While the Central Pollution Control Board (CPCB) pegged Delhi’s AQI at 494 on Monday, IQAir, an international monitoring app, showed that the pollution had touched an eye-popping level of 1,600. But what’s the reason for this big discrepancy?

An explainer on smog as thick haze from stubble burning, weather, and pollutants choke the city, prompting emergency measures under GRAP to combat the severe air quality crisis.