Why is the US Air Quality Index used in the ranking?

Many countries use different Air Quality Indexes; however, the US Air Quality Index is one of the most widely recognized AQI systems available.
The Index converts pollutant concentrations into a color-coded scale of 0-500, to easily represent the level of health hazard posed.

For more information on how the US AQI and city rankings relate, please follow the link below:
https://www.iqair.com/newsroom/why-is-the-us-air-quality-index-used-in-the-ranking