Air quality in Delhi and adjoining regions improved from "severe" to "very poor" on Wednesday as wind speed picked up due to western disturbance over north India and dispersed pollutants.

With quality of air oscillating between "poor" and "very poor", the National Green Tribunal (NGT) on Monday asked the Delhi government to deposit Rs 25 crore with the Central Pollution Control Board (CPCB) for its failure to curb air pollution in the city.

Delhi and the National Capital Region (NCR) on Sunday witnessed 'very poor' air quality with the minimum temperature recorded at 12.4 degrees Celsius, two notches below the season's average, the Met office said.

Wind directions changed by Tuesday noon, creating an outlet for the toxic air over the Delhi-NCR region and thereby improving the air quality to "very poor," from "severe" in the morning and "emergency" a day before.

The national capital and the areas around experienced the first smog episode of this season as the city turned into gas chamber two days before Diwali with air quality placed in "emergency" or "beyond-severe" category.

With air quality in Delhi-NCR finally improving to "moderate" due to the meteorological conditions, the pollution monitoring agencies say it is the first December in three years when the national capital inhaled "normal" air.

US-based passenger carrier United Airlines on Saturday said it has temporarily suspended its Newark-New Delhi flights due to poor air quality concerns in New Delhi.

The country's top green court on Friday questioned the efficacy of the Delhi government's proposed odd-even vehicle rationing scheme that was to be rolled out from Monday as the capital and its suburbs remained shrouded in a dense haze, with air pollution levels continuing to be "severe".

The alarming pollution in Delhi has led to at least 20 per cent increase in the number of persons complaining of cardiac/respiratory problems this week, officials said on Thursday.