Since 2011, WHO has been compiling and publishing ground measurements of air quality and, specifically, the annual mean concentrations of particulate matter with a diameter ≤ 2.5 μm (PM2.5) and ≤ 10 μm (PM10). The objective – beyond summarizing the current state of air quality – is to collect data on air quality that could be used to derive robust estimates of population exposure for studies of the burden of disease analysis due to ambient air pollution. The database thus fulfils part of WHO’s custodial role for indicators 11.6.2 (Air Quality in cites) and 3.9.1 (Mortality from air pollution) of the Sustainable Development Goals. The recent update of the WHO air quality guidelines, a set of evidence-based recommendations for limit values of specific air pollutants, provides clear evidence of the damage that air pollution inflicts on human health, at even lower concentrations than previously recognized. The guidelines recommend new air quality levels to protect the health of populations. Moreover reducing the levels of key air pollutants will also contribute to slowing climate change. Pollutants for which new guidelines for annual mean values have been set are PM2.5, with a guideline value half the previous one, PM10, which is decreased by 25 %, and that for nitrogen dioxide (NO2), which is four times lower than the previous guideline. This document describes the latest WHO database on ambient air quality.