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Ambient PM2.5 pollution is a substantial threat to public health in global megacities.In situ PM2.5 pollution in global megacities has seldom been investigated.This paper reviews the PM2.5 daily mass concentration of 47 global megacities in 2013, based on an official monitoring network and the chemical composition of PM2.5 reported in the literature.The results showed that the five most polluted megacities were Delhi, Cairo, Xian, Tianjin and Chengdu, all of which had an average annual concentration of PM2.5 greater than 89 μg/m3.