The sickening air pollution that led the Indian capital to shut schools and construction sites this week has prompted similar measures in nearby cities, amid fears foreign tourists may stay away.
Officials in the northern state of Uttar Pradesh say they expect the acrid smog to blanket the state within days.
For more than a week, New Delhi's skies have been filled with a thick haze that has made people's eyes sting and their throats sore. Air pollution experts blame myriad pollution sources, from diesel-burning cars and seasonal crop burning to garbage fires and stoves fuelled with kerosene and cow dung. Winter weather patterns also mean there is less wind to circulate the air.
On Monday, an Indian trade and industry lobby group warned that the pollution could have an economic impact, and released a study suggesting up to 10 per cent of workers had called in sick during the last week.
"Environment and air pollution-related issues might hurt Brand India and hit sectors like tourism and outdoor recreation, as people tend to stay away from polluted areas," said DS Rawat, secretary general of the Associated Chambers of Commerce of India.
Doctors urged people to avoid going outdoors unless they wear face masks.
"Higher air pollution levels may take days to settle," said Dr Surya Kant Tripathi, who heads the pulmonary medicine department at King George's Medical University in Lucknow, the capital of Uttar Pradesh.
Lucknow city director JP Gupta said the smog covering New Delhi was wafting over western parts of Uttar Pradesh and would soon cover the entire state, India's most populous, with around 210 million people.
In the Uttar Pradesh district of Ghaziabad, considered an eastern suburb of New Delhi, schools were ordered closed along with those in the capital on Monday and Tuesday, after which Uttar Pradesh officials said they would reassess the situation.
There are no official air pollution monitors set up in the Delhi suburban areas, but in both New Delhi and Lucknow, the levels of PM2.5 - tiny particulate matter that can clog lungs - were at least above 400 micrograms per cubic metre on Monday. That's more than 40 times what's considered safe by the World Health Organisation, and more than six times the limit set by Indian law. Some monitoring points were registering much higher levels.
New Delhi has also ordered the temporary closure of a nearby coal-burning power plant, as well as construction and demolition sites blamed for sending huge plumes of fine dust particles into the air. It also ordered that roads be doused with water to settle dust, and banned diesel-powered electricity generators for 10 days except at hospitals and mobile phone towers.
On Tuesday, India's Supreme Court will hear a plea demanding that authorities do a better job of monitoring air quality and take more serious steps to cut pollution. The Centre for Science and Environment, a New Delhi-based research and lobbying organisation, said government data shows that the smog that has covered the city for the last week is the worst in 17 years.