Delhi residents were confused by vastly different pollution readings on Tuesday. While India’s CPCB showed AQI below 400, international apps like IQAir reported over 2,000. Experts explain these discrepancies stem from different calculation methods and data sources, though both indicate dangerously poor air quality post-Diwali.

NEW DELHI: Many Delhi residents were puzzled by the wildly different readings of Delhi’s pollution level on Tuesday. While the Central Pollution Control Board (CPCB) reported AQI levels well below 400, international apps like IQAir showed figures crossing 2,000 in several areas.

For instance, at 12.30 am on Tuesday, CPCB’s data showed an AQI of 272 at Siri Fort, while IQAir reported an astonishing 2,449 for the same location. Experts say both readings are technically correct but they follow different methodologies 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 MP George, former additional director at DPCC. “Globally, different systems exist — some range from 0–10 or 0–100. IQAir, for instance, uses a US-based model.”The US Environmental Protection Agency (EPA)’s AQI, on which IQAir is based, also follows a 0–500 scale but allows values beyond 500, with anything above 301 classified as hazardous. The Indian system, however, caps readings at 500, as health risks are already severe at that level and higher values are seen to be of limited practical use.Another major difference lies in data sources. IQAir’s index is largely sensor-based, while CPCB relies on analyser-based reference-grade monitors, 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 added.Clearing the air: Why global apps showed Delhi’s AQI levels at 2,000+, but CPCB under 400Experts stressed that despite the numerical gap, both platforms were showing the same reality — that Delhi’s air quality was very poor during and after Diwali, far exceeding safe limits of both Indian and international standards.