As the air quality in Delhi plummeted to a “severe” level on Sunday, the Commission for Air Quality Management has imposed restrictions under phase three of the Graded Response Action Plan, or GRAP.

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

As of 7 pm, Delhi’s air quality index, or AQI, stood at 432, according to Central Pollution Control Board data.

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”.

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.

A ban has been imposed on BS-III petrol and BS-IV diesel four-wheelers under the third stage of GRAP, reported The Indian Express. Car owners can be fined Rs 20,000 for violating the ban.

Restrictions have also been imposed on construction and demolition activities, except for essential projects including those of the Railways, metro rail, airports, “projects of national importance”, hospitals, water supply and sanitation projects.