The air quality in Delhi remained in the “severe” category for the fourth day in a row on Saturday.

The average air quality index, or AQI, stood at 408 at 1 pm, according to the Ministry of Earth Sciences’ System of Air Quality and Weather Forecasting And Research, or SAFAR. At these levels, healthy people can experience respiratory illnesses from prolonged exposure to air pollution.

An index value between 0 and 50 indicates “good” air quality, between 51 and 100 indicates “satisfactory” air quality and between 101 and 200 indicates “moderate” air quality. As the index value increases further, air quality deteriorates.

A value of 201 and 300 means “poor” air quality while 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”.

Visuals shared by ANI on Saturday morning showed the area around India Gate engulfed in a thick layer of smog.

On Friday, primary schools in Delhi were shifted to online classes because of the rising pollution level.

A day earlier, the authorities in the national capital banned construction activities and restricted the movement of non-electric buses, as Stage 3 of the Graded Response Action Plan, or GRAP, came into force across the National Capital Region.

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

Apart from the restrictions on construction and non-electric buses, the authorities were directed to impose strict restrictions on the movement of Bharat Stage-III petrol vehicles and Bharat Stage-IV diesel vehicles that do not conform to updated emissions norms.

Residents were also urged to use public transport, work from home if possible and walk or cycle for short distances instead of using vehicles.

Also read: Why has the Supreme Court failed to curb air pollution in Delhi?