All BS-III petrol and BS-IV diesel four-wheelers will be banned from plying on in the national capital between January 10 and January 12 in view of air pollution in the city, the Delhi government said in an order issued on Monday.

The air quality in Delhi deteriorated to the “severe” category on Tuesday, according to data shared by the Ministry of Earth Sciences’ SAFAR, or System of Air Quality and Weather Forecasting And Research.

The city’s 24-hour average air quality index, or AQI, stood at 434 at 4 pm on Monday, worsening from 371 on Sunday.

“We are monitoring the situation along with the Environment department,” an unidentified travel department official told PTI. “If the air quality improves, the ban could be lifted before Friday.”

At 8.20 pm, the air quality index in the national capital stood at 418, according to real-time figures shared by SAFAR.

An air quality index between zero and 50 is considered “good”, 51 and 100 “satisfactory”, 101 and 200 “moderate”, 201 and 300 “poor”, 301 and 400 “very poor” and 401 and 500 “severe”. The AQI in the “very poor” category can lead to respiratory illness on prolonged exposure.