CPCB explains why PM readings above 1,000 µg/m³ dropped out of Delhi's Diwali AQI
In one line: CPCB says its analysers and validation rules stop at 1,000 µg/m³, so the worst Diwali hours in Delhi were left out of the official AQI, and it is reviewing the method.
Published 11 November 2026. Written by Pratidin from the reports linked at the end; every fact checked by a separate review before publishing. How we work
The Central Pollution Control Board (CPCB) said on 10 November 2026 that hourly particulate matter (PM) readings above 1,000 microgrammes per cubic metre (µg/m³) are left out of the National Air Quality Index (NAQI) under its existing method. The analysers deployed when the NAQI was introduced in 2015 were calibrated to measure 0 to 1,000 µg/m³. The automated validation system treats readings beyond that range as outside the calibrated range and excludes them, and excluded hours are not counted in the station's 24-hour average. This, the CPCB said, can create apparent data gaps during extreme episodes such as Diwali fireworks. It added that "internal deliberations and consultations with respect to processing of data beyond system design range are underway".
The explanation followed a row over blank readings on Diwali night, 8 to 9 November. The Delhi Pollution Control Committee (DPCC) later released hourly data showing PM2.5 above 1,000 µg/m³ at many stations, including 4,985 µg/m³ at Jahangirpuri and 3,744 µg/m³ at Mundka, with several others above 2,000. WION reported that 36 of Delhi's 59 stations had stopped reporting by 1 am, and that all 125 station-hours missing from CPCB records had PM2.5 above 1,000 µg/m³. DPCC's 2019 technical specifications require PM analysers with a 0 to 1,000 µg/m³ range. The Aam Aadmi Party alleged data manipulation; officials called it a technical issue. For scale, the WHO's 24-hour guideline for PM2.5 is 15 µg/m³.
The CPCB also defended the cap of the AQI itself at 500. It said the cap is a design feature, consistent with international practice such as the U.S. EPA scale, and that any value above 500 stays in the 'Severe' category (401 to 500), since a higher number would not change the health advice. The index, developed with IIT Kanpur, is calculated hourly from sub-indices for eight pollutants, and the worst sub-index sets the AQI. Most pollutants use a running 24-hour average needing at least 16 hours of data. The CPCB said private apps often differ because they use low-cost sensors or short averaging periods. Monitoring has grown from 30 continuous stations in 2014-15 to 634 now, and in Delhi from 10 to 59. The issue matters because dropping the worst hours lowers the reported daily average just when exposure is highest.
Prelims facts
- CPCB's analysers are calibrated for 0 to 1,000 µg/m³; hourly PM readings above that are excluded from the NAQI and from the 24-hour average.
- On Diwali night 2026, DPCC data showed PM2.5 of 4,985 µg/m³ at Jahangirpuri and 3,744 µg/m³ at Mundka; the WHO 24-hour guideline is 15 µg/m³.
- The NAQI, introduced in 2015, uses eight pollutants and is capped at 500; 401 to 500 is 'Severe'.
- Continuous monitoring stations have grown from 30 in 2014-15 to 634; Delhi has 59.
- CPCB says it is consulting on how to process data beyond the system's design range.
Quick recall
- Calibrated range of CPCB PM analysers?
- 0 to 1,000 µg/m³.
- What happens to hourly PM readings above 1,000 µg/m³?
- They are excluded by automated validation and left out of the 24-hour average.
- Upper cap of the NAQI and its top category?
- 500; 'Severe' is 401 to 500.
- Highest Diwali-night PM2.5 reading cited by DPCC in 2026?
- 4,985 µg/m³ at Jahangirpuri.
- Year NAQI was introduced and with which institute?
- 2015, developed by CPCB with IIT Kanpur.
- Minimum hours of data for a 24-hour average in NAQI?
- 16 hours.
- WHO 2021 24-hour guideline for PM2.5?
- 15 µg/m³.
- Continuous monitoring stations nationwide, 2014-15 vs now?
- 30 then, 634 now.
Prelims practice question
Consider the following:
1. Ammonia
2. Lead
3. Carbon dioxide
4. Ozone
How many of the above are among the eight pollutants used to calculate India's National Air Quality Index?
- Only one
- Only two
- Only three
- All four
Show answer
Answer: (c) Only three. The NAQI uses PM2.5, PM10, carbon monoxide, nitrogen dioxide, sulphur dioxide, ammonia, ozone and lead. Carbon dioxide is not one of them.
Use this in UPSC Mains: previous-year questions
Recurring theme: Urban air pollution, air quality standards and the reliability of monitoring data
- How to use this
Use the Diwali 2026 episode to argue that meeting WHO's tighter guidelines needs better monitoring, not only lower emissions.
- WHO's 2021 24-hour PM2.5 guideline is 15 µg/m³; Delhi's Jahangirpuri recorded 4,985 µg/m³ on Diwali night 2026.
- CPCB analysers are calibrated only to 1,000 µg/m³, so higher hourly readings drop out of the official 24-hour average.
- CPCB is now consulting on processing data beyond the design range, a reform India's clean air programme should adopt.
NCAP targets depend on monitoring data; the story shows a gap in that data.
The NAQI's eight pollutants include carbon monoxide, nitrogen dioxide and sulphur dioxide but not carbon dioxide or methane, which is the core of this question.
Mains practice question
Delhi's official air quality data went blank during its worst pollution hours on Diwali night in 2026. Discuss the limitations of India's air quality monitoring and index, and suggest reforms. (150 words)
Model answer
On Diwali night 2026, Delhi's PM2.5 crossed 4,000 µg/m³ at Jahangirpuri, yet many hours vanished from official records because CPCB analysers and validation rules stop at 1,000 µg/m³.
Limitations
- Instrument range: analysers calibrated for 0 to 1,000 µg/m³; readings beyond are excluded.
- Averaging bias: excluded hours drop out of the 24-hour average, so the daily figure understates exposure.
- Index cap: the NAQI stops at 500, hiding how far beyond 'Severe' the air is.
- Trust deficit: gaps at 36 of 59 Delhi stations led to allegations of manipulation.
- Comparability: private apps use low-cost sensors and short averages, so citizens see conflicting numbers.
Reforms
- Upgrade analysers and validation rules to record data beyond 1,000 µg/m³, as CPCB is now considering.
- Publish raw hourly data with flags instead of deleting it.
- Report peak exposure alongside the 24-hour AQI during episodes like Diwali.
- Calibrate and integrate low-cost sensors to widen coverage.
- Align targets with WHO's 2021 guideline of 15 µg/m³ for 24-hour PM2.5 over time.
Accurate, transparent data is the first condition for clean air policy that people believe.
The basics
Why this matters
Air pollution policy runs on numbers. Courts order curbs, the Commission for Air Quality Management triggers emergency steps, and citizens decide whether to step outside, all on the basis of the Air Quality Index (AQI). On Diwali night in 2026 those numbers went missing in Delhi exactly when the air was worst. The reason lies in how instruments are built and how the index is calculated.
What PM2.5 is
PM2.5 means particles with a diameter of 2.5 micrometres or less, small enough to enter deep into the lungs and the bloodstream. Fireworks, vehicles, biomass burning and dust all add to it. See PM2.5 and PM10.
How a station turns readings into an index
A Continuous Ambient Air Quality Monitoring Stations site measures pollutants every hour. The National Air Quality Index then converts each pollutant's average into a sub-index and reports the worst one.
- 1MeasureAnalysers record hourly concentrations; PM analysers are calibrated for 0 to 1,000 µg/m³.
- 2ValidateAutomated checks drop readings outside the calibrated range.
- 3AverageA running 24-hour average (8-hour for carbon monoxide and ozone), needing at least 16 hours of data.
- 4Sub-indexEach of eight pollutants gets a sub-index; at least three pollutants, including PM2.5 or PM10, are needed.
- 5ReportThe highest sub-index is the AQI, capped at 500.
Where the gap comes from
Step two is the problem. When PM2.5 crossed 1,000 µg/m³ on Diwali night, the hour was treated as invalid and dropped. A 24-hour average without its worst hours is lower than the true exposure. The CPCB says it is now consulting on how to process data beyond the design range. The WHO Air Quality Guidelines 2021 show how far even normal days in Delhi are from health-based limits.
Go deeper
Why it matters for UPSC
Prelims asks which pollutants make up the AQI and what PM2.5 is. Mains asks about the National Clean Air Programme, WHO guidelines and urban air pollution. This story adds a governance angle: the reliability of environmental data.
The core idea
The National Air Quality Index is built from hourly data at Continuous Ambient Air Quality Monitoring Stations. Its main driver in Delhi is PM2.5 and PM10. When readings exceed what instruments are calibrated for, they are dropped, so the 24-hour average understates exposure. The gap between Indian standards and the WHO Air Quality Guidelines 2021 shows why accurate peaks matter.
Numbers and dates to remember
- Analyser range: 0 to 1,000 µg/m³; NAQI capped at 500; 'Severe' is 401 to 500.
- NAQI introduced in 2015; eight pollutants.
- Diwali night 8 to 9 November 2026: 4,985 µg/m³ at Jahangirpuri, 3,744 at Mundka.
- 36 of Delhi's 59 stations stopped reporting by 1 am.
- Stations: 30 in 2014-15 to 634 nationwide.
Where to go next
- National Air Quality Index: how the 0 to 500 index is built.
- PM2.5 and PM10: the particles behind Delhi's worst days.
- Continuous Ambient Air Quality Monitoring Stations: the network that feeds the index.
- WHO Air Quality Guidelines 2021: the health benchmarks India is far from.
Go deeper: a design choice or a data problem?
The CPCB's case is that the method is sound. The AQI exists to tell people which health category they are in, and anything above 500 is already 'Severe'. Analysers have a calibrated range, and readings outside it are less reliable, so automated validation drops them. A 24-hour average reflects sustained exposure better than a single peak. Private apps that show '1,000 AQI' often use low-cost sensors or short averaging, so they are not comparable with the National Air Quality Index.
The critics' case is that removing the worst hours biases the daily average downwards at the very moment health risk peaks. On Diwali night, 125 station-hours missing from CPCB records all had PM2.5 above 1,000 µg/m³, and stations dropped out one after another between 7 pm and 1 am. Blank screens also feed suspicion, as seen in the allegations of data manipulation. Instruments with a wider range, or flagged rather than deleted data, would keep the record honest.
There is also a standards gap. India's 24-hour standard for PM2.5 and PM10 is far looser than the WHO Air Quality Guidelines 2021. Expanding Continuous Ambient Air Quality Monitoring Stations from 30 to 634 has improved coverage, but the November 2026 episode shows that the quality of the record matters as much as the number of stations. CPCB's own statement that it is consulting on data beyond the design range is the first step to fixing it.
National Air Quality Index
How the 0 to 500 index is built.
In one line: India's official index that turns hourly pollutant data into one number from 0 to 500 with six health categories.
How it works
Introduced in 2015 and developed by the CPCB with IIT Kanpur, the index uses eight pollutants: PM2.5, PM10, carbon monoxide, nitrogen dioxide, sulphur dioxide, ammonia, ozone and lead. Each gets a sub-index; the highest one becomes the AQI. Most pollutants use a 24-hour running average needing at least 16 hours of data; carbon monoxide and ozone use eight hours. At least three pollutants, including PM2.5 or PM10, are required.
Categories
Good (0 to 50), Satisfactory (51 to 100), Moderately polluted (101 to 200), Poor (201 to 300), Very poor (301 to 400) and Severe (401 to 500). The index is capped at 500.
Where to go next
National Air Quality Index: every story that connects to it (3)
PM2.5 and PM10
The particles behind Delhi's worst days.
In one line: Particulate matter is classified by size: PM10 is 10 micrometres or smaller, and PM2.5 is 2.5 micrometres or smaller.
Why size matters
PM10 particles lodge in the nose, throat and upper airways. PM2.5 particles go deeper, into the lungs and even the bloodstream, which links them to heart and lung disease. Sources include vehicle exhaust, burning of crop residue and waste, construction dust, industry and fireworks.
Indian standards
India's National Ambient Air Quality Standards, 2009 set PM2.5 limits of 60 µg/m³ over 24 hours and 40 µg/m³ annually, and PM10 limits of 100 and 60 µg/m³.
Why it is in the news
On Diwali night 2026, hourly PM2.5 in Delhi crossed 4,000 µg/m³ at one station, far above what analysers are calibrated to record.
Where to go next
Continuous Ambient Air Quality Monitoring Stations
The network that feeds the index.
In one line: Automatic stations that measure air pollutants round the clock and send hourly data to the CPCB.
Scale
The number of such stations has grown from 30 in 2014-15 to 634, and in Delhi from 10 to 59. In Delhi, stations are run by agencies including the CPCB, the Delhi Pollution Control Committee and the Indian Institute of Tropical Meteorology.
Limits
Each analyser works within a calibrated range. DPCC's 2019 specifications set 0 to 1,000 µg/m³ for PM analysers. Readings above this are dropped by automated validation, which is why stations appeared to go blank on Diwali night 2026. Stations also cost a lot, so cities rely on a few sites, while cheaper sensors used by private apps are less accurate.
Where to go next
WHO Air Quality Guidelines 2021
The health benchmarks India is far from.
In one line: The World Health Organization's 2021 guidelines tightened the recommended limits for major air pollutants, the first update since 2005.
The numbers
For PM2.5, the guideline is 5 µg/m³ as an annual mean and 15 µg/m³ as a 24-hour mean, down from 10 and 25 in 2005. For PM10 it is 15 µg/m³ annually and 45 µg/m³ over 24 hours. The guidelines are advisory; countries set their own legal standards.
Why it matters here
India's PM2.5 standard of 60 µg/m³ over 24 hours is four times the WHO guideline. Diwali readings in Delhi in 2026 were hundreds of times above it. The guidelines frame the health argument for tighter Indian standards and better monitoring.
Where to go next
Take the 11 November 2026 quiz: 30 Prelims-style questions with answers