The air quality in Delhi plummeted into the "severe" category on Tuesday morning, and the overall air quality index (AQI) rose to 418.
An AQI 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 and beyond category is likely to cause respiratory issues upon prolonged exposure.
A worsening of the pollution level was seen as the temperature and wind speed fell, making the dispersion of pollutants difficult.
According to the System of Air Quality and Weather Forecasting And Research (SAFAR), the air quality is likely to remain within the 'very poor' category or reach 'severe' levels over the next three days.
As the air quality has deteriorated, the Delhi government on Monday night has imposed a ban on the use of Bharat Stage (BS)-III petrol and BS-IV diesel four-wheelers in the city till January 12.
BS is an emission regulation or fuel standard mandated by the Indian government for motor vehicles. A vehicle with a low BS grade (up to IV) emits high sulfur content, contributing heavily to PM 2.5 concentration — the most prominent pollutant in Delhi's air.
Meanwhile, dense fog and smog were witnessed in Delhi and the neighboring area.
Indian Railways cancelled 277 trains on Tuesday, including because of the dense fog in northern parts of the country.