Authorities in New Delhi ordered primary schools to shut from Saturday, and told schools to stop outdoor activities for older children as air in the world’s most polluted capital had become a severe risk to health.

A filthy smog forms over the Indian capital every winter as cold, heavy air traps construction dust, vehicle emissions and smoke from crop stubble burning in neighbouring states, causing a surge in respiratory illnesses among the city’s 20 million people.

As the air quality index (AQI) topped the 400-mark, putting most districts of the capital territory in the “severe” or “hazardous” category on Friday, Chief Minister Arvind Kejriwal instructed schools to take action and warned that more measures could be introduced to restrict the number of vehicles on the roads.

India’s federal pollution control board has banned the entry of diesel trucks carrying non-essential goods into the capital, while Delhi’s administration suspended most construction and demolition work in the region earlier this week.

Delhi environment minister Gopal Rai said going forward 50 per cent of Delhi government employees would be told to work from home, and he urged private firms to take similar steps.

On Thursday, the local authority in Noida, a satellite city, ordered all schools to conduct online classes for students up to the eighth grade.

The decision to shut some schools came after concerned parents and environmentalists took to social media, and residents complained of discomfort in breathing and irritation in the eyes, nose and throat.

Healthy people can be affected, and those with existing health conditions are at greater risk when the AQI rises past 400, the federal government warned.

According to data compiled by the Swiss group IQAir, New Delhi has been the world’s most polluted capital for the past four years.

Data from Google Trends showed a spike in searches for ‘air purifiers’ in the past couple of days in the city, as residents look for ways to breathe cleaner air.

India’s National Human Rights Commission asked the chief secretaries of the neighbouring states of Punjab, Haryana, Uttar Pradesh and Delhi to provide details by Thursday on measures taken to combat the alarming pollution levels.