Air quality in the national capital region was in the severe category on Tuesday morning. The air quality index, AQI was recorded 414 at 10 AM. On Monday, the air quality was in the 'very poor' category.<br />''<br />''An AQI between Zero to 50 is considered 'good', 201 to 300 'poor', 301 to 400 'very poor' and 401 to 500 'severe'. Above 500 falls in the 'severe plus' category.<br />''<br />''With the onset of winter, a dip in the minimum temperature is making the air cold and heavy leading to accumulation of pollutants close to the ground.