What is AQI? | Understanding Air Quality Index in Simple Terms

What is AQI?

AQI stands for Air Quality Index.
It is a simple number used to tell us how clean or polluted the air is and how it may affect our health.

Instead of showing complex pollution data, AQI converts air quality into a single number that anyone can understand.

Think of AQI like a thermometer for air pollution:

• Lower numbers = cleaner air
• Higher numbers = more polluted air

Why was AQI created?

Air contains many pollutants, each measured in different units.
AQI was created to:

• Combine this data into one easy-to-read scale
• Help the public understand health risks
• Enable authorities to issue health advisories
• AQI is not a direct measurement of health risk, but a practical way to communicate air quality to the public.

Without AQI, most people would not know whether the air outside is safe to breathe.

AQI scale used in India

India follows the National Air Quality Index (NAQI) defined by the Central Pollution Control Board (CPCB).

For reference, AQI below 50 is considered healthy for everyone.

What pollutants are included in AQI?

AQI is calculated using measurements of major air pollutants, including:

• PM2.5 – Fine particles smaller than 2.5 microns
• PM10 – Coarser dust particles
• NO₂ – Nitrogen dioxide (vehicle exhaust)
• SO₂ – Sulphur dioxide
• CO – Carbon monoxide
• O₃ – Ground-level ozone
• NH₃ – Ammonia
• Pb – Lead

Among these, PM2.5 is the most critical pollutant for health.

How different pollutants contribute to the AQI number

AQI does not assign fixed percentage weightage to different pollutants.

Instead, AQI is calculated using a maximum sub-index method.

How this works

For each pollutant:

• Its measured concentration is converted into a sub-index
• This conversion uses health-based breakpoint tables
• Each pollutant gets its own sub-index value

The highest sub-index among all pollutants becomes the AQI.

A simple example

If measurements result in:

• PM2.5 sub-index = 170
• PM10 sub-index = 95
• NO₂ sub-index = 60

Then:

• AQI = 170
• PM2.5 is called the dominant pollutant

Other pollutants are still present, but AQI reflects the worst current health risk.

Why PM2.5 often determines AQI

PM2.5 dominates AQI readings in many cities because:

• It affects health at relatively low concentrations
• Its health breakpoints are stricter than many other pollutants
• It increases quickly during traffic congestion, cooking, and stagnant weather

This does not mean other pollutants are unimportant — only that PM2.5 often crosses health thresholds first.

What is PM2.5 and why does it matter?

PM2.5 particles are extremely small — about 1/30th the width of a human hair.
One micron is one-millionth of a metre.

Because of their size:

• They stay airborne for long periods
• They can reach deep into the lungs
• Some particles can even enter the bloodstream

This is why PM2.5 is closely linked to:

• Asthma
• Heart disease
• Reduced lung function
• Long-term health risks

Outdoor AQI vs Indoor Air Quality

Many people assume indoor air is always cleaner than outdoor air.
This is not always true.

Indoor air can be polluted by:

• Cooking smoke
• Incense, agarbatti, dhoop
• Mosquito coils
• Cleaning chemicals
• Poor ventilation

In polluted cities, outdoor air enters indoors, raising indoor PM2.5 levels.

This is why indoor air quality deserves as much attention as outdoor AQI.

AQI readings explain what the air quality is, but managing indoor air also involves how fresh air enters a space.
Learn how ventilation and fresh air systems work →

Does high AQI affect everyone equally?

No.

Some people are more sensitive to air pollution:

• Children
• Elderly people
• Asthma or COPD patients
• People with heart conditions
• Pregnant women

For these groups, even moderate AQI levels can cause discomfort.

How often does AQI change?

AQI can change:

• Hourly
• Daily
• Seasonally

In Indian cities:

• Winter AQI is usually worse (low wind, temperature inversion)
• Monsoon AQI is generally better (rain cleans the air)

This is why AQI tracking apps show real-time values.

Should you worry about AQI every day?

AQI is not meant to create fear.

It is a decision-making tool, not a panic signal.

Use AQI to:

• Decide outdoor activity levels
• Protect sensitive family members
• Improve indoor air quality
• Understand when precautions are needed

Key takeaway

• AQI is a simple number that represents air pollution
• PM2.5 is the most important pollutant affecting health
• Indoor air quality matters just as much as outdoor AQI
• Understanding AQI helps you make practical, informed choices

Understanding AQI helps you interpret what the air quality is at a given moment.
Managing indoor air quality also depends on how fresh air enters and leaves a space.

Learn how ventilation and fresh air systems work →

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