As per the AQI released by the CPCB at 4 pm, particulate matters -- PM 2.5 and PM 10 -- were the major contributors to "very poor" air quality on Thursday.
The Supreme Court had on October 9 banned the sale of firecrackers during this Diwali to see effects of its suspension in the light of the severe pollution and smog-like conditions prevalent in Delhi during this period.
Last year's unprecedented pollution disaster saw heavy smog hover above the capital for weeks, forcing schools to shut as authorities scrambled to contain the crisis.
Levels of PM2.5 - the fine particles linked to higher rates of chronic bronchitis, lung cancer and heart disease - soared to 778 in the days that followed Diwali, prompting the Supreme Court to warn of a public health emergency.
Levels of PM2.5 between 301 and 500 are classified as "hazardous", while anything over 500 is beyond the official index.
This time, they are taking few chances, as the country's environmental watchdog shut down a coal-fired power plant on Wednesday and banned the use of diesel generators in Delhi.
(With inputs from IANS and AFP)