India AQI Categories and How to Read Them

India's AQI turns up to eight pollutant measurements into one number from 0 to 500, and that number is set by whichever pollutant is worst, not by an average of all of them.

What is India's National Air Quality Index?

The National Air Quality Index was designed for the Central Pollution Control Board (CPCB) by an expert group and published in October 2014.¹ Its date of “launch” is reported two ways. A government reference page dates it to 17 October 2014, citing an environment ministry press release from that day;² the environment ministry's own answer to the Lok Sabha in July 2015 said it was launched in April 2015.³ The two are not necessarily in conflict: one is the publication of the method, the other its public rollout in cities.

What are the six AQI categories?

The index runs from 0 to 500 in six bands. The colours below are the ones CPCB uses in its own daily bulletin.⁴ On this site they are used only for data, and always with the category name beside them.

India's AQI categories, with CPCB's colours and health statements
Category	AQI	Possible health impact (CPCB)
Good	0–50	Minimal impact
Satisfactory	51–100	Minor breathing discomfort to sensitive people
Moderate	101–200	Breathing discomfort to the people with lungs, asthma and heart diseases
Poor	201–300	Breathing discomfort to most people on prolonged exposure
Very Poor	301–400	Respiratory illness on prolonged exposure
Severe	401–500	Affects healthy people and seriously impacts those with existing diseases

Each band is defined separately for each pollutant. For PM2.5, “Good” means a 24-hour average up to 30 µg/m³; “Severe” begins above 250 µg/m³. For PM10, the Moderate band is unusually wide, from 101 to 250 µg/m³.¹ The full CPCB breakpoint table:

CPCB AQI breakpoints, concentrations in µg/m³ unless stated
Category	PM10	PM2.5	NO₂	O₃ (8-h)	CO (8-h, mg/m³)	SO₂	NH₃
Good	0–50	0–30	0–40	0–50	0–1.0	0–40	0–200
Satisfactory	51–100	31–60	41–80	51–100	1.1–2.0	41–80	201–400
Moderate	101–250	61–90	81–180	101–168	2.1–10	81–380	401–800
Poor	251–350	91–120	181–280	169–208	10–17	381–800	801–1200
Very Poor	351–430	121–250	281–400	209–748*	17–34	801–1600	1200–1800
Severe	430+	250+	400+	748+*	34+	1600+	1800+

* For ozone, CPCB lists 748 as a one-hour value used only for calculation.

Concentrations are in µg/m³ except CO, in mg/m³. PM, NO₂, SO₂ and NH₃ use 24-hour averages; O₃ and CO use 8-hour averages. The CPCB table also lists lead (Pb), which cannot be measured in real time.¹

How is the AQI number calculated?

First, each pollutant gets its own sub-index. Find the band its concentration falls in, then place it proportionally inside that band. In CPCB's notation:¹

I = I_LO + (I_HI − I_LO) × (C − B_LO) ÷ (B_HI − B_LO)

Here C is the measured concentration, B_LO and B_HI are the edges of the band it falls in, and I_LO and I_HI are the index values at those edges.

Then comes the rule that matters most: the AQI is the highest of the sub-indices. The report chose the maximum on purpose, because adding or weighting sub-indices lets a bad pollutant hide behind several good ones.¹ So an AQI of 339 does not mean the air is “339 polluted” overall. It means one pollutant, usually PM2.5, is sitting at that point in its own scale. That pollutant is what the bulletin calls the “prominent pollutant”.

Where does “Delhi's AQI” come from?

The number quoted in news reports is CPCB's daily bulletin value, released at 4 pm and based on the average of the previous 24 hours.⁴ For a city with several monitoring stations, it is the average of the stations' values. CPCB says plainly that averaging across locations is not a scientifically sound approach but is used for simplicity.⁴ In 2024, Delhi's bulletin drew on up to 40 stations; on its thinnest day only 26 reported.⁵

Is this app showing the Indian AQI or the US AQI?

Some phone apps and purifier displays show the US EPA's index, which uses the same 0–500 shape with different breakpoints and category names. In February 2024 the EPA tightened its PM2.5 scale, effective 6 May 2024: “Good” now ends at 9.0 µg/m³ and “Moderate” at 35.4 µg/m³.⁶ India's Good band for PM2.5 goes up to 30 µg/m³.¹

The result is that the two scales disagree in both directions. Two worked examples, using PM2.5 only and each country's published breakpoints:

The same PM2.5 concentration on two scales (24-hour average)
PM2.5	India: band → AQI	US EPA (2024): band → AQI
50 µg/m³	31–60 → 83 Satisfactory	35.5–55.4 → 137 Unhealthy for Sensitive Groups
150 µg/m³	121–250 → 323 Very Poor	125.5–225.4 → 225 Very Unhealthy

Show the working for 150 µg/m³

India: 301 + (400 − 301) × (150 − 121) ÷ (250 − 121) = 323.

US: 201 + (300 − 201) × (150 − 125.5) ÷ (225.4 − 125.5) = 225.

US category colours are not shown because this site reserves colour for India's categories.

So the same 50 µg/m³ reads as a calm green-band number on one scale and a cautionary orange on the other, while at 150 µg/m³ the Indian number is the higher of the two. Neither scale is wrong; they encode different standards. To tell which one an app uses, look at its category names. “Satisfactory”, “Moderate”, “Poor”, “Very Poor” and “Severe” are India's. “Unhealthy for Sensitive Groups”, “Very Unhealthy” and “Hazardous” are the US scale's.⁴⁶

How do India's standards compare with WHO guidelines?

The AQI is a communication scale. Behind it sit legal standards. India's National Ambient Air Quality Standards, notified on 18 November 2009, set PM2.5 limits of 40 µg/m³ as an annual average and 60 µg/m³ over 24 hours.⁷ The WHO's 2021 guideline values are 5 µg/m³ and 15 µg/m³.⁸ The WHO numbers are health guidance, not law.⁹ The gap explains a common surprise: an Indian “Satisfactory” PM2.5 day, up to 60 µg/m³, can be four times the WHO's 24-hour guideline.

Why do two apps show different numbers for the same place?

Usually for one of four reasons:

Different scale. Indian versus US breakpoints, as above.
Different station. A city figure is an average; your nearest station can be far higher or lower. Station siting matters too: a 2024 Newslaundry investigation reported that many Delhi monitors breach CPCB's siting guidelines, for instance by sitting under trees.¹⁰
Different time window. The bulletin is a 24-hour average; many apps show the latest hour, which swings far more.
Different instruments. Some apps blend official monitors with low-cost sensors, which are calibrated differently.

The chapter How do you read Delhi's live air data yourself? shows where to find the official station numbers.

How do you read India's AQI?