 This study found that 7.3 billion people around the world are exposed to unsafe levels of air pollution, with most of them living in low and middle-income countries. The highest concentration of these people live in sub-Saharan Africa, where they are also the poorest. Additionally, air pollution levels are higher in lower middle-income countries, which have economies reliant on polluting industries and technologies. This article was authored by John Rentscher and Nadezda Leonova.