A single number that tells you how polluted the air is and what that means for your health. In South Asia it usually refers to the CPCB National AQI, which runs from 0 to 500.
MeasurementWhat is AQI?
The Air Quality Index (AQI) is a standardised number that summarises how polluted the air is. It converts measurements of half a dozen different pollutants into a single value with a colour and a health category that anyone can read at a glance — green for clean air, dark red for an emergency. For the hundreds of millions of people living across the Indo-Gangetic plain, where winter smog routinely pushes readings into the worst categories, it is the most important air-quality number there is.
In South Asia, “AQI” almost always means the CPCB National AQI, published by India’s Central Pollution Control Board on a 0–500 scale and quoted by newspapers, television and the SAFAR and Sameer apps. That is the scale Mausam Online shows for cities in India, Pakistan, Bangladesh, Nepal and Sri Lanka. For other countries we show the US EPA AQI, which also runs from 0 to 500 but uses different breakpoints — the two are easy to confuse, and the same air can read quite differently on each, as the section on scales below shows.
Correction: until September 2026 this entry said Mausam Online used the European AQI (0–100+) and listed that index’s bands. The city pages had already moved to the CPCB scale; this entry now matches what they show.
How AQI is calculated
Each pollutant is averaged over a set window — 24 hours for PM2.5, PM10, nitrogen dioxide, sulphur dioxide and ammonia, and 8 hours for ozone and carbon monoxide. Each average is converted into a sub-index by interpolating between published breakpoints, and the highest sub-index becomes the AQI — not an average of them, because one bad pollutant is enough to make a bad day. The pollutant that set it is named as the dominant pollutant. The CPCB standard only allows an index to be published when at least three pollutants have been measured, one of them PM2.5 or PM10, and Mausam Online applies the same rule before it shows a CPCB figure.
The pollutants that matter most in South Asia:
- PM2.5 — fine particles smaller than 2.5 µm. They penetrate deep into the lungs and the bloodstream, and are the single biggest contributor to premature deaths from air pollution worldwide.
- PM10 — coarser particles up to 10 µm: dust, construction debris, pollen and mould spores.
- Ozone (O₃) — formed on sunny afternoons when sunlight reacts with vehicle and industrial exhaust.
- Nitrogen dioxide (NO₂) — from vehicle engines and combustion.
- Sulphur dioxide (SO₂) — from coal-fired power plants, refineries and brick kilns.
- Carbon monoxide (CO) — from incomplete combustion in engines, stoves and fires.
- Ammonia (NH₃) — mostly from fertiliser and livestock; part of India’s index.
The CPCB AQI scale
| AQI | Category | Possible health impact (CPCB) | What Mausam Online advises |
|---|---|---|---|
| 0–50 | Good | Minimal impact | Air quality is great — enjoy outdoor activities. |
| 51–100 | Satisfactory | Minor breathing discomfort to sensitive people | Air quality is acceptable. Sensitive people may feel minor discomfort. |
| 101–200 | Moderate | Breathing discomfort to people with lung disease, asthma and heart disease | Sensitive groups should reduce prolonged outdoor exertion. |
| 201–300 | Poor | Breathing discomfort to most people on prolonged exposure | Limit outdoor activity. Consider a mask if sensitive. |
| 301–400 | Very Poor | Respiratory illness on prolonged exposure | Avoid outdoor exertion. Wear an N95 mask outdoors. |
| 401–500 | Severe | Affects healthy people and seriously impacts those with existing diseases | Stay indoors. Everyone may experience serious health effects. |
The scale stops at 500: a CPCB reading cannot go higher, however bad the air. For cities outside South Asia the page shows the US EPA AQI with its own six categories, from Good to Hazardous.
One number, three scales
Readers in South Asia routinely meet three different indices — often on the same day, for the same city:
| CPCB National AQI | US EPA AQI | European AQI | |
|---|---|---|---|
| Published by | Central Pollution Control Board, India | US Environmental Protection Agency | European Environment Agency |
| Range | 0–500 | 0–500 | six bands; data providers express it as a number from 0 to above 100 |
| Categories | Good, Satisfactory, Moderate, Poor, Very Poor, Severe | Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, Hazardous | Good, Fair, Moderate, Poor, Very Poor, Extremely Poor |
| Where you see it | CPCB and SAFAR bulletins, Indian newspapers and TV | Embassy monitors, IQAir and most international apps | European services and some global data feeds |
| On Mausam Online | India, Pakistan, Bangladesh, Nepal, Sri Lanka | All other countries | Only as a labelled fallback when a national figure cannot be computed |
The first two share a 0–500 range and a six-colour look, which is exactly why they are so easily confused. Their breakpoints are different. Here is the same 24-hour average of PM2.5 run through both, using the CPCB breakpoints and the US EPA breakpoints as revised in 2024:
| PM2.5, 24-hour mean | CPCB National AQI | US EPA AQI |
|---|---|---|
| 12 µg/m³ | 20 — Good | 56 — Moderate |
| 30 µg/m³ | 50 — Good | 90 — Moderate |
| 55 µg/m³ | 92 — Satisfactory | 149 — Unhealthy for Sensitive Groups |
| 90 µg/m³ | 200 — Moderate | 175 — Unhealthy |
| 150 µg/m³ | 323 — Very Poor | 225 — Very Unhealthy |
| 250 µg/m³ | 400 — Very Poor | 350 — Hazardous |
At the clean end the US scale is stricter: air the CPCB calls Good, the US calls Moderate. At the polluted end the CPCB number climbs faster, so the two cross between 60 and 90 µg/m³. An “AQI 200” therefore means about 90 µg/m³ of PM2.5 on the CPCB scale but about 125 µg/m³ on the US scale — noticeably different air behind the same number. Neither scale is more “correct”; each encodes one government’s judgement about health risk and what is achievable. The mistake is comparing a number from one with a number from the other.
Reading an AQI number without being misled
- Check the scale first. IQAir and most international apps show the US index by default; Indian bulletins show CPCB. A “US AQI 180” and a “CPCB AQI 180” are different readings. Mausam Online city pages name the scale beside the number.
- Check the averaging window. A CPCB AQI is a 24-hour picture. On a still winter evening in Delhi the hourly PM2.5 can be double the 24-hour mean, so for conditions right now, look at the hourly PM2.5 value itself.
- Check the dominant pollutant. A PM10-led index usually means dust — construction, dry soil, a dust storm. A PM2.5-led index usually means smoke and combustion. An ozone-led index points to a sunny afternoon with heavy traffic upwind.
- Remember the ceiling. The CPCB scale stops at 500. Monitors that extend the US index beyond 500 can report figures in the high hundreds or above 1,000 during extreme episodes; those numbers cannot be compared with a CPCB reading at all.
AQI in South Asia
South Asia has some of the most polluted air in the world. In IQAir’s World Air Quality Report for 2024, Bangladesh, Pakistan and India all ranked among the five most polluted countries, and Indian cities filled most of the list of the twenty most polluted cities. The causes are well understood:
- Winter temperature inversions (October–February) trap pollution in a shallow layer of cold air over the Indo-Gangetic plain — see thermal inversion.
- Crop residue burning in Punjab and Haryana in October and November sends smoke drifting east toward Delhi.
- Diwali fireworks cause short, intense spikes that can push Delhi to the top of the CPCB scale overnight.
- Construction dust and vehicle exhaust are year-round contributors in every large city.
- Coal-fired power plants and brick kilns add a steady industrial baseline.
Delhi, Lahore, Dhaka, Kanpur, Patna, Lucknow, Ghaziabad and Karachi regularly reach Poor or Very Poor in winter, and Delhi reaches Severe on the worst November days. In Lahore in November 2024, monitors that report the uncapped US index recorded readings above 1,000. A rule of thumb from Berkeley Earth equates breathing 22 µg/m³ of PM2.5 for a day with smoking one cigarette; a Severe day in Delhi, with PM2.5 above 250 µg/m³, works out to more than ten.
How to protect yourself
- Check the AQI every day — each Mausam Online city page shows the AQI, the scale it is on, the dominant pollutant, PM2.5 and PM10, with a health-advice line.
- Match the precaution to the category. From Moderate (101+), people with asthma, lung or heart disease should cut back on long outdoor exertion. From Poor (201+), everyone should limit time outdoors and sensitive people should consider a mask. From Very Poor (301+), wear an N95 outdoors. At Severe, stay indoors.
- Use the right mask. A well-fitted N95 or FFP2 filters most fine particles; cloth and loose surgical masks do little against PM2.5.
- Keep windows closed on bad days and run a HEPA air purifier in the room where you sleep.
- Move exercise indoors on Poor days and worse — you breathe six to ten times more air while exercising.
- Look after vulnerable people: children, older people, pregnant women and anyone with asthma, COPD or heart disease. If you have one of those conditions, keep your medication at hand and follow your doctor’s advice on bad-air days.
Frequently asked questions
Is AQI the same in every country? No. India uses the CPCB National AQI (0–500), the United States uses the EPA AQI (also 0–500, with different breakpoints) and Europe uses the European Environment Agency’s index. Mausam Online shows the CPCB scale for India, Pakistan, Bangladesh, Nepal and Sri Lanka and the US EPA scale elsewhere, and each city page names the scale beside the number.
Why does my phone show a different AQI from the news? Almost always because they use different scales, different averaging windows, or both. International apps usually show the US EPA index computed from recent hourly values; Indian news reports the CPCB index built on 24-hour means. Check which scale each one names before comparing them.
What is a safe AQI? The World Health Organization does not publish an AQI, but its 24-hour guideline for PM2.5 is 15 µg/m³ — about 25 on the CPCB scale, well inside Good, and about 62 on the US EPA scale, already Moderate. On the CPCB scale, Good (0–50) and Satisfactory (51–100) are the categories with little or no health impact for most people.
Does PM2.5 really matter that much? Yes. A national study published in The Lancet Planetary Health attributed about 1.67 million deaths in India in 2019 to air pollution, most of them to particulate matter. Fine particles contribute to heart disease, stroke, lung cancer, COPD, asthma and low birth weight.
Are air purifiers worth it? For indoor air, yes. A HEPA purifier sized for the room can cut indoor PM2.5 by 80–95%. Match its clean air delivery rate (CADR) to the room: a 20 m² bedroom needs roughly 200–250 m³/h.
When is AQI worst in India? From November to January, especially in north India. Crop burning, cold air trapped near the ground, calm winds and festival fireworks together push Delhi-NCR routinely into Very Poor (301–400) and at times to Severe, the top of the scale.