Understanding air quality
There isn’t one way to measure air quality. Government reference stations, satellites, and low-cost sensor networks each measure different things in different ways, with different accuracy and coverage — and separately, air also carries measurable radioactivity, using entirely different instruments again. This page covers how each of those actually works, what pollutants get measured, and how India’s Air Quality Index turns raw readings into a category anyone can act on.
How air quality is measured in India
Manual monitoring stations
Air is drawn through a filter or chemical solution over a fixed period, commonly 24 hours, then the sample is sent to a lab for gravimetric (weighing) or chemical analysis.
This is the oldest and most accurate method — it’s still the reference that automated stations are calibrated against — but it’s slow. Results can take days, and each cycle gives one data point, not continuous readings.
CAAQMS
Continuous Ambient Air Quality Monitoring Stations: automated analysers that measure pollutant concentrations in real time and transmit readings to CPCB’s servers, typically every 15 minutes to an hour.
This is what powers real-time public AQI displays. Most CAAQMS stations measure PM10, PM2.5, SO₂, NO₂, NH₃, O₃, and CO alongside meteorological data like wind, temperature, and humidity.
Satellite-based monitoring
Satellites don’t measure air at ground level directly — instruments like ISRO’s Ocean Colour Monitor (on EOS-6) or NASA’s MODIS measure Aerosol Optical Depth (AOD), how much sunlight scatters off particles in the entire atmospheric column above a point. Models then estimate ground-level PM2.5 from that signal.
It’s the only method with coverage where no ground station exists at all, but it has real limits: one daytime, cloud-free observation per day from a polar-orbiting satellite, and an estimate rather than a direct measurement.
Low-cost sensor networks
Affordable optical sensors (laser scattering, for particulate matter) and electrochemical sensors (for gases) that individuals or organizations build or buy and connect to a shared network, which aggregates and publishes the readings publicly.
These trade some accuracy for density — a city might have a handful of government reference stations and far more low-cost nodes filling the gaps between them. Different networks are mostly the same underlying model; what actually differs between them is largely hardware specification — sensor accuracy, calibration process, and which pollutants they cover.
Radiological monitoring
Radioactivity in air is a fundamentally different kind of measurement, not a chemical pollutant — it uses different instruments (Geiger-Müller counters, scintillation detectors, gamma spectrometers) and different units (becquerel (Bq) for how much radioactive material is present, sievert (Sv) for the dose a person absorbs, not µg/m³). In India this is regulated by the Atomic Energy Regulatory Board (AERB), not CPCB.
Nuclear facility safety: every nuclear power plant and fuel reprocessing site in India has an Environmental Surveillance Laboratory (ESL), monitoring air, water, soil, milk, and agricultural produce up to 30km around the site using gamma spectrometers and tritium counters — from before the facility opens through decommissioning. This is the mechanism that would detect a leak.
Radon: a separate, unrelated concern — a naturally occurring radioactive gas that seeps from soil into buildings, measured in Bq/m³. WHO treats it as a distinct indoor air concern (the second leading cause of lung cancer after smoking) and recommends keeping indoor levels below 100 Bq/m³.
The pollutants
What is PM?
PM stands for Particulate Matter — solid and liquid particles suspended in the air, classified by size rather than by what they’re made of. The number after PM is the maximum particle diameter in micrometres (µm), so PM10 covers all particles up to 10µm and PM2.5 covers the smaller ones up to 2.5µm.
PM10’s size range fully includes PM2.5’s, so a PM10 reading is never lower than the PM2.5 reading at the same time and place — they aren’t two competing measurements, and one can’t replace the other. What varies is the gap between them, and that gap says something about the source: combustion (traffic, burning) pushes PM2.5 up sharply with PM10 following close behind, while dust and construction mainly add to the coarser fraction, widening the gap between the two readings.
PM2.5 · Fine particulate matter (≤2.5 micrometres)
Comes from: Vehicle exhaust, burning of crop residue, waste, wood and solid fuel, and industrial combustion.
Health effects: Small enough to penetrate deep into the lungs and enter the bloodstream. Linked to cardiovascular and respiratory disease, and long-term exposure is linked to premature death.
Sensed by: Reference stations use Beta Attenuation Monitoring (BAM) or gravimetric filter weighing. Low-cost sensors use optical laser light-scattering (nephelometry) — counting how much light particles scatter as air passes through a laser beam.
| Category | µg/m³ (24-hr avg) |
|---|---|
| Good | 0 – 30 |
| Satisfactory | 31 – 60 |
| Moderate | 61 – 90 |
| Poor | 91 – 120 |
| Very Poor | 121 – 250 |
| Severe | Above 250 |
PM10 · Coarse particulate matter (≤10 micrometres)
Comes from: Construction and demolition dust, road dust, and the same combustion sources as PM2.5.
Health effects: Deposits in the upper airways and lungs, irritating them and aggravating asthma and other respiratory conditions.
Sensed by: Same instruments as PM2.5 (BAM or gravimetric filters for reference stations, optical light-scattering for low-cost sensors) — most particle sensors report PM1, PM2.5, and PM10 simultaneously from the same laser-scattering measurement.
| Category | µg/m³ (24-hr avg) |
|---|---|
| Good | 0 – 50 |
| Satisfactory | 51 – 100 |
| Moderate | 101 – 250 |
| Poor | 251 – 350 |
| Very Poor | 351 – 430 |
| Severe | Above 430 |
NO₂ · Nitrogen dioxide
Comes from: Vehicle engines and combustion of fuel in industry.
Health effects: Inflames the airways and reduces lung function, particularly in children, and worsens asthma.
Sensed by: Reference stations use chemiluminescence — NO reacts with ozone inside the analyser, producing light in proportion to concentration. Low-cost sensors use electrochemical cells that generate a small current from a reaction at an electrode.
| Category | µg/m³ (24-hr avg) |
|---|---|
| Good | 0 – 40 |
| Satisfactory | 41 – 80 |
| Moderate | 81 – 180 |
| Poor | 181 – 280 |
| Very Poor | 281 – 400 |
| Severe | Above 400 |
SO₂ · Sulphur dioxide
Comes from: Burning of fossil fuels containing sulphur, mainly coal-fired power plants and industrial boilers.
Health effects: Irritates the respiratory tract and can trigger bronchoconstriction, especially in people with asthma.
Sensed by: Reference stations use UV fluorescence — SO₂ molecules absorb UV light and re-emit it at a different wavelength, measured to determine concentration. Low-cost sensors use electrochemical cells.
| Category | µg/m³ (24-hr avg) |
|---|---|
| Good | 0 – 40 |
| Satisfactory | 41 – 80 |
| Moderate | 81 – 380 |
| Poor | 381 – 800 |
| Very Poor | 801 – 1600 |
| Severe | Above 1600 |
CO · Carbon monoxide
Comes from: Incomplete combustion of fuels — mainly vehicle exhaust, and indoor burning of wood or charcoal.
Health effects: A colourless, odourless gas that binds to haemoglobin more readily than oxygen, reducing how much oxygen the blood can carry to organs.
Sensed by: Reference stations use non-dispersive infrared (NDIR) absorption — CO absorbs infrared light at a specific wavelength, and the amount absorbed indicates concentration. Low-cost sensors use electrochemical cells.
| Category | mg/m³ (8-hr avg) |
|---|---|
| Good | 0 – 1.0 |
| Satisfactory | 1.1 – 2.0 |
| Moderate | 2.1 – 10.0 |
| Poor | 10.1 – 17.0 |
| Very Poor | 17.1 – 34.0 |
| Severe | Above 34.0 |
O₃ · Ground-level ozone
Comes from: Not emitted directly — forms when sunlight reacts with pollutants like vehicle and industrial emissions.
Health effects: Irritates the airways, reduces lung function, and worsens chronic respiratory disease. Typically highest on hot, sunny afternoons.
Sensed by: Reference stations use UV photometry — ozone absorbs UV light at a specific wavelength, and the amount absorbed indicates concentration. Low-cost sensors use electrochemical or metal-oxide cells.
| Category | µg/m³ (8-hr avg) |
|---|---|
| Good | 0 – 50 |
| Satisfactory | 51 – 100 |
| Moderate | 101 – 168 |
| Poor | 169 – 208 |
| Very Poor | 209 – 748 |
| Severe | Above 748 |
Breakpoints are CPCB’s official National AQI values, defined for 24-hour averages (8-hour for CO and O₃). Real-time public displays typically apply them to the latest available reading as an indicative approximation, not a strict 24-hour calculation. Pollutant sources and health effects draw on WHO’s ambient air quality guidance.
Two more official pollutants
CPCB’s National AQI actually covers eight pollutants, not six — it also defines breakpoints for ammonia (NH₃) and lead (Pb), less commonly reported but categorized the same way as the six above:
NH₃ · Ammonia
Comes from: Fertilizer use and livestock waste in agriculture, and some industrial processes.
Health effects: Irritates the eyes, nose, and throat at high concentrations; reacts in the atmosphere to form secondary PM2.5.
Sensed by: Reference stations convert NH₃ to NO first, then use the same chemiluminescence reaction as NO₂ monitors. Low-cost sensors use electrochemical cells.
| Category | µg/m³ (24-hr avg) |
|---|---|
| Good | 0 – 200 |
| Satisfactory | 201 – 400 |
| Moderate | 401 – 800 |
| Poor | 801 – 1200 |
| Very Poor | 1201 – 1800 |
| Severe | Above 1800 |
Pb · Lead
Comes from: Industrial metal processing and battery manufacturing; legacy contamination from leaded fuel.
Health effects: Accumulates in the body over time. Affects the nervous system and is especially harmful to child brain development, even at low exposure.
Sensed by: Not sensed electronically at all — air is drawn through a filter over a fixed period, and the collected particulate is sent to a lab for atomic absorption spectroscopy. There's no real-time or low-cost equivalent.
| Category | µg/m³ (24-hr avg) |
|---|---|
| Good | 0 – 0.5 |
| Satisfactory | 0.6 – 1.0 |
| Moderate | 1.1 – 2.0 |
| Poor | 2.1 – 3.0 |
| Very Poor | 3.1 – 3.5 |
| Severe | Above 3.5 |
The AQI scale
Once a concentration has been measured — by any of the methods above — it still needs interpreting. A number like “62 µg/m³ of PM2.5” means little on its own, so the Air Quality Index (AQI) turns it into a category that’s easier to act on. India’s version is defined by the CPCB, with a separate concentration range per pollutant for each category.
| Good | Minimal impact. |
| Satisfactory | Minor breathing discomfort to sensitive people. |
| Moderate | Breathing discomfort to people with lung disease, asthma, and heart disease, children and older adults. |
| Poor | Breathing discomfort to most people on prolonged exposure. |
| Very Poor | Respiratory illness on prolonged exposure. |
| Severe | Affects healthy people and seriously impacts those with existing diseases. |
Category names and associated health impacts are CPCB’s official National AQI text.