Air pollution worsened in New Delhi on Monday to the “very poor” category after a few days of clear skies, as lower temperatures and calmer winds trapped pollutants.

The Indian capital’s pollution causes illnesses such as asthma and other respiratory problems every winter as the heavy air does not let vehicular emissions, construction dust and farm-fire residues disperse easily.

The air quality in the city of 20 million was “very poor” overall and sank to “severe” in some areas, according to data from the Central Pollution Control Board. The air quality index hit 457 in the Anand Vihar locality - nine times the “good” level.

The government’s System of Air Quality and Weather Forecasting and Research blamed the foul air on a drop in temperatures and reduced wind speeds.

Delhi’s temperature has been hovering around a low of 6 degrees Celsius (42.8 Fahrenheit) for the past few days, lower than the normal minimum of 8 degrees Celsius for this time of year.

Measures such as occasional suspension of construction activities, sprinkling water on roads, and enforcing restrictions on diesel vehicles have only partly helped address Delhi’s pollution problem.

Pollution from the burning of crop residues in the neighbouring states of Punjab, Haryana and Uttar Pradesh, however, has come down this year.