Hundreds of millions of Indians in north India woke up on Sunday to toxic air following Diwali, the Hindu festival of lights, after many revellers defied bans on using firecrackers to celebrate.

The capital New Delhi was blanked with a thick haze, with the average pollution level in the capital over 9 times what is considered safe by the World Health Organization.

Delhi’s Chief Minister Arvind Kejriwal had banned use and sale of firecrackers ahead of Diwali, but the policy has been difficult to implement.

Residents in the capital let off huge amounts of fireworks to celebrate the festival well into the early hours of Sunday morning.

The city’s air pollution typically worsens in October and November due to farmers burning agricultural waste, along with coal-fired power plants in surrounding states, traffic fumes and windless days.

The raging coronavirus epidemic, with more than 400,000 confirmed cases in the city of 20 million, has also heightened alarm over the health hazard posed by the choking smog, with doctors warning of a sharp increase in respiratory illnesses.

Cities in the states of Punjab, Uttar Pradesh, Haryana, Bihar and New Delhi – which have already been suffering from some of the worst air in the world – saw even higher levels of pollution than on the morning after Diwali last year, government data analyzed by Reuters showed.

An average of air quality indices measured at different places within the major cities in these states was higher than last year, according to data from the Central Pollution Control Board.

Some Hindus on Twitter condemned activists and celebrities who advocated against using fircrackers, saying it was an attack on their religious freedom.

“Are you realizing how all of India, all places stood up in defiance against the cracker ban? It’s like a form of Hindu- freedom battle cry,” Tarun Vijay, a senior leader from Prime Minister Narendra Modi’s Bharatiya Janata Party tweeted late on Saturday.