Delhi's September AQI hits a five-year low even as PM2.5 and PM10 levels rose
How can the air quality index improve while the main pollutants get worse? The answer is in how the index is built.
Published 2 October 2026. Written by Pratidin from the reports linked at the end; every fact checked by a separate review before publishing. How we work
Delhi recorded an average Air Quality Index (AQI) of 102 in September 2026, the lowest September average in five years, according to figures released by the Union Environment Ministry on 30 September. The September average was 105 in 2025, 105 in 2024, 108 in 2023 and 104 in 2022. Between January and September 2026 the city had 194 days with AQI below 200, against 187 in 2025, 188 in 2024, 193 in 2023 and 146 in 2022, and 79 days in the poor to severe range, against 86, 86, 80 and 127 in those years. Average PM2.5 from 1 January to 28 September was 69 micrograms per cubic metre (µg/m³), the same as in 2025.
Central Pollution Control Board (CPCB) data cited by The Indian Express show that particulate pollution actually rose in September: PM10 went from 104 µg/m³ in September 2025 to 106 µg/m³, and PM2.5 from 35 µg/m³ to 39 µg/m³. The report explains the apparent paradox through the way the index is built. The overall AQI is the highest of the sub-indices of individual pollutants, not an average of them. Lower levels of nitrogen dioxide and ozone this year, according to the report, pulled down the composite index even as particulate matter increased.
The National Air Quality Index, launched on 17 October 2014, converts readings of eight pollutants (PM10, PM2.5, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead) into one number from 0 to 500 with six categories, from good (0 to 50) to severe (401 to 500). An AQI of 102 sits just inside the moderate band (101 to 200). The figures come before winter, when stubble burning, festival emissions and calm, cold weather usually push Delhi's air into the poor and severe ranges. The case shows why a single index, useful for public communication, should be read alongside pollutant-wise data when judging whether policy is working.
Prelims facts
- Delhi's average AQI in September 2026 was 102, the lowest September average in five years (105 in 2025).
- Despite this, PM10 rose from 104 to 106 µg/m³ and PM2.5 from 35 to 39 µg/m³ in September, according to CPCB data cited by The Indian Express.
- The overall AQI equals the worst sub-index among the pollutants measured, not their average.
- India's National AQI, launched on 17 October 2014, covers eight pollutants and six categories from good to severe.
- Overall AQI is computed only when at least three pollutants are measured, one of which must be PM2.5 or PM10.
Quick recall
- Delhi's average AQI in September 2026?
- 102, the lowest September average in five years.
- How is the overall AQI determined?
- It is the worst (highest) sub-index among the pollutants measured.
- When was India's National AQI launched?
- 17 October 2014.
- How many pollutants does the National AQI cover?
- Eight: PM10, PM2.5, NO2, SO2, CO, O3, NH3 and lead.
- AQI range for the 'moderate' category?
- 101 to 200.
- Minimum data needed to compute the overall AQI?
- At least three pollutants, one of which must be PM2.5 or PM10.
- Delhi's September PM2.5, 2025 and 2026 (CPCB data cited by The Indian Express)?
- 35 µg/m³ in 2025 and 39 µg/m³ in 2026.
- India's annual PM2.5 standard under the NAAQS, 2009?
- 40 µg/m³ (24-hour standard: 60 µg/m³).
Prelims practice question
With reference to India's National Air Quality Index (AQI), consider the following statements:
1. The overall AQI is the average of the sub-indices of all pollutants measured.
2. Ammonia and lead are among the pollutants covered.
3. An AQI value of 102 falls in the 'moderate' category.
Which of the statements given above are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer
Answer: (b) 2 and 3 only. 1 is wrong: the overall AQI is the worst (highest) sub-index. 2 is correct: the eight pollutants are PM10, PM2.5, NO2, SO2, CO, O3, NH3 and lead. 3 is correct: 101 to 200 is the moderate band.
Use this in UPSC Mains: previous-year questions
Recurring theme: Air quality measurement, standards and pollution control in Indian cities
- How to use this
Use it to show the gap between WHO's 2021 PM2.5 guideline and Indian standards and actual Delhi levels, and to argue NCAP must track particulates, not just the composite AQI.
- WHO's 2021 guidelines set annual PM2.5 at 5 µg/m³, far below India's standard of 40; Delhi's average PM2.5 from 1 January to 28 September 2026 was 69 µg/m³, unchanged from 2025.
- Delhi's September 2026 AQI of 102 was a five-year low, yet CPCB data cited by The Indian Express show PM2.5 rose from 35 to 39 µg/m³ and PM10 from 104 to 106 µg/m³.
- Since overall AQI is the worst sub-index of eight pollutants, NCAP should track PM2.5 and PM10 city by city, publish pollutant-wise trends and use source apportionment.
Directly tests which pollutants make up the AQI, the concept that explains Delhi's September paradox.
Mains practice question
A falling Air Quality Index does not always mean cleaner air. Explain with reference to Delhi's September 2026 data, and suggest how air quality reporting can better guide policy. (150 words)
Model answer
Delhi's average AQI in September 2026 was 102, a five-year low, yet CPCB data cited by The Indian Express show PM10 rose from 104 to 106 µg/m³ and PM2.5 from 35 to 39 µg/m³.
Why the index can mislead
- The overall AQI is the worst sub-index among eight pollutants, not their average.
- If another pollutant, such as ozone or nitrogen dioxide, falls, the composite can improve while particulates rise.
- Monthly averages hide daily peaks and local hotspots.
Why particulates matter most
- PM2.5 enters deep into the lungs and is the main health risk.
- WHO's 2021 guidelines (annual PM2.5 of 5 µg/m³) are far below India's standard of 40.
Better reporting
- Publish pollutant-wise trends with the AQI.
- Track PM2.5 and PM10 against NCAP targets city by city.
- Use source apportionment to link trends to actions by CAQM and States.
- Report health-based exposure, not only index categories.
The AQI is a communication tool; policy must be judged on the pollutants that harm health.
The basics
Why this matters
Most people learn about air pollution through one number: the AQI. Delhi's September 2026 figures show that this number can move one way while the most harmful pollutants move the other. Understanding why is a basic skill for reading environmental data.
How the index is built
The National Air Quality Index takes readings of eight pollutants, turns each into a sub-index on a 0 to 500 scale, and reports the worst one as the overall AQI. At least three pollutants must be measured, and one must be PM2.5 or PM10.
- 1MeasureMonitoring stations record concentrations of pollutants such as PM2.5, PM10, NO2 and ozone.
- 2ConvertEach concentration is converted into a sub-index, using averaging periods of 24 hours for most pollutants and 8 hours for CO and ozone.
- 3Pick the worstThe highest sub-index becomes the overall AQI.
- 4LabelThe value is placed in one of six bands, from good (0 to 50) to severe (401 to 500).
Why the paradox happened
- PM10 rose from 104 to 106 µg/m³
- PM2.5 rose from 35 to 39 µg/m³
- Average AQI fell from 105 to 102
- Lower NO2 and ozone pulled the index down, per the Indian Express report
Particulate matter is the pollutant that matters most for health. India's limits for it are set in the National Ambient Air Quality Standards, which are looser than the World Health Organization's guidelines.
Who acts on Delhi's air
Delhi and its neighbouring States share one airshed. The Commission for Air Quality Management coordinates action across the National Capital Region, while the National Clean Air Programme sets city-level targets for reducing particulate pollution across the country.
Go deeper
In one line: Delhi's average AQI in September 2026 was 102, a five-year low, even though PM10 and PM2.5 levels rose, because the AQI reports only the worst pollutant's sub-index.
Why it matters for UPSC
Air pollution is a regular GS3 topic, and Prelims has asked which gases count in the AQI. This story shows how the index works and why its components matter.
The core idea
The National Air Quality Index converts eight pollutants into sub-indices and reports the highest. Health-based limits for those pollutants are set in the National Ambient Air Quality Standards. Action in Delhi and its neighbours is coordinated by the Commission for Air Quality Management, and city targets come from the National Clean Air Programme.
Numbers and dates to remember
- September average AQI: 102 (2026), 105 (2025), 105 (2024), 108 (2023), 104 (2022).
- September PM10: 104 to 106 µg/m³; PM2.5: 35 to 39 µg/m³ (2025 to 2026).
- January to September 2026: 194 days below AQI 200; 79 poor to severe days.
- National AQI launched 17 October 2014; eight pollutants; six bands.
Where to go next
- National Air Quality Index: how one number is built from eight pollutants.
- National Ambient Air Quality Standards: India's legal limits for pollutants.
- Commission for Air Quality Management: the body for Delhi-NCR's airshed.
- National Clean Air Programme: India's plan to cut particulate pollution.
Go deeper: what should we measure progress by?
The case for a single index. The National Air Quality Index was designed, in the words used at its launch, as "one number, one colour and one description". It lets people act on health advisories without reading technical data, and it is easy to compare across days and cities.
The limits. Because it reports only the worst sub-index, the AQI can improve when a less harmful pollutant falls, even if particulates rise, as Delhi's September data show. Monthly averages also hide daily spikes. PM2.5 is the main health risk, and India's annual limit of 40 µg/m³ under the National Ambient Air Quality Standards is eight times the WHO's 2021 guideline of 5 µg/m³.
What policy uses. The National Clean Air Programme measures success by reductions in particulate concentrations (its target is set for PM10), not by AQI categories. The Commission for Air Quality Management runs the Graded Response Action Plan, which triggers stage-wise curbs as AQI worsens.
The takeaway. Use the AQI for communication and daily response, but judge long-term progress on pollutant concentrations, especially PM2.5, measured against both Indian standards and WHO guidelines.
National Air Quality Index
How eight pollutants become one number.
In one line: India's National Air Quality Index, launched on 17 October 2014, turns readings of eight pollutants into a single number from 0 to 500.
The pollutants and bands
The eight are PM10, PM2.5, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead. The six bands are good (0 to 50), satisfactory (51 to 100), moderate (101 to 200), poor (201 to 300), very poor (301 to 400) and severe (401 to 500).
The rules
Each pollutant gets a sub-index; the worst sub-index is the overall AQI. At least three pollutants must be measured, one of them PM2.5 or PM10. Most pollutants use 24-hour averages; carbon monoxide and ozone use 8-hour averages.
Where to go next
National Ambient Air Quality Standards
India's legal limits for the pollutants in the AQI.
In one line: The National Ambient Air Quality Standards (NAAQS), last revised in 2009, set legal limits for 12 pollutants in outdoor air in India.
Key limits
For PM2.5 the limit is 40 µg/m³ as an annual average and 60 µg/m³ over 24 hours. For PM10 it is 60 µg/m³ annually and 100 µg/m³ over 24 hours.
How they compare
The World Health Organization's 2021 guidelines are much stricter: 5 µg/m³ annual and 15 µg/m³ over 24 hours for PM2.5. Critics argue India's standards should be tightened; others say interim targets are more realistic for now.
Why it is in the news
Delhi's September PM2.5 of 39 µg/m³ is below India's 24-hour limit but far above WHO's guideline.
Where to go next
National Ambient Air Quality Standards: every story that connects to it (2)
Commission for Air Quality Management
The statutory body that manages Delhi-NCR's shared airshed.
In one line: The Commission for Air Quality Management (CAQM) is a statutory body for the National Capital Region and adjoining areas, set up under an Act of 2021.
Origin
It was first created by an ordinance in October 2020 and then by the Commission for Air Quality Management in National Capital Region and Adjoining Areas Act, 2021. It replaced the Environment Pollution (Prevention and Control) Authority, which had been set up in 1998 under the Environment (Protection) Act on the Supreme Court's direction.
Powers
It can issue binding directions to the Centre's and States' agencies in its area, and it runs the Graded Response Action Plan (GRAP), which imposes curbs in stages as air quality worsens.
Where to go next
Commission for Air Quality Management: every story that connects to it (2)
National Clean Air Programme
India's national plan to cut particulate pollution city by city.
In one line: The National Clean Air Programme (NCAP), launched by the Environment Ministry in January 2019, is India's plan to reduce particulate pollution in cities that do not meet the standards.
How it works
It covers 131 non-attainment cities, which consistently exceed the NAAQS. Each city has an action plan for dust, vehicles, industry and waste burning. The target was revised to a reduction of up to 40% in PM10 levels by 2025-26, compared with 2017-18.
Why it matters here
NCAP judges progress by particulate concentrations, not by AQI categories, so Delhi's rising September PM levels matter more for NCAP than the lower AQI.
Where to go next
National Clean Air Programme: every story that connects to it (2)
Prelims-style quiz
Consider the following gases:
1. Carbon dioxide
2. Ozone
3. Ammonia
4. Methane
How many of the above are among the pollutants used in India's National Air Quality Index?- Only one
- Only two
- Only three
- All four
Show answer
Answer: (b) Only two. Ozone and ammonia are included. Carbon dioxide and methane are greenhouse gases but are not part of the AQI's eight pollutants.
Consider the following statements about Delhi's air in 2026:
1. The average AQI in September 2026 was lower than in September 2025.
2. Average PM2.5 from January to late September 2026 was lower than in 2025.
3. The number of poor to severe days from January to September 2026 was lower than in 2022.
How many of the statements given above are correct?- Only one
- Only two
- All three
- None
Show answer
Answer: (b) Only two. 1 is correct: 102 against 105. 2 is wrong: it was 69 µg/m³, the same as in 2025. 3 is correct: 79 days against 127 in 2022.
Consider the following statements:
Statement-I: Delhi's average AQI can fall in a month even when PM2.5 and PM10 levels rise.
Statement-II: The overall AQI is the highest of the sub-indices of the pollutants measured.
Which one of the following is correct in respect of the above statements?- Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
- Both Statement-I and Statement-II are correct and Statement-II does not explain Statement-I
- Statement-I is correct but Statement-II is incorrect
- Statement-I is incorrect but Statement-II is correct
Show answer
Answer: (a) Both Statement-I and Statement-II are correct and Statement-II explains Statement-I. Both are correct. Because the AQI reports the worst sub-index, a fall in other pollutants such as ozone or NO2 can lower the index even while particulates rise; this is the explanation in the Indian Express report.
Which one of the following statements about the Commission for Air Quality Management (CAQM) is correct?
- It was created by an executive order of the Delhi Government in 2017
- It is a statutory body set up under a 2021 Act for the National Capital Region and adjoining areas
- It is an attached office of the Central Pollution Control Board
- It only covers the National Capital Territory of Delhi
Show answer
Answer: (b) It is a statutory body set up under a 2021 Act for the National Capital Region and adjoining areas. CAQM was set up under the Commission for Air Quality Management in National Capital Region and Adjoining Areas Act, 2021, replacing the earlier Environment Pollution (Prevention and Control) Authority. Its jurisdiction covers the NCR and adjoining areas, not Delhi alone.
Consider the following statements:
1. The World Health Organization's 2021 guideline for annual average PM2.5 is 5 µg/m³.
2. India's National Ambient Air Quality Standards set the annual PM2.5 limit at 40 µg/m³.
Which of the statements given above is/are correct?- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Show answer
Answer: (c) Both 1 and 2. Both are correct. WHO lowered its annual PM2.5 guideline from 10 to 5 µg/m³ in 2021; India's 2009 NAAQS set it at 40 µg/m³.