How precise is the ranking?

The rankings are based on real-world monitoring data from a variety of sources. All monitoring methods have a degree of error. The rankings presented here represent annual average concentrations taken from multiple monitoring sites, and data is checked and validated. However, even after this process, the data have some uncertainty. Where cities and countries in the ranking have similar PM2.5 concentrations, the ranking may be affected by measurement error and the ranking position should be considered to be indicative rather than absolute.