Cities across India woke up to a blanket of smog the morning after Diwali on Thursday, firecrackers being the main contributing factor.

The air quality in Mumbai, Chennai and Bengaluru had been deteriorating since Wednesday. On Thursday evening, Mumbai’s air quality index, or AQI, had dropped to the “poor” category at 204, much below 50 which represents good air quality with little potential to affect public health. An AQI of over 300 represents hazardous air quality.

At 11 am on Friday, the air quality index in at the Mumbai US Consulate area was down to 245. “External emissions like firecrackers are the main reason for the deterioration, combined with the change in the weather pattern,” Gufran Beig, the project director at the System of Air Quality and Weather Forecasting and Research, or Safar, told The Times of India. However, this year’s Diwali has been much quieter than the past few years.

Experts have predicted that the AQI in Mumbai will drop to later on Friday because of the emissions hanging in the air. In 2016, the city had recorded an AQI of 314 the day after Diwali. Safar believes that the air quality will begin to improve from Saturday.

In Chennai, which celebrated the festival on Wednesday, the air quality was as bad as in Delhi. At 4 pm on Thursday, the AQI in the Tamil Nadu capital was 302, a tad better than Delhi’s 319. The primary pollutant was PM 2.5, which are particulate matter smaller than 2.5 microns.

“It is the duty of the state Pollution Control Board to restrict the emissions in the spread area of Chennai by limiting crackers to maintain the desired air quality,” K Karthikeyan, former member secretary of the Tamil Nadu Pollution Control Board, told The Hindu.

Bengaluru, too, choked on Thursday. There was a thick blanket of smog over the city. PM 2.5 was recorded at 13.46 microgrammes per cubic metre on the outskirts of the city, while in some areas it was as high as 71.12.

However, the air quality improved on Friday morning. At 11 am, the air quality index in the Peenya industrial area was moderate at 98. B Nagappa, an officer with the Karnataka State Pollution Control Board, said the particulate matter was hovering over the city because of lack of dispersal.