India directed schools to move to hybrid mode for some grades, asked government offices to stagger timings for staff and imposed restrictions on vehicles in Delhi and adjoining regions as air quality deteriorated in the country’s north on Monday.

Delhi recorded “very poor” air on Monday with an air quality index (AQI) reading of 379 in the 24 hours to midday, the Central Pollution Control Board said.

Pollution was expected to worsen on Tuesday to “severe” levels above an AQI reading of 400, which poses a risk to healthy people and seriously impacts those with existing diseases.

The restrictions were imposed “considering the highly unfavourable meteorological conditions including calm winds”, said the Commission for Air Quality Management (CAQM), which handles air quality in the national capital region.

Its order, among other measures, directed schools to conduct classes in hybrid mode — online and in-person — for students up to Grade 5 and asked the federal government to decide on staggering timings of its office hours.

North India battles intense air pollution every winter as cold air and low temperatures trap vehicular pollutants, construction dust, and smoke from farm fires set off illegally in the adjoining states of Punjab and Haryana.

Delhi recorded its highest pollution during this season last month when AQI readings shot up to 494, prompting the government to close schools and advise offices to allow 50 per cent of employees to work from home.

A thick blanket of toxic smog engulfed most parts of northern India on Nov 18 and readings of air quality in the capital New Delhi hit their highest this year after dense fog overnight.

The smog, a toxic blend of smoke and fog, happens each year in winter as cold air traps dust, emissions, and smoke from illegal farm fires in some surrounding states.

Visibility dropped to 100 metres in Delhi and Chandigarh, a city northwest of the capital, but authorities said flights and trains continued to operate with some delays.

India’s pollution control authority said the national capital territory’s 24-hour AQI reading was at 484, classified as “severe plus”, the highest this year.

According to Swiss group IQAir’s live rankings, New Delhi was the most polluted city in the world with the air quality at a “hazardous” 1,081 and the concentration of PM2.5 — particulate matter measuring 2.5 microns or less in diameter that can be carried into lungs, causing deadly diseases and cardiac issues — was 130.9 times the World Health Organisation’s recommended levels.