India, Nov. 5 -- Delhi's official air quality index dropped from 366 to 309 between November 2 and 3, but a detailed analysis of monitoring data raises questions about the reliability of pollution readings during one of the city's most toxic weeks of the year.

Missing data, suspicious measurement patterns, and algorithmic loopholes in how the city's average AQI is calculated appear to have combined to produce readings that may not accurately reflect ground conditions, an HT analysis of Central Pollution Control Board data shows.

To understand the issue, it is essential to unpack the mechanism to calculate AQI, which contains built-in flexibility designed to ensure readings are available even when some stations malfunction. But this flex...