How Is AQI Calculated? Complete Guide to Air Quality Index

The Air Quality Index (AQI) is a simplified way of describing

air pollution using a numerical value and a corresponding category such as
Good, Satisfactory, Moderate, Poor, Very Poor or Severe.AQI is calculated from measurements of individual air pollutants. The
concentration of each pollutant is converted into a pollutant-specific
sub-index, and the overall AQI is determined from the available pollutant
sub-indices.The exact calculation depends on the country’s AQI system. This guide focuses
on the Indian National Air Quality Index.

Quick Answer: How Is AQI Calculated?

In India’s National Air Quality Index system, the AQI is calculated from
concentrations of specified air pollutants.

The general process is:

Measure pollutant concentrations.
Calculate a sub-index for each pollutant.
Compare each pollutant concentration with the corresponding AQI
concentration breakpoints.
Determine the AQI value.
The pollutant with the highest sub-index generally determines the
overall AQI.

In simple terms, AQI converts several difficult-to-understand pollution
measurements into one easier-to-understand number.

What Does AQI Stand For?

AQI stands for Air Quality Index.

It is an index rather than a direct measurement of a single pollutant.
An AQI value is calculated using measurements of one or more pollutants.

This distinction is important because an AQI of 150 does not mean that
there are “150 units” of pollution in the air.

Instead, the number represents the position of the measured air quality
within the AQI calculation system.

Which Pollutants Are Used to Calculate India’s AQI?

India’s National Air Quality Index uses multiple pollutants to assess
ambient air quality.

The pollutants included in the Indian AQI framework include:

Particulate Matter (PM10)
Particulate Matter (PM2.5)
Nitrogen Dioxide (NO₂)
Sulphur Dioxide (SO₂)
Carbon Monoxide (CO)
Ozone (O₃)
Ammonia (NH₃)
Lead (Pb)

Not every AQI location necessarily has usable measurements for every
pollutant at every moment.

Why Are There Multiple Pollutants?

Air pollution is not a single substance.

A city can have high particulate pollution while gaseous pollutants are
relatively low. Another location can have a different pollution profile.

Using multiple pollutants gives the index a broader representation of
ambient air quality.

What Is an AQI Sub-Index?

Each pollutant is first evaluated separately.

For example, suppose an air-quality monitoring station measures:

PM2.5
PM10
NO₂
CO

The concentration of each pollutant is converted into its corresponding
AQI sub-index.

You can think of this as:

Pollutant concentration → Pollutant sub-index

After the available pollutant sub-indices have been calculated, the overall
AQI is determined.

How the AQI Calculation Works

The basic calculation can be understood as a series of steps.

Step 1: Measure pollutant concentration

Air-quality monitoring equipment measures concentrations of pollutants in
the ambient air.

Depending on the pollutant, measurements may be reported in units such as
micrograms per cubic metre (µg/m³) or milligrams per cubic metre (mg/m³).

Step 2: Find the appropriate concentration range

The measured concentration is compared with the pollutant-specific
concentration breakpoints used by the AQI system.

Each AQI category corresponds to a particular concentration range for
each pollutant.

Step 3: Calculate the pollutant sub-index

The concentration is converted to an index value using the relevant
breakpoint interval.

Step 4: Compare pollutant sub-indices

The resulting sub-indices are compared with one another.

Step 5: Determine the overall AQI

The pollutant producing the highest sub-index generally determines the
overall AQI.

The AQI Calculation Formula

A common linear interpolation approach used in AQI systems can be expressed
as:

I = [(I_HI − I_LO) /
(BP_HI − BP_LO)]
× (C − BP_LO) + I_LO

Where:

I = pollutant sub-index
C = measured pollutant concentration
BP_HI = upper concentration breakpoint
BP_LO = lower concentration breakpoint
I_HI = AQI value corresponding to the upper breakpoint
I_LO = AQI value corresponding to the lower breakpoint

The precise averaging periods, concentration breakpoints and rounding rules
should be taken from the applicable Indian AQI methodology rather than
assumed from another country’s system.

A Simple Example of AQI Interpolation

The easiest way to understand the formula is with a simplified example.

Suppose, for illustration only, that a pollutant has:

Lower concentration breakpoint = 50
Upper concentration breakpoint = 100
Lower AQI = 50
Upper AQI = 100
Measured concentration = 75

The measured concentration is halfway between the two concentration
breakpoints.

Therefore, the corresponding sub-index would also fall approximately
halfway between the two AQI breakpoints.

This example demonstrates the principle of interpolation. It is
not an official Indian pollutant breakpoint.

Why Does the Highest Pollutant Sub-Index Determine AQI?

Imagine a monitoring station produces these hypothetical sub-indices:

Pollutant	Sub-Index
PM2.5	165
PM10	122
NO₂	74
CO	55

In this simplified example, PM2.5 has the highest sub-index.

Therefore, PM2.5 would be the dominant pollutant determining the overall
AQI.

This also gives you useful information that the single AQI number alone
does not provide: which pollutant is driving the air-quality
category.

What Are the AQI Categories in India?

India’s National Air Quality Index uses six broad categories.

AQI	Category	General Description
0–50	Good	Air quality is considered good.
51–100	Satisfactory	Air quality is generally acceptable, although some pollutants may affect sensitive individuals.
101–200	Moderate	Some people may experience health effects, particularly sensitive groups.
201–300	Poor	Health effects become more significant with increased pollution.
301–400	Very Poor	Prolonged exposure can present greater health concerns.
401–500	Severe	Air pollution is at a very high level.

Does AQI Equal PM2.5?

No.

This is one of the most common misunderstandings about AQI.

PM2.5 is one pollutant. AQI is an index calculated using pollutant
measurements.

PM2.5 can be the pollutant responsible for the highest sub-index at a
particular location, but AQI and PM2.5 are not interchangeable measurements.

Can PM2.5 Alone Determine AQI?

It can sometimes be the dominant pollutant, but AQI calculation is designed
around multiple pollutants.

If PM2.5 produces the highest applicable sub-index, it can determine the
reported AQI for that monitoring location.

However, this does not mean the AQI system itself is simply a PM2.5
measurement.

Why Does AQI Change During the Day?

AQI can change because pollutant concentrations change.

Factors that can affect ambient air pollution include:

Traffic
Industrial emissions
Construction activity
Dust
Weather conditions
Wind speed and direction
Temperature
Atmospheric mixing
Rainfall
Seasonal pollution sources

Because the concentrations change, the pollutant sub-indices can change,
which can cause the overall AQI to change.

Why Can AQI Be Different Between Two Nearby Locations?

Air pollution is not necessarily distributed evenly across a city.

Two monitoring stations can measure different concentrations because they
experience different traffic levels, emission sources, wind patterns,
building density and local conditions.

Therefore, an AQI reported by one monitoring station should not automatically
be assumed to represent every street or neighborhood in a large city.

How Is AQI Different From an Air Quality Monitor Reading?

Measurement	What It Tells You
PM2.5	Concentration of fine particulate matter.
PM10	Concentration of particulate matter in the specified size range.
CO₂	Carbon dioxide concentration; often useful for assessing ventilation indoors.
TVOC	An indicator associated with volatile organic compounds.
AQI	An index calculated from pollutant measurements according to a defined methodology.

How Do Home Air-Quality Monitors Calculate AQI?

This is where consumers need to be particularly careful.

A consumer air-quality monitor may measure PM2.5 and then display an
“AQI” value calculated by its software.

However, the device’s AQI may not necessarily use the same methodology as
India’s official National Air Quality Index.

Some consumer devices and apps may use an AQI standard from another country,
a proprietary index, or a simplified conversion based primarily on PM2.5.

Therefore, when comparing a home monitor with an official government AQI
value, check which AQI standard the device uses.

Indian AQI vs US AQI

AQI is not a universal mathematical scale.

India and the United States use different AQI methodologies, pollutant
breakpoints, averaging periods and health-category descriptions.

Consequently, the same pollutant concentration can potentially produce
different index values under different national AQI systems.

This is why an air-quality monitor should identify the AQI standard being
used rather than simply displaying “AQI” without explanation.

What Does AQI Actually Tell You?

AQI is designed to simplify complex air-quality information.

Instead of asking the public to interpret several pollutant concentrations
separately, the index provides a single number and category.

For example:

AQI: 85

Category: Satisfactory

But for a more complete understanding, you should also look at the
individual pollutant concentrations and identify which pollutant is
driving the AQI.

Limitations of AQI

1. AQI is a summary number

A single AQI value cannot describe every detail of the air-quality
environment.

2. It may hide individual pollutant differences

Two locations can have similar AQI values but different pollutant profiles.

3. AQI depends on the methodology

Different countries and organizations can use different AQI systems.

4. Monitoring locations matter

A station’s measurement may not perfectly represent conditions at every
location nearby.

5. Low-cost sensors can introduce additional uncertainty

Consumer air-quality monitors may have different levels of accuracy and
can be affected by environmental conditions, sensor technology, calibration
and placement.

How to Read AQI Correctly

When you see an AQI number, don’t stop at the number itself.

Check which AQI standard is being used.
Check the AQI category.
Identify the pollutant responsible for the highest sub-index.
Look at individual PM2.5 and PM10 concentrations.
Consider whether the data comes from a nearby monitoring station.
Check the measurement time and averaging period.

AQI vs PM2.5: Which Should You Watch?

Both can be useful, but they answer different questions.

AQI gives you an easier-to-understand overall air-quality
category.

PM2.5 tells you the measured concentration of fine
particulate matter.

If you are using an indoor air-quality monitor, it can be useful to look
at the actual PM2.5 measurement as well as any AQI value displayed by the
device.

Frequently Asked Questions

What is a good AQI in India?

Under India’s National Air Quality Index, an AQI from 0 to 50 is classified
as Good.

What AQI is considered poor in India?

India’s AQI categories classify 201–300 as Poor, 301–400 as Very Poor and
401–500 as Severe.

Is AQI calculated only from PM2.5?

No. India’s National Air Quality Index considers multiple pollutants.
PM2.5 may be the pollutant producing the highest sub-index at a particular
time and therefore determining the reported AQI.

Why is my air-quality monitor’s AQI different from the government AQI?

Possible reasons include different sensors, measurement locations,
averaging periods, calibration, data timing and—very importantly—different
AQI calculation standards.

Does AQI measure CO₂?

CO₂ is not one of the pollutants used in India’s National Air Quality Index.
Indoor CO₂ monitoring is nevertheless useful for a different purpose,
particularly understanding ventilation.

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About the Author

ManBearPig is a part-man, part-bear, part-pig creature and a full-time expert on global warming—mostly because he has personally experienced the consequences of being extremely, extremely, super cereal.

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His research philosophy is simple:

“I’m super cereal.”

ManBearPig believes climate change is too important to discuss without facts—and too ridiculous to discuss without a little humor. Through this website, he explores global warming, climate science, environmental changes, energy, and the strange ways humans respond when their own planet starts sending them warning signals.

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