Every winter, as toxic smog cloaks Delhi-NCR, a set of emergency measures comes into force. The scale of the restrictions tightens at each stage under the Graded Response Action Plan (Grap), from one to four, as air quality worsens.
These measures – which include a construction ban and curbs on entries of polluting vehicles into the Capital – along with policies meant to mitigate the region’s air pollution crisis rely on air quality data. So they can only be as effective as the data is accurate. It is why the Central Pollution Control Board (CPCB) has a set of rules to guide how many monitoring stations a city must have and where they can be located.
On November 2, Delhi’s air quality index was 324, this winter season’s first ‘very poor’ air day, based on data available from 58 of the 59 stations listed in CPCB’s daily bulletin. The five big NCR cities around the capital have 25 more stations – eight in Ghaziabad, seven in Noida, four in Gurugram, and three each in Faridabad and Greater Noida.
How many stations should a city have?
There are two kinds of air quality monitoring stations: the manual ones, which collect air samples for 24 hours at a time, twice a week, and continuous stations, known as CAAQMS (continuous ambient air quality monitoring stations), which take readings through the day.
{{/usCountry}}There are two kinds of air quality monitoring stations: the manual ones, which collect air samples for 24 hours at a time, twice a week, and continuous stations, known as CAAQMS (continuous ambient air quality monitoring stations), which take readings through the day.
{{/usCountry}}The daily AQI comes from the continuous ones.
In Delhi, the stations are run mainly by the city’s pollution control committee DPCC, CPCB, the India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology (IITM).
The starting point for how many monitoring stations a city must have is population. Under criteria that CPCB drew up with the state boards in 2019, using 2011 Census figures, a city must have at least:
Four stations, one of them continuous, for a population of 100,000 to 500,000 people
- Six stations, three of them continuous, for 500,000 to 1 million people
- Eight, including five continuous, for 1 million to 5 million
- Sixteen, among them 12 continuous, for populations over 5 million
- But these conditions were drawn using the 2011 Census, the latest official count of India’s population. Fifteen years on, the actual population figures are likely to be far higher for some of the cities.
In Gurugram, the 2011 Census counted the city’s population at 886,519, which puts its minimum requirement at three CAAQMS and three manual stations, according to figures the environment ministry gave Parliament in 2024. The NCR city’s population is expected to be far higher now. But, under the rules, Gurugram has an adequate number of continuous monitors, at four.
The seven other big cities in CPCB’s daily air quality bulletin all had more than 5 million people in 2011 once their suburbs were counted, placing them in the band with the necessity to have the most number of stations.
Mumbai (26), Hyderabad (14) and Bengaluru (14) have more than the 12 continuous stations that the band requires. Chennai, with nine, and Ahmedabad, with 10 including Vatva, which the bulletin lists separately, have fewer. Pune lists nine and Kolkata seven, but the bulletin counts their suburbs separately: with Pimpri-Chinchwad’s eight, the Pune area has 17, and with Howrah’s five and Barrackpore’s one, the Kolkata area has 13.
Much of this could also depend on where a city’s boundary is drawn. In a late-2023 filing, in a case the NGT took up over a news report that questioned why Kolkata had fewer air quality monitors than Delhi and Mumbai, CPCB put the minimum for the Kolkata municipal area, home to about 4.5 million people in 2011, at five continuous stations. For the wider urban agglomeration, including Howrah and Barrackpore, of about 14 million, it put the minimum at 12. The area had 11 then. With more stations in Howrah since, it now has 13.
In 2019, the NGT recorded that, by CPCB’s criteria, India needed about 800 more continuous stations and 1,250 more manual ones.
For Delhi-NCR, the Commission for Air Quality Management, which oversees air pollution mitigation measures for the region, earlier this year said the network of monitors will be increased on the basis of area too.
Under these norms, “one monitoring station is envisaged per 25 sq. km.” in Delhi and the cities that adjoin it (Ghaziabad, Noida, Greater Noida, Gurugram, Faridabad and Sonipat), it said in May, repeating norms it first set out in February. In practice, that would mean one station for every square of a grid 5km a side. Other district headquarters and cities in NCR are to get one per 50 sqkm, it said.
In October, Union environment minister Bhupender Yadav said Delhi’s expansion followed a 5km-by-5km grid, and that the grid would be extended to other NCR cities.
Also read: Supreme Court says vehicular emission major contributor to Delhi-NCR air pollution
Where a station should be set up
Most stations are meant to capture the air quality of an area: a residential neighbourhood, a commercial district or an industrial zone, according to CPCB, which says the network is designed around population and around pollution from different activities such as transport, commerce and industry.
In the most populous cities, the 12 continuous stations are to be split among four residential areas, three commercial areas, two industrial areas and three places with high road traffic.
For a station to reflect the air quality of its area, CPCB’s guidelines say its air intake (the opening through which the monitor draws in outdoor air) must be between 3 metres and 10 metres off the ground. It must be more than 20m away from trees. Any building must be further away than twice its height above the intake. For instance, if a building rises 10 metres above the intake, the monitor must be more than 20 metres from it.
The monitor must have unrestricted airflow on three of its four sides, and be located at least 25m from household chimneys, 200m from unpaved roads, and it cannot be in “a confined place, corner or a balcony”.
The busier a nearby road, the further back monitors for ozone and nitrogen oxides gases must be. They need to be at least 10m from a road that sees up to 10,000 vehicles a day, and at least 250m from one with 110,000 or more.
These rules exist because a monitor’s surroundings change what it records. US rules, which India’s closely mirror, explain that buildings and trees can soak up gases such as ozone and nitrogen dioxide and block the flow of air, and that trees give particles surfaces to settle on. CPCB’s guidelines call a site representative if its data reflect “the concentrations of various pollutants and their variations in the area”.
The stations meant to capture air quality near traffic-dense areas work the other way round. They are put beside busy streets on purpose, to record what pedestrians on pavements along these stretches would be exposed to.
For these stations, the air intake must be 3m above the street level, high enough to avoid dust thrown up from the road.
There are also ‘background stations’ that are supposed to measure pollutants blowing into and out of a city. In cities of more than 5 million people, two of the four manual stations are supposed to go upwind and downwind of it.
In September 2020, the Comptroller and Auditor General (CAG) inspected 13 continuous stations in Delhi and found that all of them were close to trees on more than one side. Two were next to roads with heavy traffic, and three were near tall buildings and construction sites. In its 2022 report, the CAG said this indicated “possible inaccuracies in the data generated by these CAAQMSs, rendering the AQI values unreliable”. The Delhi government’s environment department replied in October 2021 that it would act to comply with CPCB’s norms.
The inspection was part of an audit covering the period up to March 31, 2021, and the report followed in 2022 after the government’s reply. It was tabled in the Delhi Assembly only on April 1, 2025.
A joint DPCC-CPCB committee found in 2022 that most stations met CPCB’s siting-rules, according to the environment department’s action-taken report, cited by the Delhi Assembly’s Public Accounts Committee, PTI reported.
Three stations, at Anand Vihar, RK Puram and Jahangirpuri, had site-concerns but were not moved. CAQM cited their location in dense, polluted areas and the need for unbroken long-term data.
In March 2026, the committee said several stations still did not fully follow CPCB’s rules and recommended that the department comply and remove obstacles.
Also read: Air quality scales ‘severe’ on all monitors Delhi had last Diwali
AQI and what next
A station’s readings count towards the day’s AQI only if it has data for at least three pollutants, each with at least 16 hours of readings, according to CPCB’s rules. One of them must be particulate matter 2.5 or 10 (PM 2.5 or PM10), particles 2.5 or 10 micrometres across or smaller.
The number of stations that make the cut changes from day to day. On November 2, Bengaluru’s AQI came from four of its 14 stations. And CPCB’s bulletin said there was “insufficient data” to record the AQI for three NCR towns – Baghpat, Bahadurgarh and Jind – that day.
Delhi’s network has grown from 18 continuous stations in April 2017 to 38 by March 2021, and to 59 on CPCB’s bulletin now. According to CAQM, another 46 stations are proposed across NCR, 14 of which are meant for Delhi, 16 for Haryana, 15 for Uttar Pradesh and one for Rajasthan. In May this year, the CAQM urged NCR states to install them quickly.