CPCB
DOC

Central Control Room for Air Quality Management

How is AQI Calculated?

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Govt.
of India

Central Pollution Control Board

National Air Quality Index

Ministry of Environment, Forest & Climate Change

Methodology Summary

How is AQI Calculated?

The Air Quality Index transforms complex air quality data for multiple pollutants into a single number, category name, and colour so that the public can easily understand local air quality. India’s National AQI defines six categories based on ambient pollutant concentrations and their likely health impacts. AQ sub-indices and health breakpoints are defined for eight pollutants: PM2.5, PM10, NO2, SO2, CO, O3, NH3, and Pb, for which short-term National Ambient Air Quality Standards are prescribed.

Key Principle

Each pollutant gets a sub-index using notified health breakpoints.

The worst sub-index becomes the overall AQI.

The pollutant with the maximum sub-index is the dominant pollutant.

The Six AQI Categories

Good0–50

AQI category and colour code as defined by CPCB for public communication.

Satisfactory51–100

AQI category and colour code as defined by CPCB for public communication.

Moderately Polluted101–200

AQI category and colour code as defined by CPCB for public communication.

Poor201–300

AQI category and colour code as defined by CPCB for public communication.

Very Poor301–400

AQI category and colour code as defined by CPCB for public communication.

Severe401–500

AQI category and colour code as defined by CPCB for public communication.

Calculation Methodology

1

Sub-indices for individual pollutants at a monitoring location are calculated using the 24-hourly average concentration, or 8-hourly average for CO and O3, against the relevant health breakpoint concentration range. The worst sub-index is the AQI for that location.

2

Overall AQI is calculated only if data are available for a minimum of three pollutants, of which one must be either PM2.5 or PM10. Otherwise, data are considered insufficient. A minimum of 16 hours of data is needed to calculate a sub-index.

3

Sub-indices for monitored pollutants are calculated and disseminated even when data are inadequate for the overall AQI. The individual pollutant sub-index then reflects the status of that pollutant.

4

The web-based system provides AQI on a real-time basis.

5

For manual monitoring stations, an AQI calculator is provided where data can be entered manually to obtain the AQI value.

Sub-Index Formula

Linear Interpolation Formula

I_p = [ (I_Hi − I_Lo) / (BP_Hi − BP_Lo) ] × (C_p − BP_Lo) + I_Lo

IₚSub-index for pollutant p

CₚTruncated concentration of pollutant p

BPHi / BPLoBreakpoint concentration greater than / less than or equal to Cₚ

IHi / ILoAQI value corresponding to BPHi / BPLo

Worked Example

For PM2.5, the sub-index is 51 at 31 µg/m³, 75 at 45 µg/m³, and 100 at 60 µg/m³. After calculating sub-indices for the monitored pollutants, the overall AQI is the maximum of those values. The pollutant yielding that maximum value is called the dominant pollutant.

AQI Breakpoints Table

AQI Category (Range)	PM10 24-hr	PM2.5 24-hr	NO2 24-hr	O3 8-hr	CO 8-hr (mg/m³)	SO2 24-hr	NH3 24-hr	Pb 24-hr
Good (0–50)	0–50	0–30	0–40	0–50	0–1.0	0–40	0–200	0–0.5
Satisfactory (51–100)	51–100	31–60	41–80	51–100	1.1–2.0	41–80	201–400	0.6–1.0
Moderately Polluted (101–200)	101–250	61–90	81–180	101–168	2.1–10	81–380	401–800	1.1–2.0
Poor (201–300)	251–350	91–120	181–280	169–208	10.1–17	381–800	801–1200	2.1–3.0
Very Poor (301–400)	351–430	121–250	281–400	209–748*	17.1–34	801–1600	1201–1800	3.1–3.5
Severe (401–500)	430+	250+	400+	748+*	34+	1600+	1800+	3.5+

* When the 8-hourly average of O3 exceeds 208 µg/m³, the 1-hourly average value is used instead for computing the sub-index.

Official PDF Document

Source: Central Pollution Control Board (CPCB), Ministry of Environment, Forest & Climate Change, Government of India — National Air Quality Index.

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Methodology summary page for the CPCB National Air Quality Index calculation framework.

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NOTE: This page summarizes how the National Air Quality Index is calculated and includes the PDF.