Alarm over Delhi's rising pollution spiked on Tuesday as residents woke up to a sepia haze in the morning, eyes smarting and an irritation in the nose and throat. The air quality index, which measures the concentration of poisonous particulate matter in the air, hit the "severe" level of 451 on a scale where the maximum reading is 500 and where anything above 100 is considered unhealthy.

Announcing that junior schools would be shut, Deputy Chief Minister Manish Sisodia said: "I would also request people to avoid morning walks. The situation is close to a severe crisis." The Delhi government has put out health advisories for high-risk groups, mainly the children and elderly, saying they should avoid outdoor activities.

In neighbouring Ghaziabad in Uttar Pradesh, primary sections of all schools will be closed today and tomorrow. In Haryana, schools will now start at 9 am till the end of November due to the heavy smog.

Chief Minister Arvind Kejriwal, likening Delhi to a "gas chamber", had suggested that schools should be closed for a few days. "Every year, during this time of the year, Delhi becomes a gas chamber for almost a month," Mr Kejriwal said in tweets.

Mr Kejriwal sought an appointment with Union Environment Minister Harsh Vardhan, who is in Germany for a climate change summit. The Chief Minister was told that a meeting would be possible only on Thursday, when Dr Vardhan returned.

It was "equal to smoking 50 cigarettes a day," Arvind Kumar, chairman for chest surgery at Sir Ganga Ram hospital, told Reuters.

Noting the deadly smog, the Indian Medical Association called for an annual half marathon on November 19 to be cancelled to protect runners and volunteers from high levels of deadly particulate matter.

The US embassy's real-time air quality index said that levels of the fine pollutants known as PM2.5 that are most harmful to health reached 703 -- well over double the threshold of 300 that is classed as hazardous. It stood at 999 for the RK Puram area, beyond which no readings are available.

Vehicle emissions and dust from construction sites have been blamed for the spike, besides firecrackers and farm burnings. The Delhi government blamed the centre for not stepping in to stop the burning of stubble of wheat and other grain in neighbouring Punjab and Haryana.