Delhi's air quality was on the brink of turning severe on Tuesday morning which witnessed worst pollution of the season as stubble burning in neighbouring states intensified, authorities said.
The air quality of Delhi was recorded at 397, just three points below the severe level and highest of this season, according to the data of the Central Pollution Control Board.
Seventeen areas of the national capital recorded severe air quality, the data said.
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".
The Centre-run System of Air Quality Forecasting And Research (SAFAR) attributed heavy stubble burning in the last 24 hours and calm winds as the reason behind the further deterioration of air capital.
The SAFAR said about 28 per cent of pollution by PM2.5 (presence of particles in the air with a diameter of less than 2.5 micrometres) on Tuesday was caused due to regional factors like stubble burning.
PM2.5 was recorded at 251, the highest of this season. Fine particulates can be a matter of more serious health concern than PM10.
The PM10 level (particles in the air with a diameter of less than 10 micrometres) in Delhi stood at 453, according to the CPCB data.
The SAFAR said the pollution is likely to increase further tomorrow and day after and then slow down.
"Air quality index will be in the upper level of very poor for the next three days. The main reason is heavy biomass burning in the past 24-hours and local calm winds," the SAFAR said.
The Indian Institute of Tropical Meteorology (IITM) also spotted a large number of biomass fire spots through satellite imageries in neighbouring states of Delhi.
The prevailing meteorological conditions are less favourable for dispersal of pollutants for the next two days due to moderate ventilation index and low wind speed, it said.
"As per Air Quality Forecast the overall Air Quality of Delhi is likely to remain close to upper end of very poor category in the next two days," the IITM said.
A thick haze due to smog continued to hover over the national capital, authorities said.