Diwali is a week away but the capital's air quality has already plunged to "very poor" levels, breaching the hazardous "red zone" on Sunday.
This much pollution recorded for the first time this season. If sustained for three straight days, the air quality would further worse during and after diwali.
Delhi's air quality index, separately calculated by CPCB and SAFAR, showed a common reading of 318 on Sunday.
An AQI of more than 300 is considered "very poor". While the city's air quality falls to even more hazardous levels during the winter, it's unusual for levels to rise this early in the season.
Even US embassy's air quality monitoring stations, located at the relatively clean Chanakyapuri, showed "unhealthy" levels as per their AQI measurements.
Delhi was the worst among all cities monitored by CPCB. On Sunday, air quality was visibly poor with light smog over the city.
Contributing to the pollution were weather conditions, heavy pre-Diwali traffic and smoke from crop fires.
"Diwali traffic has been a factor and it won't improve soon," said Anumita Roy Chowdhury , executive director of centre for Science and Environment.