India's National Air Quality Index Explained
India launched its National Air Quality Index in 2015 under the slogan "One Number, One Colour, One Description." On the surface it looks like the US AQI — a 0-to-500 scale, colour-coded, reporting the worst pollutant. But the categories are spaced very differently, because they were drawn for a country where a "normal" day in a major city can carry several times the particulate load of a typical US city. This guide explains the system and the mismatch.
What the National AQI is
The CPCB index runs from 0 to 500 across six categories and is built from sub-indices for eight pollutants: PM2.5, PM10, NO₂, ozone (O₃), carbon monoxide (CO), sulphur dioxide (SO₂), ammonia (NH₃), and lead (Pb). Each pollutant's sub-index is computed by linear interpolation within its breakpoint band, and the overall AQI is the maximum sub-index. CPCB requires data for at least three pollutants — one of which must be PM2.5 or PM10 — before an AQI is reported.
The six categories
| AQI | Category | Colour | In plain English |
|---|---|---|---|
| 0–50 | Good | Green | Minimal impact. |
| 51–100 | Satisfactory | Light green | Minor breathing discomfort possible for sensitive people. |
| 101–200 | Moderate | Yellow | Breathing discomfort for people with lung disease, asthma, and heart conditions. |
| 201–300 | Poor | Orange | Breathing discomfort to most people on prolonged exposure. |
| 301–400 | Very Poor | Red | Respiratory illness on prolonged exposure. |
| 401–500 | Severe | Maroon | Affects healthy people and seriously impacts those with existing disease. |
The sub-index breakpoints
Here are the concentration bands for the main pollutants. PM, NO₂, and SO₂ use a 24-hour average; CO and ozone use an 8-hour average. (Ozone also has 1-hour breakpoints for higher values, not shown.)
| Category | PM2.5 (24-hr, µg/m³) | PM10 (24-hr, µg/m³) | NO₂ (24-hr, µg/m³) | CO (8-hr, mg/m³) | O₃ (8-hr, µg/m³) |
|---|---|---|---|---|---|
| Good | 0–30 | 0–50 | 0–40 | 0–1.0 | 0–50 |
| Satisfactory | 31–60 | 51–100 | 41–80 | 1.1–2.0 | 51–100 |
| Moderate | 61–90 | 101–250 | 81–180 | 2.1–10 | 101–168 |
| Poor | 91–120 | 251–350 | 181–280 | 10.1–17 | 169–208 |
| Very Poor | 121–250 | 351–430 | 281–400 | 17.1–34 | 209–748 |
| Severe | 251+ | 431+ | 401+ | 34+ | 749+ |
Why the categories don't match the US AQI
This is the part that trips travellers up. The two scales share the 0–500 range but space the categories very differently. The clearest example is PM2.5: a 24-hour PM2.5 of 60 µg/m³ sits at the top of India's "Satisfactory" band (AQI ~100), while the same concentration is solidly "Unhealthy" (around AQI 153) on the US scale. India's "Moderate" band (101–200) spans what the US splits across "Unhealthy for Sensitive Groups" and "Unhealthy."
The wider bands reflect baseline exposure: ambient PM2.5 in major Indian cities is routinely a multiple of typical US urban levels, so a scale calibrated to US breakpoints would read "Hazardous" for much of the year in some cities. The categories were set to communicate meaningful day-to-day differences against that higher baseline — not to be interchangeable with other countries' numbers.
Reading it as a visitor
- Don't compare the number to a US AQI. An Indian AQI of 150 is materially worse air than a US AQI of 150.
- Watch the category, then the dominant pollutant. In Delhi and much of the north, PM2.5 is almost always the driver, especially in the autumn/winter crop-burning and inversion season.
- For a stable cross-border benchmark, look at raw PM2.5 in µg/m³. The WHO 24-hour guideline of 15 µg/m³ is the same everywhere.
Air quality on your home screen
Smog Report shows real-time air quality with widgets and Live Activities, and lets you switch between regional scales including India's National AQI. Free on iOS.
Download for iOSPrimary sources: CPCB — National Air Quality Index · CPCB — Live AQI portal