Residents in the Indian capital New Delhi and its suburbs have been waking up in recent weeks to hazy mornings with the sky a shade of grey due to air pollution.

A thick layer of smog envelops the capital in winter as cold, heavy air traps construction dust, vehicle emissions and smoke from crop stubble burning in neighbouring northern states, causing a surge in respiratory illnesses among the city’s 20 million people.

The air quality index in nearly all monitoring stations in New Delhi was between 300 to 400 or in the “very poor” category on Monday, which experts say leads to respiratory illness if there is prolonged exposure. However, it was an improvement on last week’s reading of 400-500, which is described as “severe”.

Indian authorities on Friday shut factories and construction sites, restricted diesel-run vehicles and deployed water sprinklers and anti-smog guns to control haze and smog enveloping the skyline of the capital region.

The Delhi government closed primary schools and restricted outdoor activity for older students as the air quality index exceeded 470, more than 10 times the global safety threshold, according to the state-run Central Pollution Control Board. Primary schools will reopen on Wednesday.

Delhi State Environment Minister Gopal Rai said private demolition and construction will still remain banned, but public works relating to highways and power transmission will be permitted.

Almost every year, New Delhi tops the list of the many Indian cities gasping for breath after the Diwali festival celebrations with their massive lighting of firecrackers. The crisis is exacerbated particularly in the winter when the burning of crop residues in neighbouring states coincides with cooler temperatures that trap deadly smoke, which travels to New Delhi and leads to a surge in pollution.

Air quality could worsen later this week, said the System of Air Quality and Weather Forecasting and Research (SAFAR) on its website.