Which of the following pollutants are considered for Air Quality Index…

Concept: India's National Air Quality Index (AQI), launched by the Central Pollution Control Board (CPCB) in 2014, is a single daily number built from the sub-indices of a fixed set of eight pollutants — PM10, PM2.5, NO2, SO2, CO, O3, NH3 and Pb. A sub-index can be computed only for a pollutant that carries a short-term National Ambient Air Quality Standard (NAAQS, 2009) — a 24-hour, 8-hour or 1-hour limit — because the index has to be reported for a single day. NAAQS fixes those ambient concentration standards; the CPCB AQI framework separately defines each pollutant's sub-index breakpoints and the health categories they map to.

Application: NAAQS notifies twelve parameters, so merely being a notified pollutant is not enough. Each item in the list has to be tested against the eight-pollutant AQI set.

• CO (carbon monoxide): NAAQS prescribes 8-hour and 1-hour limits, and CO is one of the eight AQI sub-index pollutants.

• NO2 (nitrogen dioxide): NAAQS prescribes annual and 24-hour limits, and NO2 is one of the eight AQI sub-index pollutants.

• C6H6 (benzene): NAAQS prescribes only an annual limit of 5 µg/m3 and no short-term limit, so no daily sub-index can be formed for it.

• Benzo(a)pyrene: NAAQS prescribes only an annual limit of 1 ng/m3 in the particulate phase and no short-term limit, so no daily sub-index can be formed for it.

• O3 (ozone): NAAQS prescribes 8-hour and 1-hour limits, and O3 is one of the eight AQI sub-index pollutants.

Among the pollutants listed in this question, the ones used for AQI estimation are therefore carbon monoxide, nitrogen dioxide and ozone — the stem items labelled A, B and E.

Cross-check: the other five AQI pollutants — PM10, PM2.5, SO2, NH3 and Pb — do not appear in this question at all, while the two excluded items, benzene and benzo(a)pyrene, belong to the annual-standard group of NAAQS along with arsenic and nickel; that annual-only group is precisely what separates the twelve NAAQS parameters from the eight AQI pollutants. Two CPCB operating rules reinforce the reading: an AQI is issued for a station only when at least three pollutants are monitored, one of which must be PM10 or PM2.5, and the AQI displayed is the worst (highest) of the available sub-indices.