Delhi's air quality worsened further on Sunday, with an AQI of 441, making it the second-most polluted city in the country.

The city's 24-hour Air Quality Index (AQI), recorded at 4 pm daily, stood at 441, in the "severe" category. On Saturday, the AQI was 417.

According to the Central Pollution Control Board (CPCB), a "severe" AQI poses risks to healthy individuals and severely impacts those with pre-existing health conditions.

Four cities in the country recorded the AQI in the "severe" category. Bahadurgarh in Haryana topped the list with an AQI of 445, followed by Delhi (441), Bhiwani in Haryana (415) and Bikaner in Rajasthan (404), according to the CPCB's data.

Of the 40 monitoring stations in the capital, data from 34 provided by the CPCB showed that 32 stations recorded the air quality in the "severe" category, with AQI levels of above 400.

An AQI between 0 and 50 is considered "good", 51 and 100 "satisfactory", 101 and 200 "moderate", 201 and 300 "poor", 301 and 400 "very poor", 401 and 450 "severe" and above 450 "severe plus".

With the third stage of the Graded Response Action Plan (GRAP) restrictions coming into force in Delhi-NCR from Friday, a stringent ban has been imposed on construction and demolition activities, mining-related operations have been suspended, shifting to online classes for students up to Class 5 is being considered and daily water sprinkling on major roads has been mandated.

The GRAP for Delhi-NCR is divided into four stages of air quality -- Stage 1 for "poor" air quality (AQI ranging from 201 to 300), Stage 2 for "very poor" air quality (AQI from 301 to 400), Stage 3 for "severe" air quality (AQI from 401 to 450) and Stage 4 for "severe plus" air quality (AQI above 450).

According to the Centre's Decision Support System for Air Quality Management, vehicular emissions contributed approximately 15.8 per cent to Delhi's pollution on Sunday.

The system also reported that stubble burning was the main contributor to the capital's air pollution on Saturday, accounting for 25 per cent of the total pollution.

Meanwhile, the prominent pollutant was PM2.5, according to the CPCB.

PM2.5 refers to fine particles with a diameter of 2.5 micrometres or less, about the width of a human hair.

These particles are so small that they can penetrate deep into the lungs and even enter the bloodstream, posing significant health risks.

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