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 the 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.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."