Delhi residents were bracing for the fall-out from Diwali, one of the year’s biggest Hindu festivals, which is expected to add to the chronic air pollution problem in the Indian capital.

The festival will be celebrated widely in northern India on November 7, and 8, and thousands of fireworks will be let off, contributing to already high polution levels.

The air quality in the city is expected to deteriorate to the “severe plus emergency” level, according to a government-affiliated pollution monitoring agency.

In the last 36 hours, air quality levels have improved, albeit only slightly, with ratings dropping from “emergency” to “severe.”

There are six Air Quality Index (AQI) categories on a scale from 0-500 – Good, Satisfactory, Moderate, Poor, Very Poor and Severe.

One study estimated that more than 15,000 people died prematurely in the national capital in 2016 from illnesses connected to fine particulate matter pollution.

The AQI in Delhi on Tuesday morning was marginally above 400. Monday’s smog was worse. The level of PM2.5 pollutants in the air in South Delhi’s Okhla area was 644, or 20 times the World Health Organization’s declared safe level. According to the India Meteorological Department, shallow fog in the morning mixed with the pollutants and created the smog.

The early onset of winter’s cool and foggy weather coupled with pollution created by fireworks during Diwali and ongoing high levels of pollution have triggered this situation, experts say.

The SAFAR, or System of Air Quality and Weather Forecasting And Research, under the Ministry of Earth Science, said in a report that Delhi’s air quality was expected to deteriorate to the “severe plus emergency” category after Diwali.

The highest levels of PM10 and PM2.5 are expected between 11pm to 3am on the night of November 7-8. The report warned people to refrain from physical activity outdoors.

The air quality will be bad on November 8 “even if 50% of the total load of toxic firecrackers as compared to Diwali-2017 is added.” As a result, “the prevailing weather conditions will aggravate the high smoke level and (put air quality) in the ‘severe’ range for at least 2 days (November 8-9). This scenario is likely to hold the locally generated crackers emissions within the NCT (National Capital Territory), slowing down the dispersion, resulting in increased levels of PM2.5 and PM10 pollution.”

Assuming that only half of last year’s amount of firecrackers are used, the lead pollutant PM2.5 – particles in the air with a diameter of less than 2.5 micrometers – is expected to reach 575 a day after Diwali, the SAFAR report said.

Though policy discussions on air pollution control have commenced, the Delhi government has not managed to do anything to address the problem.

To control the situation that escalated last year, the Supreme Court has banned the manufacture and sale of pollution-causing fireworks in New Delhi. The apex court has also said that people in New Delhi and its neighboring areas can still use approved “green fireworks,” but only between 8pm and 10pm on Diwali.

Paddy stubble burning that often takes place in this season in the neighboring states of Haryana and Punjab was also blamed for increasing the pollution levels in the national capital. This time is no different.