Delhi woke up to a thick layer of smog on Monday morning, with people in many parts of the city lighting firecrackers till late at night on the Diwali festival despite a ban imposed by the government.

At 7 am on Monday, many parts of the national capital recorded air quality in the “very poor” category, according to the Central Pollution Control Board. Several private air quality monitoring organisations, however, recorded pollution levels in the hazardous category.

An air quality index between zero and 50 is considered “good”, 51 and 100 “satisfactory”, 101 and 200 “moderate”, 201 and 300 “poor”, 301 and 400 “very poor” and 401 and 500 “severe”. An AQI in the “very poor” category can lead to respiratory illness on prolonged exposure.

In September, the Delhi government had reimposed a ban on the production, sale, storage and use of all types of firecrackers in the city as part of its plan to control pollution levels in the winter months. This was the third consecutive year that the government has imposed a ban on sale and use of firecrackers.

Later in September, the Supreme Court had rejected a plea seeking to allow the use of barium in green firecrackers.

Also read:

Why is the Supreme Court’s regulation of firecrackers so ineffective in curbing air pollution?

At 7 am on Monday, monitoring stations in Delhi’s Anand Vihar, Bawana, Dwarka Sector 8, Jahangirpuri, Pusa and Shadipur recorded air quality in the “very poor” category, according to the Central Pollution Control Board.

On Sunday night, Delhi recorded levels of PM2.5 fine particles at 763 micrograms per cubic metre and PM10 particles at 874 micrograms per cubic metre.

Particulate matter smaller than 2.5 microns (or about ten-thousandth of an inch) is particularly dangerous to human health. Such particles are small enough to travel deep into the respiratory system, potentially impairing lung function.

To be considered safe, the National Ambient Air Quality Standards require PM2.5 concentration in air to be less than 60 micrograms per cubic metre in any given 24-hour period.

Data from the World Air Quality Index Project, a global nonprofit that covers 1,000 cities in the world, showed that at 8 am, air quality was hazardous at Anand Vihar (672), Jawaharlal Nehru Stadium (496), PGDAV College (554), Okhla (419) and Sri Aurobindo Marg (319).

The World Air Quality Index Project uses a scale developed by the United States Environmental Protection Agency, which categorises air quality above 300 as hazardous.

Also read:

Why does the same part of Delhi show different air quality readings at the same time?

Delhi Environment Minister Gopal Rai said that air pollution in the city rose on Sunday as firecrackers were lit at some places in a targeted manner, ANI reported.

“The way some Bharatiya Janata Party leaders were inciting people...the city is suffering the consequences today,” he said.

Rai said it was unfortunate that the BJP was not willing to act responsibly in spite of the Supreme Court’s directions.

In winters, Delhi’s air quality drops to “very poor” or “severe” categories due to several factors, including lower temperatures, wind speeds, industrial pollution and stubble burning in neighbouring states of Haryana and Punjab. The Air Quality Index also often drops further around Diwali as ban on firecrackers is rarely enforced.

Earlier this month, air quality in Delhi had turned severe, after which the Commission for Air Quality Management had implemented measures under the highest level of the Graded Response Action Plan to curb pollution.

Ahead of Diwali, however, air quality in the national capital had improved to moderate in most places due to a spell of rain.

Also read: This Diwali, Delhi had best AQI in eight years – before firecrackers took over