India AQI vs US AQI: why CPCB and IQAir differ

India's National Air Quality Index

The Central Pollution Control Board (CPCB) launched the National Air Quality Index in 2014. It covers eight pollutants, PM2.5, PM10, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead, and reads them in six bands:

• 0–50: Good
• 51–100: Satisfactory
• 101–200: Moderate
• 201–300: Poor
• 301–400: Very poor
• 401–500: Severe

The US AQI

The US AQI, set by the US Environmental Protection Agency, is what IQAir, Airleaf and most international apps show. Its bands are narrower at the low end: 0–50 good, 51–100 moderate, 101–150 unhealthy for sensitive groups, 151–200 unhealthy, 201–300 very unhealthy and above 300 hazardous. In 2024 the EPA tightened its fine-particle table, so only 9 µg/m³ of PM2.5 or less now counts as good.

Where the two scales cross

For fine particles, the pollutant that sets the index on most bad days in India, the two tables give these values for the same concentration:

• 40 µg/m³: India 66 (satisfactory), US 112 (unhealthy for sensitive groups)
• 55 µg/m³: India 92 (satisfactory), US 149 (unhealthy for sensitive groups)
• 68 µg/m³: India 125 (moderate), US 160 (unhealthy)
• 80 µg/m³: India 166 (moderate), US 168 (unhealthy), about level
• 100 µg/m³: India 232 (poor), US 182 (unhealthy)
• 150 µg/m³: India 323 (very poor), US 225 (very unhealthy)

What that means in practice

Below roughly 80 µg/m³ of PM2.5 the US scale gives the bigger number, often by 30 to 60 points. Above it, India's does. So on an ordinary polluted day in Mumbai, Kolkata or Bengaluru an app will usually look worse than CPCB, while in a Delhi winter smog CPCB's number can be the higher one.

Averaging matters too. CPCB's index uses 24-hour averages for particles (8 hours for ozone and carbon monoxide), so it describes the past day. The US AQI that apps show is usually hourly or close to it, so it moves within the hour when smoke or smog arrives or clears.

And the stations differ. CPCB's figures come from its own network of monitoring stations, while many apps blend government stations, low-cost sensors and models. Two readings a few kilometres apart can both be honest.

Which one should you use?

Pick one and read it against its own bands. India's scale is the one government advisories follow, including the stages of Delhi's Graded Response Action Plan (GRAP). The US scale is stricter on moderate pollution, so it flags more days as a risk for children, older people and anyone with asthma or a heart condition. If you are in one of those groups, the US bands are the more cautious guide.

Where Airleaf's numbers come from

Airleaf shows the US AQI for every Indian city in its directory: from ground stations via OpenAQ where one has reported recently nearby, and otherwise from the CAMS air-quality model. Each city report says which, and shows the PM2.5 reading behind the number and the past week.