New Delhi,  The air quality index of the national capital turned foul and mounted to 'very poor' category at 304 on Tuesday morning, with northwest district recording the most polluted air, according to the Central pollution Board's data.

Tuesday was the season's worst air day so far.

The air pollution reaches a crescendo in Delhi-NCR 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, thus creating health hazards for the residents.

As per the pollution control board, out of the 36 pollution monitoring stations, the air quality index in as many as 19 stations is in the 'very poor' category, 14 stations recorded the index in the 'poor' category, one recorded it in the 'moderate' category and two were non-functional.

The area near northwest Delhi's Wazirpur recorded the highest AQI at 379, followed by 364 in Dwarka Sector 8 and Mundka. Lodhi road logged the cleanest air in the capital city at 193.

Delhi's neighbouring areas -- Ghaziabad, Faridabad, Noida and Greater Noida -- have also recorded 'very poor' quality of air. Greater Noida's air is currently the most polluted amongst all. Another neighbour Gurugram, on the other hand, has the lowest AQI at 289 in the 'poor' category.

Nationwide, as many as nine cities have very poor quality of air. Rajasthan's Bhiwadi tops the charts, followed by Greater Noida, Panipat, Bhagpat, Ghaziabad, Faridabad, Delhi, Agra and Noida.

The System of Air Quality Weather Forecasting and Research (SAFAR) categorizes air quality in the 0-50 range as good, 51-100 as satisfactory, 101-200 as moderate, 201-300 as poor, 301-400 as very poor and above 400 as severe.

According to the India Meteorological Department, the spike in pollutants is a result of low wind speeds, along with an increase in stubble burning in the neighbouring states of Punjab and Haryana.

Pawan Gupta, a Universities Space Research Association scientist at NASA's Marshall Space Flight Centre, said that farm fires are more widespread in Haryana, Punjab, Uttar Pradesh and also in parts of border areas in Pakistan. NASA's satellite imagery also corroborated his contentions.