The recommended World Health Organisation safe daily maximum is 25.

(AFP) – India s capital New Delhi was enveloped in heavy, toxic smog Sunday -- the worst levels in recent years -- with 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 particles measuring less than 2.5 microns hit the highest level of this season, exacerbated by light rains late Saturday, India s state-run System of Air Quality Weather Forecasting and Research (SAFAR) said.

The reading for pollutants in the atmosphere hit 810 micrograms per cubic metre Sunday morning, 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 25.

"Pollution has reached unbearable levels," Delhi Chief Minister Arvind Kejriwal tweeted Sunday.

Visibility was so poor that major carriers Air India and Vistara said Sunday flights were being delayed or diverted to and from Delhi s airports.

Some cricket players and coaches have also been training in masks ahead of the Twenty20 international between Bangladesh and India later Sunday.

"It s actually scary -- you can t see things in front of you," protester Jaivipra told AFP 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 from respiratory problems.

"Patients are coming with more lung and respiratory diseases, like more (are) affected with asthma," Reshma C.M. said.

Pollution blame game 

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 to cut traffic.

Global non-profit Vital Strategies  senior vice president for environmental health, Daniel Kass, said while temporary curbs were helpful, they had 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 told AFP.

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.