et al., 2011) that influence health outcomes. In this way, environmental factors are undoubtedly part of a self-perpetuating cycle that operates across multiple domains, but delineating exactly how this occurs—and how this may differ across place and time—will require further research.

Many of the environmental factors relevant to health are directly amenable to policy. Therefore, identifying which of these factors are important contributors to the U.S. health disadvantage could point to policy interventions that might reduce the disadvantage. For example, cross-national comparisons show that levels of active transportation, such as walking or cycling, can be effectively modified by specific land use and transportation policies (Pucher and Dijkstra, 2003; Pucher et al., 2010b). Although many of the data reviewed in this chapter are highly suggestive of an important role for environmental factors, more empirical evidence is needed to draw definitive conclusions. Important areas for future cross-national research on environmental factors and health include (1) characterizing levels and distributions of environmental risk factors using comparable measures across countries; (2) documenting inequalities in the distribution of these environmental factors; (3) identifying the extent to which these environmental factors affect health and the extent to which their effects are modulated by individual-, community-, or country-level factors; (4) examining directly the contribution of environmental factors to health differences between the United States and other high-income countries; and (5) studying national, regional, and local country policies that may curb levels of adverse environmental exposures, reduce the extent to which they are inequitably distributed, or buffer their effects.

The contribution of environmental factors to the U.S. health disadvantage is likely to result from dynamic and reinforcing relationships between environmental and individual-level factors. Environmental factors also operate over a person’s life course, so that the environments one experiences early in life may influence health trajectories over time. Environmental factors are in turn linked to upstream social and policy determinants. In many ways, the environment can be thought of as the mid- or “meso-” level of influence linking macrolevel factors (e.g., economic and social policy) and microlevel processes (e.g., individual behavior). A comprehensive understanding of the causes of the U.S. health disadvantage will require recognizing how the environment interacts with these other factors and helps perpetuate or mitigate the disadvantage across a broad set of health domains.

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