Air quality index (mean of reporting cities), Tamil Nadu

The unweighted mean of the National Air Quality Index (AQI) of every city in the geography that appears in CPCB's daily AQI bulletin, for the 24 hours ending 16:00 IST. Lower is better: 0–50 Good, 51–100 Satisfactory, 101–200 Moderate, 201–300 Poor, 301–400 Very Poor, 401–500 Severe.

Numerator

Sum of the Index Values of the reporting cities

Denominator

Number of reporting cities

Methodology

Source: CPCB's daily National Air Quality Index bulletin, published about 16:05 IST for the 24 hours ending 16:00 IST. Each city row's Index Value is CPCB's National AQI (0–500): for every participating continuous monitoring station CPCB computes a sub-index for each pollutant measured (PM2.5, PM10, NO2, SO2, CO, O3, NH3, Pb; at least three, one of them PM2.5 or PM10), takes the highest sub-index as the station AQI, and reports the arithmetic mean of the city's station AQIs. CPCB itself notes that averaging across locations 'is not a scientifically sound approach' and is done for simplicity. IHO takes every city row in the bulletin, assigns it to a state with a curated city-to-state crosswalk (built from CPCB's station list and Delhi-NCR bulletin, and from the bulletin's own state labels such as 'Aurangabad (Maharashtra)'), and reports the arithmetic mean of the Index Values of the cities in each state, each city counting once whatever its population or number of stations. The India figure is the mean over all reporting cities in the bulletin, not the mean of the state figures, so states with many monitored cities weigh more. A city name the crosswalk does not know stops the release rather than being dropped. From 18 July 2021, when Bihar's Aurangabad station could have joined the bulletin, a row printed only as 'Aurangabad' may be Bihar's or Maharashtra's (on 34 days in 2022 the bulletin printed two such rows); those rows count towards India but towards neither state.

Caveats

This is a summary convention, not an exposure measure: it is not population-weighted, a mean of AQIs is not the AQI of mean concentrations (each AQI is the worst pollutant's sub-index, and the worst pollutant differs between cities), and it says nothing about rural air. A state figure based on one city is just that city's AQI; such values are marked low-sample. Coverage is urban and uneven: only cities with continuous monitoring stations appear, the network is concentrated in the Indo-Gangetic plain, Maharashtra, Rajasthan, Karnataka and Haryana, and several states and union territories have one city or none on a given day. Cities with insufficient data drop out of a day's bulletin and come back, so the set of cities behind a state's figure changes from day to day. The network grew from about 10 cities in May 2015 to about 100 in 2020 and about 250–270 in 2024–26, so long-run comparisons, especially for India as a whole, partly reflect which cities were added (many smaller towns from 2021 onward) rather than a change in the air. A city's Index Value is the 24-hour average to 16:00 IST over whichever of its stations reported that day; some bulletins mark stations whose data ended at 15:00.