Why some apps showed Delhi’s AQI at 2,000 while the official CPCB reported it below 400

Many Delhi residents were puzzled on Tuesday when the city’s air quality index (AQI) showed wildly different numbers across platforms. While the Central Pollution Control Board (CPCB) reported readings below 400, international tracking apps such as IQAir showed levels crossing 2,000 in several areas of the city, a TOI report stated.

At 12.30 am on Tuesday, CPCB’s data showed an AQI of 272 at Siri Fort, while IQAir reported a reading of 2,449 for the same location. This difference led to widespread confusion among users checking air quality levels online.

Different scales, different results

Experts clarified that both sets of readings were correct, but they were based on different methods and scales.

“The Indian AQI, developed in 2015, is calculated using six key parameters — PM2.5, PM10, ozone, nitrogen dioxide, sulphur dioxide, and carbon monoxide. The graded scale, from good to severe+, ranges from 0 to 500,” said M.P. George, former additional director at the Delhi Pollution Control Committee (DPCC).

“Globally, different systems exist, some range from 0–10 or 0–100. IQAir, for instance, uses a US-based model,” he added.

The US Environmental Protection Agency (EPA)’s AQI, on which IQAir is based, also follows a 0–500 scale but allows values beyond 500. In the US model, anything above 301 is classified as hazardous. In contrast, India’s system caps readings at 500 since health risks are already severe at that level, and higher values offer limited practical insights.

Sensors versus analysers

Experts also pointed out differences in how data is collected. IQAir’s index is mainly based on sensor data, whereas CPCB uses analyser-based, reference-grade monitors. These are considered more accurate and standardised.

“I would trust CPCB’s data, as the calibration and reference methods are known. We don’t know the exact algorithms IQAir uses,” Dr George said.

Both show the same reality

Despite the numerical gap, experts stressed that both readings pointed to the same situation, Delhi’s air was highly polluted. The city’s air quality remained in the “very poor” category during and after Diwali, exceeding safe limits under both Indian and international standards.