Delhi's air quality deteriorated further with six areas in the city recording severe pollution as authorities predicted further rise in the next two days.
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 is considered to be "severe".
Twenty-five areas of the national capital recorded "very poor" air quality, while in four areas it was in the "poor" category, it said.
The PM2.5 (fine particulate matter in the air with a diameter of less than 2.5 micrometre) level was recorded at 208 and the PM10 level at 397, it added.
In Gurgaon, the air quality was "poor", it said.
The Centre-run System of Air Quality and Weather Forecasting (SAFAR) said the air quality index dropped speedily as expected, but remained "very poor".
"Although increasing-AQI trend is predicted from tonight (Tuesday night) for the next two days, it will remain in 'very poor' range only. This is mainly due to decline in wind speed along with adverse meteorological conditions. The contribution from long-range dust or stubble biomass is negligible," the SAFAR said.
According to the Indian Institute of Tropical Meteorology (IITM), the maximum ventilation index is likely to be 3,500 sqm/second on Tuesday.
A ventilation index lower than 6,000 sqm/second with average wind speed less than 10 kmph is unfavourable for dispersion of pollutants, the IITM said.
Delhi's air quality has been in the "very poor" category for the past one week but on Saturday it had improved to "poor" category for a brief period. It again slipped into "very poor" category Sunday, the authorities said.