# **3 Discussion**

Using large-scale spatially resolved wealth and pollution data we find that ambient air pollution is, on average, positively correlated with wealth in LMICs around the world. This appears to be driven by urbanization, which is highly positively correlated with both wealth and ambient air pollution in most LMICs. However, there is substantial heterogeneity in this result which appears driven partly by the the weaker correlation between urbanization and ambient air pollution in countries where the primary source of ambient air pollution is non-anthropogenic.

Within cities the theoretically anticipated relationship between pollution and wealth is ambiguous. It will be driven by patterns of residential sorting that are likely influenced by cultural, geographic, and historic differences. As a result, one would expect city-level results to vary widely across different cities in different countries. We find this to be the case. Sorting across different terrains, particularly elevation, appears to be important in places where cities are built on varied geographies. Concentration of economic activity also appears to explain some of the relationship between wealth and pollution levels. Cities with more economic concentration have a stronger and more positive relationships between wealth and pollution. This may be due to the relatively wealthy wishing to live closer to areas of concentrated economic opportunity – which may also be the most polluted areas. This is analogous to the mechanism that appears to drive the correlations between pollution and wealth at a country level – living in close proximity to economic opportunity can come at the cost of living closer to emissions sources.

There are several limitations to our results. First, we do not account for indoor air pollution, which is negatively correlated with urbanization<sup>32</sup> because rural households are more likely to cook with pollution-generating fuels. Relatively higher concentrations of indoor air pollution in rural areas suggests that the total burden of air pollution may be less positively correlated with wealth than the ambient concentrations examined here. In the extreme case, sufficiently high levels of indoor concentrations in rural areas could completely offset the positive correlation of ambient concentrations with wealth and lead to a regressive total burden of air pollution.

A second limitation is that we do not observe how ambient pollution levels translate to personal exposures and impacts. Personal exposures are mediated through the built environment,<sup>33</sup> patterns of commuting, and workplace conditions. Individuals living in neighborhoods with the same ambient pollution levels may not be personally exposed to the same amount of pollution because of differences in these conditions. For example, less wealthy neighborhoods may have lower quality housing and may therefore experience higher levels of infiltration than urban areas.<sup>32</sup> Greater wealth may also allow individuals to better insulate themselves from negative impacts for a given level of personal exposure<sup>34</sup> if, for example, wealth is correlated with better health care access. Therefore, even in countries where ambient pollution is higher in wealthier 11