The Commission for Air Quality Management on Monday night reimposed stage 4 of the Graded Response Action Plan, or GRAP in Delhi and the National Capital Region as the air quality worsened to the “severe” category.

The move came hours after the panel invoked stage 3 measures on account of “very poor” air quality in Delhi and its surrounding areas.

Stage-IV is the highest level of GRAP, a set of incremental measures meant to control air pollution in the National Capital Region. At this level, all construction activities, even of highways and roads, are banned, and trucks are prohibited from entering the National Capital Region, except if they are carrying essential commodities or running on clean fuels.

Classes for students up to standard 5 had already transitioned into hybrid mode – or both physical and online classes – in Delhi, Gurugram, Faridabad, Ghaziabad, and Gautam Budh Nagar when the third stage of GRAP was implemented.

Under stage four, state governments can also decide on allowing public and private offices to work at 50% strength, and allowing the remaining employees to work from home.

The Commission for Air Quality Management noted in its order on Monday that the Supreme Court had on December 5 directed it to reintroduce Stage-IV measures if the air quality index were to cross 400. The panel said that Delhi’s air quality breached this level around 10 pm on Monday.

An air quality index of more than 400 may cause respiratory problems even for healthy people, and cause serious health problems for those with lung or heart disease.