New Delhi's air quality has once again become a major point of concern, especially after the Diwali celebrations, with a visible discrepancy in pollution levels reported by different monitoring platforms. Residents and experts alike have been puzzled by the stark contrast between the Air Quality Index (AQI) figures presented by the Central Pollution Control Board (CPCB) and international apps like IQAir. On Tuesday, for example, IQAir reported an AQI of 442, classifying it as 'hazardous,' while the CPCB rated the city's air quality at 350, which falls into the 'very poor' category. In some locations, this difference was even more pronounced, with IQAir reporting values exceeding 2,000 while CPCB reported values below 400. This begs the question: who has got Delhi's AQI right?
The answer, according to experts, is that both readings are technically correct, but they employ different methodologies and scales. The Indian AQI, developed in 2015, is calculated based on six key parameters: PM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide. It uses a graded scale ranging from 0 to 500, where 0-50 is considered "Good," 51-100 is "Satisfactory," 101-200 is "Moderate," 201-300 is "Poor," 301-400 is "Very Poor," and 401-500 is "Severe". The Indian system caps readings at 500 because health risks are already considered severe at that level, and higher values are seen as having limited practical use. To calculate AQI, data for a minimum of three pollutants must be present, of which one should be either PM10 or PM2.5.
On the other hand, IQAir utilizes a US-based model based on the US Environmental Protection Agency (EPA)'s AQI. While the US EPA's AQI also follows a 0-500 scale, it allows values beyond 500, with anything above 301 classified as hazardous. This difference in scale is one reason why IQAir often reports much higher values than the CPCB.
Another significant difference lies in the data sources. The CPCB relies on analyzer-based reference-grade monitors, which are considered more accurate and standardized. These are part of the National Air Monitoring Program (NAMP), covering 240 cities with over 342 monitoring stations. The AQI is calculated by a web-based system that captures data from these continuous monitoring stations automatically. In contrast, IQAir's index is largely sensor-based. While this allows for more widespread monitoring, the calibration and reference methods used by these sensors may not be as rigorous as those used by the CPCB.
It's also important to note that the formula to calculate the AQI is the same as per the Indian CPCB and US-EPA. The AQI is calculated using equations separately for each parameter. For example, if you wish to calculate AQI on the basis of four parameters, use the equation four times, and the worst sub-index communicates the AQI. A subindex is a linear function of the concentration of pollutants.
Despite the numerical discrepancies, experts agree that both platforms reflect the same underlying reality: Delhi's air quality is very poor and exceeds safe limits. Following Diwali, a combination of firecrackers, cold stagnant air, construction dust, vehicle emissions, and crop-burning smoke has led to a surge in pollution levels.
While the CPCB's data is considered more reliable due to its standardized methodology and accurate monitoring systems, the higher values reported by IQAir serve as a stark reminder of the severity of the pollution. Regardless of the specific number, the consensus is that Delhi's air quality poses a significant health risk, particularly for those with respiratory issues.
Ultimately, whether the AQI is 350 or 1,100, the message is clear: Delhi's air pollution requires urgent and sustained action.
