# **1 Introduction**

Air quality poses a significant threat to health in most parts of the world. At current levels air pollution is responsible for approximately 10 million premature deaths each year.<sup>1</sup> While the number of deaths associated with indoor air pollution has declined in recent years, increases in deaths due to ambient pollution have offset this improvement and led to an overall increase in total air pollution deaths. More than half of the world’s population now lives in areas where ambient air pollution levels are increasing. The majority of those facing increasing levels of air pollution live in low- and middle-income countries (LMICs)<sup>2</sup> where more than 90% of air pollution-related deaths occur.<sup>3</sup> The health burden of air pollution is also unequally shared _within_ countries. With a few exceptions, low income populations in Europe are exposed to higher pollution levels than wealthy populations in the same country.<sup>4</sup> In the United States low income populations<sup>5–8</sup> and racial and ethnic minority populations<sup>8–13</sup> tend to be more exposed to ambient particulate pollution. These differences across groups have persisted over the past several decades despite universal reductions in pollution levels.<sup>14,15</sup> While the within-country relationship between income or wealth and air pollution has been well documented in Europe and the US, evidence from LMICs is scarce.<sup>16</sup> This scarcity of evidence in the parts of the world with the largest populations, the highest levels of ambient pollution, and where pollution levels are increasing fastest limits our understanding of the present and future patterns of vulnerability. Lack of evidence may hinder effective policy-making to reduce the harms of this exposure. Furthermore, with the exception of recent studies in Eastern Europe<sup>4</sup> and China,<sup>17</sup> examinations of income and ambient air pollution outside of high-income countries have been limited to urban areas.<sup>18–23</sup> These city-level studies have documented pollution exposures declining with income<sup>18–21,23</sup> or non-linear relationships with pollution highest at wealth extremes.<sup>22</sup> Population-scale estimates of the relationship between wealth and pollution in LMICs that include observations from both rural and urban areas are needed. Conclusions about the distribution of pollution exposures across wealth levels in Europe and the United States may not apply where settlement patterns differ.<sup>17</sup> Urban centers tend to be economic drivers that generate substantially higher incomes than are typically found in rural areas.<sup>24,25</sup> While this gradient exists in most countries it can be especially pronounced in LMICs. Higher levels of economic activity can simultaneously lead to higher levels of pollution<sup>26–28</sup> and wealth. If urban residents are on average wealthier and experience more ambient pollution than rural residents, then wealth could be positively correlated with ambient pollution exposure across the full population. This is true even in settings where within urban pollution is negatively correlated with neighborhood 2