On Tuesday, October 21, 2025, Delhi residents checking air quality readings encountered a perplexing situation: while the Central Pollution Control Board (CPCB) reported Air Quality Index (AQI) levels below 400, several international apps, such as IQAir, displayed figures exceeding 2,000 in some areas. This discrepancy led to widespread confusion, but experts clarify that both readings can be technically correct, arising from differing methodologies and data sources.
The Indian AQI, developed in 2015, calculates air quality using six key parameters: PM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide. The resulting graded scale ranges from 0 to 500, categorized from 'good' to 'severe+'. In contrast, IQAir, relies on the US Environmental Protection Agency (EPA)'s AQI, which also uses a 0-500 scale but allows values beyond 500, classifying anything above 301 as hazardous. The Indian system, however, caps readings at 500 because health risks are already considered severe at that level, deeming higher values of limited practical use.
For example, at 12:30 AM on Tuesday, the CPCB reported an AQI of 272 at Siri Fort, while IQAir showed an astonishing 2,449 for the same location. This difference highlights how the capping of the AQI in the Indian system can lead to significantly different readings compared to international systems that allow values to rise above 500.
Another key difference lies in the data sources. IQAir's index is largely sensor-based, providing real-time data that captures rapid changes in air quality. However, these sensors may be more susceptible to interference or errors. The CPCB relies on analyzer-based reference-grade monitors, which are considered more accurate and standardized, although they may have slightly delayed results. M.P. George, former additional director at DPCC, stated a preference for trusting CPCB's data due to the known calibration and reference methods, while the exact algorithms used by IQAir are not fully transparent.
It’s also important to note that the CPCB calculates AQI based on 24-hour average concentrations, whereas IQAir reports AQI more frequently. This difference in calculation timeframe can also contribute to the variations in reported AQI values.
Despite the numerical discrepancies, experts emphasize that both platforms conveyed the same core message: Delhi's air quality was very poor during and after Diwali, far exceeding safe limits according to both Indian and international standards. While the numbers may differ, the underlying reality of hazardous air quality remains consistent across platforms. Therefore, it is essential to consider the methodologies and data sources behind different AQI readings to understand the complete picture of Delhi's air quality.
