The Central Pollution Control Board (CPCB) has defended its air quality reporting after questions were raised over the Air Quality Index (AQI) readings recorded during and after Diwali.
The CPCB clarified that brief pollution spikes caused by firecrackers do not necessarily reflect the air quality levels used for public health advisories. In a statement issued on November 10, the pollution watchdog said the AQI is calculated using pollutant concentrations averaged over specified periods. It explained that the index reflects exposure over time and that short-lived increases in pollution, such as those during firecracker bursting, are not included in the 24-hour average used for the relevant pollutants.
The clarification comes amid concerns over differences between CPCB’s AQI figures and readings reported by other platforms, including claims that the official index was capped at 500 even when monitoring stations recorded higher concentrations.
The CPCB said the AQI scale ranges from zero to 500, with categories ranging from ‘Good’ and ‘Satisfactory’ to ‘Moderate’, ‘Poor’, ‘Very Poor’ and ‘Severe’.
An AQI of 500 or above represents the highest level of health concern on the scale. The board said the upper limit is a feature of the index’s design and does not change the health category assigned to the reading. It added that the AQI methodology was developed in 2015 in line with prevailing international practices, including the United States Environmental Protection Agency’s AQI scale.
Explaining how the index is calculated, the CPCB said it is generated every hour using sub-indices for eight pollutants: PM2.5, PM10, carbon monoxide, nitrogen dioxide, sulphur dioxide, ammonia, ozone and lead. The sub-index for each pollutant is calculated using its average concentration over the preceding 24 hours, except for ozone and carbon monoxide, for which an eight-hour average is used. The highest sub-index among the pollutants determines the overall AQI for that hour. The CPCB also sought to explain why some private applications may show higher or lower readings.
It said several platforms use data from different sources, including low-cost sensors that may not follow the National Ambient Air Quality Standards (NAAQS), 2009, or the prescribed calibration and quality assurance procedures.
Some platforms may also use different averaging periods. Such readings, the board said, cannot be directly compared with the official AQI, which is based on calibrated and quality-assured data from its monitoring network.
On reports that private applications sometimes show lower AQI levels than the official figures, the CPCB said this could happen because the official index continues to account for pollution exposure over the preceding 24 hours. It maintained that the resulting figure provides a more consistent indication of sustained exposure and is more relevant for public health advisories.
The board further said particulate matter analysers were initially calibrated for a measurement range of zero to 1,000 micrograms per cubic metre. Readings beyond the calibrated range are treated as outside the range and are not used in computation. During extreme pollution events, including Diwali firecracker bursts, this may create data gaps at particular stations.