Delhi's air quality inched closer to the 'severe' category on Monday morning with calm winds and cloudy conditions trapping pollutants, according to monitoring agencies.

A thick layer of smog blanketed Delhi and its suburbs, reducing visibility to just 600 metres at 8 am at the Safdarjung Observatory, the national capital's primary weather station. The visibility was 800 metres at the Indira Gandhi International Airport.

An official at the India Meteorological Department said a slight increase in wind speed and light rainfall may bring marginal relief during the day. Unfavourable atmospheric conditions are likely to persist for two to three days, the official said.

Delhi's air quality index (AQI) stood at 400 at 9 am. The 24-hour average AQI, recorded at 4 pm every day, was 395 on Sunday, 389 on Saturday, 415 on Friday, 390 on Thursday, 394 on Wednesday, 365 on Tuesday, 348 on Monday and 301 on November 19.

An AQI between zero and 50 is considered 'good', 51 and 100 'satisfactory', 101 and 200 'moderate', 201 and 300 'poor', 301 and 400 'very poor', 401 and 450 'severe' and above 450 'severe-plus'.

The national capital has recorded 10 severe air quality days this November so far. The city recorded just three severe air quality days in November last year, while it experienced 12 such days in 2021, the maximum in the month since the Central Pollution Control Board (CPCB) began monitoring.

There were nine such days in November 2020, seven in 2019; five in 2018; seven in 2017; 10 in 2016, and six in 2015, according to CPCB.

According to a joint project by the Delhi government and the Indian Institute of Technology-Kanpur, biomass burning was the top reason for Delhi's foul air, contributing 31 to 51 per cent to the capital's air pollution in the last few days.

Delhi Environment Minister Gopal Rai has directed agencies and departments concerned to ensure strict implementation of curbs on polluting vehicles and check the increasing incidents of biomass burning.