This paper presents progress in elaborating an indicator of population exposure to PM2.5 with the objective to produce internationally harmonised indicators for all OECD and G20 countries. The paper takes stock of the various methodological options, including those based on data from ground-based monitoring, remote sensing, and a hybrid approach. A calculation methodology is described and examples of the indicator are presented for all OECD and G20 countries for the 1990-2013 time period. Possible next steps are identified with the aim of updating the indicator on a regular basis.