An AQI between 201 and 300 is considered 'poor', 301 and 400 'very poor' and 401 and 500 is 'severe'
Delhi's air quality significantly deteriorated Sunday with a fall in temperature which slowed dispersion of pollutants, as authorities warned pollution level might further increase in the next two days.
The Central Pollution Control Board (CPCB) recorded an overall air quality index (AQI) of 377, the highest pollution level in nearly 20 days.
Ghaziabad, Noida and Faridabad recorded 'severe' air quality, CPCB data showed.
An AQI between 201 and 300 is considered 'poor', 301 and 400 'very poor' and 401 and 500 is 'severe'.
The air quality in seven areas -- Anand Vihar, Mundka, R K Puram, Rohini, Vivek Vihar, Jahangirpuri and Nehru Nagar recorded 'severe air quality while 20 areas recorded 'very poor' air quality, according to the CPCB.
The overall PM2.5 -- fine particulate matter in the air with a diameter of less than 2.5 micrometre -- level was recorded at 221 and the PM10 level at 389, it said.
The Centre-run System of Air Quality and Weather Forecasting (SAFAR) said the overall air quality of Delhi will further deteriorate over the next two days.
"Winds are calm and dispersion is low. Western disturbances influence may impact Delhi's air quality by introducing the moisture and making air heavy. The expected fall in temperature and moderate fog is likely to increase pollution over coming two days is expected to the upper level of very poor," the SAFAR said.
The SAFAR said that the air quality might improve on Wednesday if sufficient amount of rainfall occur, which is expected.
"However, marginal shower often deteriorates air quality because high moisture content over shadows the wash out effect. In all probability, the AQI will remain within the limit of very poor and will not touch severe," it said.
"Levels of gaseous pollutants, NOx and CO are forecast to be enhanced -- up to moderate range -- after a long time due to a fall in boundary layer height and reduced vertical mixing," the SAFAR said.