# CPCB explains why PM readings above 1,000 µg/m³ dropped out of Delhi's Diwali AQI

URL: https://upsc.help/current-affairs/2026-11-11/cpcb-aqi-1000-cap-diwali-gaps/
Date: 11 November 2026 (2026-11-11)
Source: The Hindu, National
UPSC papers: GS3
Publisher: upsc.help (Pratidin), free and open-source UPSC Civil Services current affairs. Not affiliated with UPSC. Every fact was checked against the source reports by a separate review before publishing.

**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.

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?

(a) Only one
(b) Only two
(c) Only three
(d) All four

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.

## UPSC previous-year questions this connects to

- UPSC Prelims 2016, Prelims: In the cities of our country, which among the following atmospheric gases are normally considered in calculating the value of Air Quality Index?
1. Carbon dioxide
2. Carbon monoxide
3. Nitrogen dioxide
4. Sulfur dioxide
5. Methane
Select the correct answer using the code given below.
(a) 1, 2 and 3 only
(b) 2, 3 and 4 only
(c) 1, 4 and 5 only
(d) 1, 2, 3, 4 and 5 (https://upsc.help/pyq/q325-cities-our-country-among-following-atmospheric-gases/)
- UPSC Mains 2021, GS3: Describe the key points of the revised Global Air Quality Guidelines (AQGs) recently released by the World Health Organisation (WHO). How are these different from its last update in 2005? What changes in India's National Clean Air Programme are required to achieve these revised standards? (https://upsc.help/pyq/q094-describe-key-points-revised-global-air-quality/)
  How to use this story in the answer: Use the Diwali 2026 episode to argue that meeting WHO's tighter guidelines needs better monitoring, not only lower emissions.
- UPSC Mains 2020, GS3: What are the key features of the National Clean Air Programme (NCAP) initiated by the Government of India? (https://upsc.help/pyq/q093-key-features-national-clean-air-programme-ncap/)

## 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.

## 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.

## Sources

- https://www.business-standard.com/india-news/cpcb-explains-why-pm-readings-above-1-000-g-m-excluded-from-official-aqi-126111100157_1.html
- https://thefederal.com/category/states/north/delhi/cpcb-clarifies-on-excluding-extreme-pollution-readings-above-1000-from-aqi-262505
- https://thenewsmill.com/2026/11/cpcb-explains-delhi-aqi-data-gaps-confirms-cap-at-500-in-index-design/
- https://www.wionews.com/india-news/delhi-diwali-pollution-missing-data-pm25-levels-1794281831859
- https://outlookindia.com/national/missing-diwali-pollution-readings-delhi
- https://www.legacyias.com/the-hindu-upsc-news-analysis-for-11-november-2026/