NEW DELHI: Many Delhi residents were forced to spend Christmas indoors this year as air quality remained at "severe to emergency" levels for a fourth day, in the Indian capital's worst smog crisis this year.

Senior officials predicted the severe pollution would last another three days due to unusually stagnant and cold air. They issued a three-day emergency response on Monday that included closing area factories and banning construction works.

"We are preferring to stay at home due to pollution despite Christmas celebrations and a public holiday," said Amit Azad, a financial consultant. He bought an air purifier this week after developing a cough because of the smog.

A Delhi government official blamed the pollution for lighter-than-usual traffic on already holiday-thinned streets, while a Delhi airport official said some domestic and international flights were delayed for up to two hours due to poor visibility.

Delhi's air quality index, which measures the concentration of poisonous particulate matter, averaged 420 on Tuesday morning, slightly better than 449-450 the previous two days - the worst this year - data from the government's Central Pollution Control Board showed. A reading above 100 is considered unhealthy.

Delhi is ranked among the world's worst cities in air quality, after years of breakneck growth in auto sales and coal-fired power generation.

Environmentalists say the federal and city governments, while focusing on temporary responses, are side-stepping more effective, long-term measures that would hit at the sources of pollution and promote cleaner fuels.

"There is a lack of political will to take required measures on a sustained basis to contain pollution levels," said Sunil Dahiya, a senior campaigner with Greenpeace India. He called for higher car parking fees and reduced dependence on coal-fired power plants.

Dahiya also said the government should immediately close down schools and urge people to curtail outdoor activities given the serious health effects of the current pollution levels.