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. The US Index’s “Good” range (<12μg/m3) is relatively comparable to the WHO AQG (<10μg/m3).