Residents living in the capital of India woke up to the affects of the Diwali firework celebrations on Monday, in the already heavily polluted city of 16 million.
The air pollution following celebrations triggered warnings that even healthy people were at risk of respiratory problems.
"My eyes are irritated, I'm coughing and I find it difficult to breathe," said 18-year-old Delhi student Dharmendra, who uses only one name as is common in India. Because of the pollution, "I don't go out so much nowadays."
Residents were advised to stay indoors with health warnings issued for the young, elderly and those with respiratory or heart conditions.
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In recent years the capital has been trying to clean up its air quality.
It has barred cargo trucks from city streets, required drivers to buy newer cars that meet higher emissions standards and carried out several weeks of experimental traffic control, limiting the number of cars on the road.
But other pollution sources, including construction dust and cooking fires fuelled by wood or kerosene, continue unabated.
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The US embassy in New Delhi said its air pollution index had late on Sunday breached the "hazardous" level upper limit of 500, at which it stops measuring levels of PM2.5. The index had rocketed to 1,126 by 2 am (2030 GMT).
PM2.5 monitoring is a method used by the World Health Organisation (WHO) to measure fine particulate matter of 2.5 micrometers or less in diameter. It is dangerous because the tiny particles, including soot, dirt, dust and smoke, can be inhaled into the lungs and get into the bloodstream.
"Last night, levels hit the severe category, which is the worst. You could see and sense how the visibility had come down and there was a choking haze all around," said Anumita Roychowdhury at the Centre for Science and Environment.
Air quality is usually poorest in Delhi's winter months of December and January, but Roychowdhury said data was not available to assess if Delhi's air was worse on average than in prior years because it has not been consistently recorded.
India is home to four of the world's 10 cities with the worst air pollution, the World Health Organisation said in May. New Delhi is ranked 11th.
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China, another country that suffers from hazardous amounts of smog, shuts schools and offices if air quality deteriorates to extreme levels for three consecutive days, but India has not yet taken similar steps.
"Where is the plan for short- and medium-term action, and for emergency action?" Roychowdhury said. "We need one very urgently."