A thick blanket of toxic smog engulfed most parts of northern India on Monday and readings of air quality in the capital New Delhi hit their highest this year after dense fog overnight. Visibility dropped to 100 m (109 yards) in Delhi and Chandigarh, a city northwest of the capital, but authorities said flights and trains continued to operate with some delays. India's pollution control authority said the national capital territory's 24-hour air quality index (AQI) reading was at 484, classified as "severe plus", the highest this year.