Delhi's air quality turned "very poor" on Monday amid an increase in stubble burning, bursting of firecrackers and moderately unfavourable meteorological conditions which allowed accumulation of pollutants.

However, the 24-hour average air quality index (AQI) of 312 was still the second best for the Diwali day in seven years. Before this, the city recorded an AQI of 281 on Diwali in 2018.

Delhi had recorded an AQI of 382 on Diwali last year, 414 in 2020; 337 in 2019; 319 in 2017; and 431 in 2016, according to the Central Pollution Control Board data.

The neighbouring cities of Ghaziabad (301), Noida (303), Greater Noida (270), Gurugram (325) and Faridabad (256) reported poor to very poor air quality.

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", and 401 and 500 "severe".

The relatively better air doesn't mean it's good air. The PM2.5 levels at 25 monitoring stations in the national capital were five to six times the national standard of 60 microgram per cubic metre at 4 pm.

PM2.5 are fine particles that are 2.5 microns or less in diameter and can travel deep into the respiratory tract, reaching the lungs and entering the bloodstream.

On Sunday evening, the city reported a 24-hour average AQI of 259, which was lowest for the day before Diwali in seven years.

Pollution levels crept up at night amid a drop in temperature and wind speed as people burst firecrackers in several parts of the national capital and the number of farm fires rose to 1,318, the highest this season so far.

The air quality situation remained largely stable during the day due to moderate wind speed and warm conditions.

However, low temperature, calm winds and emissions from firecrackers at night may push the air quality deep into the "very poor" category or even the "severe" zone by early Tuesday morning.

SAFAR, a forecasting agency under the Union Ministry of Earth Sciences, had earlier predicted that the air quality may deteriorate to "very poor" on Monday morning (Diwali) due to calm winds and low temperatures which allow rapid accumulation of pollutants in the air.

It will continue to remain "very poor" if no firecrackers are burst, it said.

In case firecrackers are burst like last year, the air quality may plunge to "severe" levels on the night of Diwali itself and continue to remain in the "red" zone for another day, the System of Air Quality and Weather Forecasting and Research (SAFAR) had predicted.