The air quality of the national capital and the surrounding regions has improved marginally owing to a slight increase in surface winds, but is likely to deteriorate mid-week, the System of Air Quality and Weather Forecasting and Research, a Central government agency said on Monday.

The national capital city's air quality index stood at 237 at 1.30 p.m., which falls in the 'poor' category.

The air pollution reaches a crescendo in Delhi and surrounding regions every winter, when pollution from stubble burning combines with the suspended water droplets in the lower atmosphere to form a thick blanket of noxious smog.

"A change in the surface wind direction is forecasted which is likely to bring calm surface wind conditions leading to low ventilation and deterioration of AQI. Very poor AQI is predicted for 21st and 22nd October," the air quality forecasting system added.

SAFAR recorded 1,090 stubble burning fire counts around Haryana, Punjab, and neighbouring regions. "Though transport-level wind direction is favourable, an increase in transport level wind speed and local Delhi surface winds leads to a decrease in the stubble burning percentage contribution in Delhi by PM2.5," it added.

The SAFAR model estimate of the stubble burning share in PM2.5 is 10 per cent for Monday, as against 17 per cent on Sunday and 19 per cent a day before. Pawan Gupta, a Universities Space Research Association scientist at NASA's Marshall Space Flight Centre said out of Punjab, Haryana and Gujarat, the former is recording the greatest number of farm fires this season.

As per the Central Pollution Control Board (CPCB), the air quality index in as many as 31 pollution monitoring stations in Delhi is in the 'poor' category, one station recorded the index in the 'very poor' category, three recorded it in the 'moderate' category and one logged it in 'satisfactory' category.

The pollution monitoring station in North West Delhi's Jahangirpuri area recorded the most polluted air at 310, followed by 277 at Dwarka Sector-8 and 375 at Bawana area. Punjabi Bagh area logged the least air quality index at 91.

Meanwhile, Delhi's neighbouring regions — Gurugram, Ghaziabad, Faridabad, Noida and Greater Noida — also recorded 'poor' quality of air. Greater Noida and Gurugram's air is currently the most polluted amongst all.

Nationwide, as many as four cities have very poor quality of air. Haryana's Yamunanagar tops the charts at 363, followed by Uttar Pradesh's Moradabad, Varanasi and Bulandshahr. Karnataka's Mysuru city recorded the cleanest air in the country..