The air quality index in Delhi has worsened and people in the national capital breathed the most polluted air in 63 days. Overall Air Quality Index (AQI) in the city slipped into the ‘severe’ category on Jan 9, which stood at 434. The poorest air quality logged in Delhi, since November 4. A combination of low wind speed, low temperatures, burning of waste and fog led to the worsening air quality. Delhi govt banned BS-3 petrol and BS-4 diesel vehicles in the capital till January 13.