The Central Pollution Control Board (CPCB) has explained why India's official Air Quality Index (AQI) stops at 500, even as some private mobile applications show readings exceeding 1,000 and, in certain cases, 1,800. In a clarification issued on November 10 following concerns over post-Diwali pollution data, the board said the 500-point ceiling is a feature of the index's design and does not mean pollution is being under-reported.

The CPCB said the National Air Quality Index (NAQI), launched in 2015, follows a scale of 0 to 500. Any reading above 500 remains within the 'Severe' category, and extending the numerical scale would not change the health category assigned to it.

"The capping at 500 is thus a design feature of the index, as has been evolved in the 2015 methodology of NAQI in line with the prevailing international practice, including the US EPA AQI scale," the board said.

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Why Do Private Apps Show Higher AQI?

The CPCB attributed differences between official readings and private app figures to variations in monitoring equipment, data sources, averaging periods and calculation methods.

It said some private platforms use low-cost sensors, instantaneous hourly readings and different AQI breakpoints. Such figures cannot be directly compared with the official NAQI, which uses 24-hour averages and calibrated, quality-assured data.

The board also explained that private apps may show higher readings during short pollution spikes but lower figures when concentrations fall.

"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, since the NAQI continues to reflect the exposure of the preceding 24 hours," it said.

How Is The Official AQI Calculated?

The CPCB calculates the AQI every hour using data for eight pollutants: PM2.5, PM10, carbon monoxide (CO), nitrogen dioxide (NO2), sulphur dioxide (SO2), ammonia (NH3), ozone and lead.

The highest sub-index among the pollutants determines the AQI for that hour. Data for at least three pollutants, including PM2.5 or PM10, is required for the calculation.

The index has six categories: Good, Satisfactory, Moderately Polluted, Poor, Very Poor and Severe. The CPCB said the Severe category indicates the risk of respiratory effects even among healthy people and serious health impacts on those with lung diseases.

Why Can Extreme Pollution Readings Be Missing?

CPCB said its particulate matter analysers were calibrated for concentrations ranging from 0 to 1,000 microgrammes per cubic metre (µg/m³). Its automated data-validation algorithm automatically designates measurements as out-of-range measurements when they go above this calibrated range and does not incorporate those data points into the averages.

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This range can, in cases of severe pollution episodes, such as Diwali firecracker burning, also go beyond this range. These can be recorded as blanks on monitors at the respective stations. These hours are not included in the 24-hour average of the pollutant.

According to the board, the country's Continuous Ambient Air Quality Monitoring Stations (CAAQMS) increased from 30 in 2014-15 to 634 as of November 10, including 59 in Delhi.

The CPCB argued that NAQI still serves as the official AQI for publishing and reporting the data. It also asked people to adhere to health advice associated with the current AQI category, especially for children, the elderly and those with respiratory or cardiac conditions.