Delhi’s air quality remained in the “severe plus” category on Sunday, with 25 of the city’s 40 monitoring stations recording Air Quality Index readings above 450, according to data from the Sameer application at 12.05 pm.

The national capital’s average AQI stood at 457, the application which provides hour updates from the Central Pollution Control Board, showed. Data from one monitoring station was unavailable at the time.

The cities adjoining the capital also reported hazardous air quality levels on Sunday. Noida recorded an AQI of 466, Ghaziabad 457 and Greater Noida 434.

A day earlier, Delhi’s overall AQI had touched 431, rising sharply from 349 on Friday and 307 on Thursday.

An index value between 301 and 400 indicates “very poor” air.

Between 401 and 450 indicates “severe” air pollution, while anything above the 450 threshold is termed “severe plus”.

An AQI in the “severe” and “severe plus” category signifies hazardous pollution levels that can pose serious risks even to healthy individuals.

The deterioration in the air quality has been attributed to unfavourable weather conditions, including very low wind speeds caused by a western disturbance, the Hindustan Times reported.

On Saturday, as the air quality slipped into the “severe plus” category, the Commission for Air Quality Management imposed Stage 4 restrictions under the Graded Response Action Plan to curb pollution in the Delhi-National Capital Region.

Earlier in the day, the authorities had reimposed Stage 3 curbs, which were upgraded to Stage 4 by night as pollution levels worsened.

This was the first time this season that Stage 4 restrictions, triggered when the AQI breaches the 450 mark, were enforced in Delhi-NCR.

The curbs include a ban on the entry of trucks into the region, a halt on construction activities for both public and private projects, and a shift to hybrid mode for all schools except Classes 10 and 12.

GRAP is a set of incremental anti-pollution measures that are triggered to prevent further worsening of air quality once it reaches a certain threshold in the Delhi-NCR region.

Delhi has been recording air quality in the “poor” or worse categories since mid-October.

Air quality deteriorates sharply in the winter months in Delhi, which is often ranked the world’s most polluted capital.

Stubble burning in Punjab and Haryana, along with the lighting of firecrackers during Diwali, vehicular pollution, falling temperatures, decreased wind speeds and emissions from industries and coal-fired plants contribute to the problem.

Watch: Delhi’s pollution crisis needs drastic action