CPCB explains Delhi AQI cap and monitoring data gaps

The Central Pollution Control Board says its official Air Quality Index is capped at 500 by design and that extreme readings can disappear from station averages when older monitors exceed their measurement range. The explanation follows questions about gaps in Delhi’s monitoring data and differences between government figures and private app readings after Diwali.

CPCB said the National Air Quality Index was developed in 2015 as a public communication tool. Its 500 ceiling marks the top of the Severe category, which the board said represents the highest level of health concern on the scale. The index is intended to convey risk through its category and associated health advice, rather than indicate unlimited increases in pollution through higher numbers.

The board said older particulate-matter analysers were calibrated to measure concentrations up to 1,000 micrograms per cubic metre. Under automated validation, hourly readings above that range are treated as outside the instruments’ calibrated limits and excluded. During extreme episodes, those exclusions can leave gaps in data for an individual station and remove the affected hours from that station’s 24-hour pollutant average,...