New Delhi: To understand the air pollution situation in Delhi-NCR, people track data from the Central Pollution Control Board (CPCB) and Switzerland-based company IQAir. However, the Air Quality Index (AQI) on the two platforms often shows a major difference for the same city and at almost the same time. The reason is not necessarily a change in air quality, but the different methods used to measure pollution and convert the data into AQI.
In India, the CPCB’s National Air Quality Index is based on eight pollutants. These include PM2.5, PM10, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead. A separate sub-index is prepared for each pollutant, and the highest value among them is considered the AQI for that monitoring station. For this, data for at least three pollutants must be available.
Under the CPCB system, the 24-hour average is generally important for PM2.5 and PM10. IQAir, on the other hand, uses the AQI scale of the United States Environmental Protection Agency (US EPA) and prepares its index on the basis of six key pollutants. Its AQI Plus mainly reflects pollution levels over the past one hour. Therefore, when pollution rises suddenly, IQAir’s number can change rapidly.
The method of compiling city-level data also differs between the two platforms. In Delhi, the average of AQI data received from CPCB and DPCC monitoring stations is taken, while IQAir uses the median of hourly pollution data from the city’s monitoring stations. IQAir also includes data from low-cost sensors along with government stations.
India’s national AQI ranges from 0 to 500. A level between 401 and 500 is considered severe. On the other hand, figures on the US scale can also go above 500. In such a situation, a direct comparison of the numbers on the two platforms is not appropriate.
Therefore, while checking AQI, it is important not to depend on only one number, but also to look at the scale, measurement period, monitoring station and dominant pollutant. Different figures do not mean that the data from one platform is necessarily wrong. Both describe the air situation through different methodologies.
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