A sharp spike in Delhi's air pollution was witnessed in the past 24 hours with 30 out of 36 monitoring stations reporting a severe air quality index.

The GRAP (Graded Response Action Plan) has 4 stages — stage I when AQI ranges between 201 and 300, stage II when AQI is 301-400, stage III when AQI is 401-450 and stage IV when AQI is 450-500.

Toxic smog blanketed northern India on Thursday, becoming too thick to see through in several places, as high levels of pollution combined with humidity, low wind speed, and a drop in temperature, officials said.

Dense smog engulfed Delhi and its surrounding areas on Wednesday morning with visibility dropping and the air quality settling in the "very poor" category. According to data by the Central Pollution Control Board,