Which Gases India’s Air Quality Index Tracks

The full list at a glance

• The Central Pollution Control Board (CPCB) uses eight pollutants for India’s National AQI.
• PM10 is particles up to 10 micrometres. Road and construction dust are common sources. They irritate the lungs.
• PM2.5 is particles up to 2.5 micrometres. Smoke adds fine particles. Gases can also react to make them. They reach deep into the lungs.
• NO2 is nitrogen dioxide. Traffic and fuel burning supply nitrogen oxides. It harms breathing and helps make ozone.
• SO2 is sulphur dioxide. Coal burning and industry release it. It irritates airways.
• CO is carbon monoxide. Incomplete burning, including vehicle exhaust, releases it. It reduces how much oxygen the blood carries.
• O3 is ground-level ozone. Sunlight drives reactions between nitrogen oxides and other gases. It irritates lungs and harms plants.
• NH3 is ammonia. Fertiliser and animal waste release it. It can react to make fine particles.
• Pb is lead. Smelting and battery plants can release lead-bearing dust. It harms the nervous system, especially in children.

• PM10, PM2.5, NO2, SO2, NH3 and Pb use 24-hour averages. CO and O3 use 8-hour averages.
• These are possible sources, not a source test for each reading.
• Sources: CPCB National AQI report, WHO pollutant and health guide, US EPA on airborne lead and US EPA on ammonia and fine particles.

Direct emissions and pollution made in air

• A primary pollutant enters the air from a source. Exhaust can release CO. Dust can release PM10.
• A secondary pollutant forms through reactions in the air. Ground-level ozone is a clear example. Some PM2.5 forms this way too.
• NO2 can be released directly or form from nitric oxide.
• The categories describe formation, not how dangerous a pollutant is.
• Sources: US EPA pollutant types and US EPA nitrogen-oxide assessment.

Measure air, then make a sub-index

• A station measures each pollutant’s ambient concentration. That means how much is in the surrounding outdoor air.
• CPCB compares each concentration with health-based breakpoints.
• The result is that pollutant’s sub-index.
• The averaging windows smooth short spikes. They also make readings more comparable.
• Source: CPCB’s National AQI explanation and breakpoint table.

The highest sub-index decides the result

• Imagine a station reports three valid sub-indices. PM2.5 is 142, NO2 is 75 and CO is 40.
• PM2.5 has the worst sub-index. So the AQI is 142.
• The index does not add the three or take their average.
• This is an example of sub-indices, not raw concentrations.
• Source: CPCB National AQI calculation method.

Enough data must be available

• CPCB needs readings for at least three pollutants. One must be PM10 or PM2.5.
• Its method also calls for at least 16 hours of data to calculate a sub-index.
• Missing data are not treated as clean air.
• All eight pollutants are eligible. They are not all measured at every station.
• Lead is hard to measure in real time. CPCB says past-day lead data can still support AQI checks.
• Source: CPCB report, data rules and lead note.

Read the number and colour together

• Our example score, 142, falls in the Moderate band.
• Risk generally grows as the number rises.
• People with lung or heart disease can feel breathing trouble even in the Moderate band.
• Sources: CPCB category colours and CPCB live AQI category guide.

Emissions are not the same as the air you breathe

• Emissions are pollutants released from a source, such as a vehicle or factory.
• Ambient concentration is the amount measured in outdoor air at a place and time.
• Weather, wind and chemical reactions can change it.
• AQI uses the measured concentration, not an estimate of one source’s emissions.
• A high AQI does not identify one polluter by itself.
• Sources: CPCB calculation method and WHO on pollutant sources and reactions.

AQI is not a greenhouse-gas inventory

• CO2 and methane help warm the planet. They are not among India’s eight AQI inputs.
• This is about the index’s purpose. It is not a claim that these gases never cause harm.
• Do not use a simple rule that all greenhouse gases are excluded.
• Ground-level ozone is a greenhouse gas and an AQI pollutant.
• A low AQI does not mean climate emissions are low. The two measures answer different questions.
• Sources: CPCB AQI scope and NASA on greenhouse gases and ozone.

Who reports these readings?

• CPCB coordinates India’s National Air Quality Monitoring Programme. It also publishes National AQI information.
• Check the station, time and available pollutants when reading a live value.
• Nearby places can have different air.
• Sources: CPCB monitoring programme and CPCB live AQI portal.