Share of reporting cities with poor or worse air, Tamil Nadu

The percentage of cities in the geography in CPCB's daily AQI bulletin whose National AQI was 201 or more (Poor, Very Poor or Severe) for the 24 hours ending 16:00 IST.

Numerator

Reporting cities with an Index Value of 201 or more

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 counts the city rows in the geography whose Index Value is 201 or more (CPCB's Poor, Very Poor and Severe categories, which CPCB associates with breathing discomfort for most people on prolonged exposure or worse) and divides by the number of city rows in the geography. Each printed category is checked against its band; the share uses the Index Value. The India figure counts across all reporting cities.

Caveats

With few reporting cities the share moves in large steps: in a state with one city it can only be 0% or 100% (marked low-sample). The 201 threshold is a category boundary, so a city at 199 and one at 205 are treated very differently. 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.

Comparability

Comparable day to day within a geography when the reporting-cities count is stable. Long-run comparisons are affected by network growth, which added many smaller towns, often in cleaner regions, after 2020. Strongly seasonal: the share peaks in November–January in north India.