CPCB Rejects AQI Under-Reporting Charge, Cites Methodology and Hardware Limits

The Central Pollution Control Board has rejected allegations that it under-reported Delhi’s air pollution during Diwali, saying differences between its readings and private air quality apps came from scientific methodology and hardware limits. In a clarification issued through a press briefing on Tuesday, the board responded to media reports that questioned its Air Quality Index reporting during and after Diwali.

The dispute centred on apparent data gaps at some Delhi monitoring stations, a perceived AQI “cap” of 500 and wide gaps between official values and private platforms. CPCB classified Delhi’s citywide average AQI as “very poor” on Diwali night, even as many stations were already in the “severe” category above 400.

Private platforms showed far higher peaks. AQI.in reported a peak AQI of 2,283 immediately after Diwali and 637 by Monday morning. CPCB stated such comparisons can be misleading because its National Air Quality Index is built to reflect exposure over 24 hours rather than sudden hourly jumps.

How Is AQI Calculated?

In India, AQI is calculated by the National Air Quality Index (NAQI) which was launched in 2015. According to the CPCB, the AQI is generated hourly from eight different parameters namely PM2.5, PM10, CO, NO2, SO2, NH3, Ozone and Lead.

"The sub-index for each parameter is calculated from its hourly average over the previous 24 hours, using the corresponding health breakpoints (for Ozone and CO, the average of the previous 8 hours is used instead), in line with the prescribed standards. The worst i.e. highest sub-index among all pollutants is taken as the AQI for that hour," the board said.

The methodology used by the NAQI is very similar to the the United States Environmental Protection Agency (US EPA) as well as China.

The CPCB also clarified that the NAQI data is published without human intervention and is fully automated.

"The NAQI is computed and disseminated in near real time through a fully automated system without human intervention, which has been in operation since inception and ensures uniformity and comparability of air quality information over time," the CPCB said.

The Difference In Readings

The board traced the issue to older particulate matter analysers installed when the NAQI was introduced in 2015. Those instruments were calibrated for a measurement range of 0-1,000 micrograms per cubic metre.

CPCB stated its automated data validation system treats any hourly reading above that calibrated range as an error. Those extreme readings are then left out of the final calculation.

“During extreme episodes, which are occasional, like Diwali firecracker bursting, this may appear as a data gap at the particular station concerned, and those hours do not contribute to the 24-hour average of relevant pollutant for the concerned station,” the board stated.

CPCB also drew a distinction between hourly spikes and the 24-hour NAQI average. Private apps respond instantly when pollution jumps, while the official index is designed to show sustained health risk.

“It is generally perceived that private apps show higher AQI. This may hold during short peaks, when hourly values rise sharply. However, when concentrations fall, such apps show lower AQI than the NAQI (National Air Quality Index), since NAQI continues to reflect the exposure of the preceding 24 hours,” the CPCB stated.

The board added that NAQI converts complex pollutant data into a single colour-coded index meant to communicate long-term health risks. “The NAQI provides a consistent and reliable indication of the likely health impacts of sustained exposure to pollutants, which is the scientifically relevant measure for public health advisories,” the CPCB stated.

Monitoring Network Upgrade

The board stated monitoring technology has moved on since 2015. It added that older instruments are being replaced continuously and the monitoring network is also being expanded on an ongoing basis.

CPCB linked that upgrade to the risk of future data dropouts during extreme pollution episodes. The board stated the changes were aimed at improving both reporting and coverage.

“CPCB remains committed to transparent, scientific and reliable reporting of air quality and to continuous improvement of the monitoring network and methodology in consultation with experts,” the board stated.

Post Diwali, AQI Still An Issue

According to CPCB data today, the AQI in Post-Diwali Delhi remained an issue as a reading of 303, in the 'Very Poor' category, was recorded at Chandini Chowk at 12 AM.

An AQI between 0 and 50 is classified as 'good', 51 and 100 as 'satisfactory', 101 and 200 as 'moderate', 201 and 300 as 'poor', 301 and 400 as 'very poor', and 401 and 500 as 'severe'.

According to media reports the government is planning to use artificial rain through cloud seeding to curb the rising population in the capital with trials having already been undertaken in late October and early November near the Indira Gandhi International Airport.