As air quality in the National Capital Region plummeted to the “severe plus” category, the authorities in Delhi on Sunday announced a ban on trucks entering the national capital and a temporary halt on construction activities for public projects.

Chief Minister Atishi also announced that physical classes would be discontinued for all students, except for Classes 10 and 12. “All schools will hold online classes, until further orders,” she said on social media.

The daily average air quality index, or AQI, stood at 484 at 8 am on Monday, according to data from the Central Pollution Control Board. On Sunday, the AQI hit 441 at 4 pm, which further rose to 457 at 7 pm.

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”. At these levels, healthy people can experience respiratory illnesses from prolonged exposure to air pollution.

In view of the rising AQI levels, the Commission for Air Quality Management in the National Capital Region and the surrounding areas ordered the implementation of Stage 4 of the Graded Response Action Plan, or GRAP, across the National Capital Region from Monday.

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.

Stage 1 of GRAP is activated when the AQI is in the “poor” category. The second, third and fourth stages are activated when the AQI crosses the “very poor”, “severe” category and “severe plus” category.

Apart from a ban on trucks except for those carrying essential items or using clean fuel and suggesting online classes for schools, the commission on Sunday recommended that offices in the National Capital Region work at 50% capacity, with the rest working from home.

“Non-essential light commercial vehicles registered outside Delhi will also be prohibited, except for EVs [Electric Vehicles] and CNG [Compressed Natural Gas] and BS-VI [Bharat Stage-IV] diesel ones,” the commission said.

“Delhi-registered BS-IV or older diesel medium and heavy goods vehicles are banned, except for those in essential services,” it added.

The commission also urged children, the elderly and those with respiratory or other chronic diseases to avoid outdoor activities and stay indoors, as much as possible.