As many as four cities in the country, including Delhi, had a “severe” air quality index (AQI) on Sunday, with the national capital (AQI 441) being the second-most polluted city.

Haryana’s Bahadurgah had the worst air quality (445) and Bhiwani, also in Haryana, was third (415) followed by Rajasthan’s Bikaner (404), data from the Central Pollution Control Board (CPCB) showed.

Delhi’s 24-hour AQI, recorded at 4 pm daily, was found to be in the “severe” zone on Saturday as well, when it stood at 417.

CPCB’s Sunday data for Delhi, obtained from 34 out of 40 pollution monitoring stations in the city, showed that as many as 32 centres reported a “severe” air quality, with levels of more than 400 at each facility.

The AQI is divided into the following categories: “good” (0-50), “satisfactory” (51-100), “moderate” (101-200), “poor” (201-300), “very poor” (301-400), “severe” (401-450) and “severe plus” (>450).

According to the central government’s Decision Support System for Air Quality Management, vehicular emissions contributed almost 16% (15.8%) to the Capital’s pollution on the day.

The system also found stubble burning from the neighbouring states as the main contributor to Delhi’s air pollution on Sunday, accounting for 25% of the total pollution.

On the other hand, the prominent pollutant was PM2.5, as per the CPCB.

PM2.5 refers to fine particles that have 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.

(With inputs from PTI)