INDIA – The air quality in India‘s capital Delhi worsened on Sunday Dec. 14, to new severe level, with authorities urging vulnerable people to stay indoors.

The air quality index in Delhi hit a severe score of 491 out of 500 as of Sunday morning, data from the Central Pollution Control Board showed, up from Saturday’s evening reading of 441.

The Commission for Air Quality Management on Saturday invoked Stage IV, the highest level of the Graded Response Action Plan amid severe pollution, according to a statement from India’s Environment, Forest and Climate Change Ministry.

The action plan restricted the entry of older diesel trucks, halted construction activities, including public projects, and implemented hybrid schooling.

Authorities advised residents to remain indoors, particularly children and individuals with respiratory or heart conditions, and to wear masks if they need to go outside.

The commission said the deterioration of air quality was caused by high humidity and a change in wind direction, which prevents pollutants from dispersing and contributes to the formation of smog.