India's pollution control authority has classified the air in Delhi as "severe plus", after the city passed 450 according to its measurements on Monday morning.

Delhi's air quality worsens to 493 AQI, exceeding WHO limits, as smog blankets northern India, prompting restrictions and health concerns.

Delhi recorded air pollution more than 17 times above an acceptable level as worsening smog triggered curbs on trucking and shuttered construction sites and schools.

A thick, toxic smog has enveloped India's capital, home to more than 33 million people. One technology firm which monitors air pollution ranked New Delhi as the most polluted city in the world.

The presence of pollutants such as PM 2.5 and PM 10, nitrogen dioxide, sulfur dioxide, and ozone in the air poses serious health risks, particularly for children and the elderly.

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

The Supreme Court agreed for an urgent hearing of a plea on Monday seeking directions to curb air pollution in the national capital and adjoining areas.

By Tanvi Mehta NEW DELHI (Reuters) -Toxic smog engulfed much of northern India on Monday and pollution readings in New Delhi hit their highest of this pre-winter season, after dense fog overnight. Visibility dropped to 100 metres in the capital and at least eleven flights were diverted.