Coeff. of Haze (US) (Green, 1966)

SO₂ and PM (smoke shade)

Weighted sum of two pollutants

Oak Ridge AQI (Thom & Ott, 1976)

SO₂, NO₂, PM, CO, photochemical oxidants

Weighted (ratio of observed pollutant to standard) combination of 5 pollutants

Taiwan AQI (Lohani, 1984)

Statistical factor analysis approach for AQI. Authors compared the air quality index based on factor analysis method and Pindex method. The ratings (or trends) obtained by both these methods are exactly same, but the AQI based on factor analysis shows a wider range—indicates it’s the superior approach

USEPA’S Pollution Standard Index (1999) (USEPA, 2019)

SO₂, NO₂, PM, CO, O₃

Highest concentration of any ONE pollutant from group compared to its standard. Doesn’t consider synergistic effects of pollutants. Reported as separate indices specific to each pollutant

API, Canada (pre-2007)

Range from very good (0) to very poor (100), with 50 as the threshold

Air Quality Health Index (AQHI), Canada (Wong et al., 2012)

O₃, NO₂, PM_2.5

Sum of excess mortality risk from pollutants. Calculated hourly based on 3-hr rolling average

Ministry of Environment, Forestry & Climate Change, India (Central Pollution Control Board, 2014)

CO, NO₂, PM₁₀, PM_2.5, SO₂, O₃, NH₃

Revamped AQI to incorporate pollutants monitoring in real time. The proposed API to adopt the HIGHEST sub-index (specific pollutant concentration is related to “breakout values” obtained by comparison of pollutant concentration and revised AQ regulatory standards and/or any corroborated dose-response relationship reported in the literature)

API in China (Gong et al., 2015)

SO₂, NO₂, PM₁₀

Uses HIGEST pollutant sub-index with “breakout” values. 3 tiers of regulatory AQ standards

(Buke & Kone, 2016)

SO₂, NO₂, PM₁₀

Compilation of equally weighted variables normalised to target values (from EU guidelines for compliance in 2022)