While the national capital enjoyed relatively clean air for three months, the air quality dipped to ‘poor’ category last week, and further dropped to ‘very poor’ category this week.

Latest images from US space agency NASA show several occurrences of stubble burning by farmers in neighbouring states of Haryana and Punjab and adjoining areas of Pakistan.

The overall Air Quality Index (AQI) in Delhi touched the 301 mark by 10 pm on Sunday.

An AQI between 0 and 50 is considered ‘good’, 51 and 100 ‘satisfactory’, 101 and 200 ‘moderate’, 201 and 300 ‘poor’, 301 and 400 ‘very poor’, and 401 and 500 ‘severe’.

A layer of thick smog lingered over Delhi on October 13-14 as the air quality plummeted to ‘very poor’ category, days before strict measures to fight air pollution come into force under the Graded Response Action Plan (GRAP).