urban design and transportation infrastructure may affect health. PERRY HYSTAD is an assistant professor in the College of Public Health and Human Sciences at Oregon State University. He specializes in spatial epidemiology and the chronic health effects of air pollution.
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. Accessed 27 February 2014.
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