Delhi's streets were covered in shredded paper from used firecrackers as a thick blanket of smoke settled over the city on Wednesday, when Deepavali was celebrated in north India.
Pollution levels measured in PM2.5 - particulate matter that is 100 times thinner than a human hair and causes respiratory troubles - touched 2,000 micrograms per cubic metre at a particular hour in parts of the capital on Deepavali night, over four times worse than Singapore's worst haze days.
"It has gone to severe levels in many places and it was bad, but it could have been much worse," said Dr Gufran Beig, chief scientist at India's state-run System of Air Quality Weather Forecasting and Research. "The good news is that the winds have dispersed it very fast."
He said the pollution level was 40 to 50 per cent higher than last year as the festival fell in November, when temperatures are much lower, but that "our calculations show that actually less firecrackers were burst". Pollution peaks as temperatures drop because harmful particles get trapped in the lower layers of the atmosphere.
The agency - which recorded the 24-hour average of PM2.5 at 350 - asked people with lung or heart ailments to avoid "prolonged or heavy exertion" until today. By yesterday, smog had settled over the city.
The World Health Organisation has already called India's capital, with a population of nearly 17 million, the most polluted city in the world, a contention rejected by the Indian government.
Rapid vehicle growth, increased construction activity around Delhi, a preference for diesel cars due to lower fuel costs, and wood fires burning in neighbouring states all contribute to pollution. The city adds 1,400 new cars every day, doubling pollution levels over the last decade.
The government is experimenting with car-free days on certain busy stretches and, since last Saturday, it has been charging trucks entering Delhi a green tax of 700 rupees (S$15) to 1,300 rupees. One survey said truck traffic fell 30 per cent in three days.
Still, experts say it is not enough. "What you need is a proper scientific assessment (of the pollution problem) and a two- to three-year air quality management plan," said Mr Sumit Sharma at the The Energy and Resources Institute in Delhi.
Over the last couple of years, a number of initiatives have been non-starters or are facing delays, including an initiative to ban vehicles older than 15 years that was supposed to have been implemented last year.
At this time, pollution levels are already exceeding last year's. The Centre for Science and Environment calculates that PM2.5, on average, increased from 230 for the period of Nov 1 to 9 last year to 275 over the same period this year.
But some Delhi residents are just happy that Deepavali did not result in a visit to the hospital this year.
As in previous years, health warnings were given by the government, with Delhi chief minister Arvind Kejriwal begging residents not to turn the Hindu festival of lights into a festival of smoke. The government said the warnings managed to reduce pollution in some areas.
Ms Juhi Garg, a chartered accountant, said: "This Deepavali was better than the last few years. Usually in our colony (neighbourhood), kids start bursting firecrackers three days in advance but that didn't happen this year. "