India's capital New Delhi was enveloped in heavy, toxic smog on Sunday - the worst levels in recent years - with hundreds of flights diverted or delayed as politicians blamed each other for failing to tackle the crisis.
Every winter, the megacity of 20 million people is blanketed by a poisonous smog of car fumes, industrial emissions and smoke from stubble burning at farms in neighbouring states.
Concentrations of fine particles (2.5 microns or less in diameter) in the air hit the highest level of this season, India's state-run System of Air Quality Weather Forecasting and Research (SAFAR) said.
The air quality index for PM2.5 hit 810 early in the day, well beyond the "hazardous" zone, according to the US embassy in Delhi, which independently monitors pollution levels.
The recommended World Health Organisation safe daily maximum is a reading of 25.
"Pollution has reached unbearable levels," Delhi chief minister Arvind Kejriwal tweeted Sunday.
Visibility was so poor at Delhi's airports that 37 flights were diverted, and hundreds of departures and arrivals were delayed, officials said.
The Twenty20 international between Bangladesh and India went ahead as planned on Sunday, with a near-capacity crowd in attendance despite health warnings from government agencies.
"It's actually scary - you can't see things in front of you," protester Jaivipra said at a rally in Delhi on Sunday calling for politicians to do more to curb pollution.
Nurses at the demonstration said they were seeing more people suffering as a result of the smog.
"Patients are coming with more lung and respiratory diseases, like more (are) affected with asthma," Reshma C.M. said.
The conditions sparked a blame game between state and federal politicians over who was responsible for the conditions, which authorities said Friday reached "emergency" levels.
In a tweet last week, Kejriwal called on the state governments of neighbouring Punjab and Haryana to take action.
"Delhi has turned into a gas chamber due to smoke from crop burning in neighbouring states," he tweeted.
Federal environment minister Prakash Javadekar accused Kejriwal of politicising the issue and presenting the two states "in a bad light and as villains".
Schools in Delhi have already been ordered closed until Tuesday, and construction halted.
From Monday, there will be an odd-even car licence plate scheme in effect to thin out traffic.
Global non-profit Vital Strategies' senior vice president for environmental health, Daniel Kass, said while temporary curbs were helpful, they had a limited impact over time.
"They are insufficient to address the most important aspect of air pollution, which is what people live with day-to-day," Kass said.
He said a range of measures needed to be imposed at local and national levels for air quality to improve.
Apart from changing agricultural practices, he said the measures should include more public transport investment, emission controls on two-wheelers, switching electricity generation sources, and accelerating the conversion of home-heating from charcoal to natural gas.
Last year, a UN report found 14 of the world's 15 most polluted cities were in India, with one US study saying it kills a million people prematurely every year.
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