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.