Delhi's AQI Is 'Severe', But Why Does The AQI Number Stop At 500? CPCB Explains

News india Delhi's AQI Is 'Severe', But Why Does The AQI Number Stop At 500? CPCB Explains

Last Updated:November 10, 2026, 22:46 IST

The CPCB attributed differences between official readings and private platforms to variations in measurement techniques and calculation methods.

Delhi’s average Air Quality Index (AQI) currently lying between 314 and 382, falling in the ‘Very Poor’ to ‘Severe’ categories. Amid concerns over pollution levels, the Central Pollution Control Board (CPCB) has explained why India’s AQI is capped at 500, even when pollution rises further, saying the limit is a deliberate feature of the index and is in line with international practices.

In a clarification issued on November 10, the CPCB said an AQI reading of 500 or above falls under the ‘Severe’ category, with the health advisory remaining the same beyond that level. Increasing the numerical value would not change the assigned health impact category.

The clarification comes amid questions over the reporting of air quality during and after Diwali, including differences between official readings and those displayed by private platforms, as well as data gaps at some monitoring stations in Delhi.

Why is the AQI capped at 500?

According to the CPCB, the National Air Quality Index (NAQI) ranges from zero to 500 and has six categories: ‘Good’, ‘Satisfactory’, ‘Moderately Polluted’, ‘Poor’, ‘Very Poor’ and ‘Severe’.

The upper limit of 500 represents the highest level of health concern. The board said an AQI of 500 or above indicates broadly the same health risk, falling under the ‘Severe’ category.

The associated health warning states that such conditions may cause respiratory effects in healthy people and serious health impacts on those with lung diseases.

“The capping at 500 is thus a design feature of the index," the CPCB said, adding that the methodology, developed in 2015, follows prevailing international practices, including the US Environmental Protection Agency’s AQI scale.

How does CPCB calculate AQI?

The NAQI was launched in 2015 and developed by the CPCB through the Indian Institute of Technology Kanpur, under the guidance of an expert group comprising medical professionals, air quality experts, academics, NGOs and State Pollution Control Boards.

The index is calculated every hour using eight pollutants: PM2.5, PM10, carbon monoxide (CO), nitrogen dioxide (NO2), sulphur dioxide (SO2), ammonia (NH3), ozone and lead.

For most pollutants, the calculation uses hourly averages over the preceding 24 hours. For ozone and carbon monoxide, the previous eight-hour average is considered. The highest sub-index among the pollutants determines the AQI for that hour.

The CPCB said at least 16 hours of data out of 24 are required to generate a sub-index for most pollutants, except CO and ozone. At least three pollutants must be available, including either PM2.5 or PM10.

The board added that similar methodologies are followed in the US and China. The NAQI is calculated through an automated system without human intervention.

Why do private apps show different AQI readings?

The CPCB attributed differences between official readings and private platforms to variations in measurement techniques and calculation methods.

According to the board, some private platforms use instantaneous hourly readings and low-cost sensors that may have limitations in calibration and data quality. Their AQI breakpoints may also differ from those used by the NAQI.

Such platforms may show sharper increases during short pollution spikes. However, when pollution levels begin falling, their readings can drop faster than the official index, which continues to reflect exposure over the preceding 24 hours.

The CPCB said the NAQI is designed to measure exposure over time, making prolonged pollution more relevant to health than brief spikes, such as those caused by firecracker bursts.

Why do some monitoring stations show data gaps?

The CPCB said particulate matter analysers installed when the NAQI was introduced in 2015 were calibrated for a measurement range of 0-1,000 micrograms per cubic metre (µg/m³).

Under the automated validation system, hourly readings beyond this range are treated as outside the calibrated limit and excluded from calculations. During extreme pollution episodes, this can result in data gaps at individual monitoring stations, with those hours not contributing to the relevant pollutant’s 24-hour average.

The board said it was reviewing the methodology and consulting experts on how readings beyond the system’s design range should be processed.

It also said the number of Continuous Ambient Air Quality Monitoring Stations had increased from 30 across the country in 2014-15 to 634 at present. Delhi’s monitoring network has expanded from 10 stations in 2014-15 to 59.

The CPCB said the NAQI is India’s official index for reporting air quality and is available on its website and through the SAMEER mobile application. It advised citizens, particularly children, elderly people and those with respiratory or cardiac conditions, to follow the health advisories linked to the prevailing AQI category.

About the Author

Shuddhanta PatraSenior Sub-Editor

Shuddhanta Patra is an Indian journalist with about nine years of experience in covering national politics, geopolitics, defence, diplomacy and international affairs. Her reporting focuses on South As...Read More

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