Delhi’s air quality deteriorated to the “severe” category on Sunday morning after having a shown a slight improvement a day earlier, data from the System of Air Quality and Weather Forecasting and Research showed.

The Capital had an air quality index of 430 on Sunday, according to the SAFAR website. On Saturday, Delhi’s air quality had improved from the “severe” category to “very poor”, and the air quality index stood at 398, ANI reported.

An air quality index from zero to 50 is considered as “good”, 51 to 100 is “satisfactory”, 101 to 200 is “moderate”, 201 to 300 is “poor”, 301 to 400 is “very poor” and 401 to 500 is “severe”.

The India Meteorological Department has issued a forecast of generally cloudy sky with light rain on Sunday, PTI reported. Humidity levels on Saturday ranged from 74% to 95%, according to the weather agency.

Delhi has recorded 23 days in the “severe” category till now this year, according to the Hindustan Times. The Capital had 11 days of “severe” air quality in November, the most such days in the month since the Central Pollution Control Board began maintaining air quality in 2015.

Pollution in the Capital had increased sharply earlier this month as well, after which the Supreme Court gave a 24-hour ultimatum to the Centre and Delhi government to take steps to control it.

Soon after the hearing, Delhi Environment Minister Gopal Rai announced the closure of schools in the city till further orders. Schools had reopened on December 18 after the Commission for Air Quality Management in the National Capital Region and Adjoining Areas has allowed a phased reopening.