# **1. Introduction**

Air pollution has wide-ranging and profound impacts on human health and well-being. Poor air quality has been shown to be responsible for over 4 million death each year from outdoor pollutants, another 2.3 million from indoor air pollution, a wide range of cardiovascular, respiratory, and neurological diseases, as well as reductions in cognitive abilities (WHO, 2018). Studies show that the vast majority of the world’s population face unsafe air pollution levels. Yet, our understanding of what exact air pollution levels should be considered _unsafe_ is still evolving. Based on the latest medical evidence the WHO updated its air quality guidelines in 2021, significantly increasing the stringency of its 2005 guidelines.

A growing evidence base also highlights that the exposure to and impact of air pollution are not equally distributed. As health and productivity suffer, evidence from the US has shown that air pollution reinforces socio-economic inequalities. Especially poor and marginalized groups are often exposed to higher pollution levels. In addition, these groups also tend to be more vulnerable to the impacts of pollution, since low-paying jobs are more likely to require physical and outdoor labor, meaning that people are faced with heightened exposure. Pollution sources, such as industrial plants or transport corridors, are disproportionately placed in low-income neighborhoods. And as they increase air pollution further, this drives down housing prices, which in turn reinforces their status as low-income neighborhoods. Constraints in terms of the accessibility, availability and quality of health care provision further increase air pollution related mortality among low-income groups.

In short, socio-economic marginalization makes people more exposed and vulnerable to air pollution. Yet, apart from substantial evidence for the US, little evidence exists documenting the global scale of poor people’s exposure to harmful air pollution. A better understanding of the interplay between air pollution and poverty could be crucial for several reasons. Studies from high-income countries on the mortality and morbidity associated with air pollution may not be directly transferable to low-income countries and communities, where the nature of occupations and health care differ substantially. The health and productivity implications of unsafe air pollution will also impact the socio-economic development prospects of low- and middle-income countries. This is especially pertinent in low-income countries, which – as this study shows – still tend to have relatively low pollution levels compared to more industrialized middle-income countries. In these countries, it will be important to ensure that development progress does not come hand in hand with intensifying air pollution and the associated adverse effects.

Against this context, this study explores the global prevalence of unsafe outdoor air pollution, and how it interacts with poverty. It uses global high-resolution data on ambient air pollution (outdoor concentrations of PM2.5), population distribution, and poverty to provide aggregate exposure estimates (see Section 3.1 on data sources). It also shows that pollution levels are most hazardous in middle-income countries, where economies tend to rely more heavily on polluting industries and technologies. This study does not cover indoor air pollution, which is another pervasive risk to health and well-being, especially in developing countries.

Overall, this study contributes to the literature in two ways. It offers the first global population PM2.5 exposure estimates based on the WHO’s recent guidelines, revised in September 2021, with detailed national and subnational estimates. It also offers the first global estimates of PM2.5 exposure and its interaction with national and subnational poverty levels.

The rest of this study is organized as follows. Section 2 reviews the existing evidence on air pollution, health impacts and development outcomes. Section 3 summarizes the data sets and analytical methods used in this study. Section 4 presents results, and section 5 concludes.

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