Pollution in New Delhi rose to a "severe" level on Thursday after residents of the Indian capital celebrated the Hindu festival of Diwali by setting off fireworks long into the night, ignoring restrictions imposed by the Supreme Court.
Two federal government pollution indices showed air at "very poor" and "severe" levels, indicating that prolonged exposure could lead to respiratory illnesses.
The indices mostly measure the concentration of tiny poisonous particulate matter, or PM 2.5, particles that are less than 2.5 microns in diameter, which can be carried deep into the lungs.
The US embassy said its measurements showed PM 2.5 levels in its part of central Delhi had soared to 689, posing a serious health risk.
A level of 50 or less is considered safe.
A task force under the federal pollution control board was scheduled to meet to assess the situation, a city government spokesman said.
Few Delhi residents wear face masks when they go out in heavy smog and joggers and passers-by were out as usual near parliament early on Thursday.
Last month, the Supreme Court allowed the use of "green" firecrackers for Diwali, but only for two hours in the evening.
However, there were no "green" fireworks available for sale and countless fireworks were let off through the evening on Wednesday.
Authorities have been reluctant to ban fireworks to avoid offending members of the majority Hindu community, for whom Diwali is one of the biggest festivals.
"The Supreme Court order on fireworks was not followed and health warnings from the government were limited to few newspapers and some websites," said Greenpeace campaigner Sunil Dahiya.
The apparent lack of concern about the toxic air - whether through ignorance or apathy - allows politicians to ignore the problem, say environmental activists and others.
Tiny particulate matter can cause major health problems.
In recent weeks, Delhi doctors have reported an increase in patients with respiratory problems.
Adding to the smog has been smoke from the surrounding countryside, where farmers at this time of the year burn the stubble in their fields to prepare for winter sowing.
More gentle winds and cool air, which can trap pollution, exacerbate the problem.
For a second year, New Delhi's chief minister has likened the city to a "gas chamber". Last year, he declared a public health crisis, shut schools for a week and told residents to remain indoors.