The Supreme Court ruled last month that only environmentally friendly crackers - that emit less smoke and soot - can be sold in Delhi, in a bid to cut the smog that has scarred the city's worldwide reputation.
The index measures air quality taking into account particulate matter - PM 2.5, or very fine particles smaller than 2.5 micrometres in diameter, and PM 10 which are less than 10 micrometers in diameter - as well as other pollutants, including nitrogen dioxide, ozone and carbon monoxide. This is almost double the pollution levels of past year post Diwali, when the AQI was around 367.
SAFAR's data showed that PM 2.5 level was more than eight times the permissible limit, while PM 10 was six times the permissible limit. The 24-hour average Air Quality Index (AQI) at midnight stood at 423.
The Supreme Court had instructed that only "green" fireworks be burst between 8 pm and 10 pm on Diwali.
The capital city was followed by Kolkata with recording the same AQI.
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Twenty three areas in Delhi recorded "severe" air quality while eight recorded "very poor" quality air, according to Central Pollution Control Board.
Last month, the Supreme Court allowed the use of "green" firecrackers for Diwali, but only for two hours in the evening. A "severe plus emergency" level AQI essentially means that even healthy people may suffer from respiratory illnesses on prolonged exposure. According to the SAFAR, the moisture in the air on Wednesday was normal, thus reducing the pollution load slightly.
The top court had directed the police to ensure that banned firecrackers were not sold and in case of violation, the station house officer (SHO) of the area would be held responsible.
But despite the order, certain places recorded violations where firecrackers were burnt before and after the set time frame.
Twitter users took to the microblogging site posting pictures of people burning crackers wearing masks and slammed the "mass contempt" of Supreme Court's order.