The air quality in Delhi on Saturday was hovering around the “severe” category for the first time this winter season, according to the Central Pollution Control Board data.

The national capital’s 24-hour average air quality index stood at 397 at 4 pm, the worst since January, PTI reported. This indicates “very poor” conditions that pose a risk of respiratory problems on prolonged exposure. An air quality index ranging between 401 and 500 falls under the “severe” category.

The air quality index on Saturday worsened from 354 on Thursday, 271 on Wednesday, 302 on Tuesday and 312 on Monday.

Levels of the PM2.5 fine particles was 215 micrograms per cubic meter on Saturday evening, showed real-time figures from the Ministry of Earth Sciences’ SAFAR, or System of Air Quality and Weather Forecasting And Research.

Particulate matters smaller than 2.5 microns (or about ten-thousandth of an inch) are 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.

“Air quality is likely to deteriorate further and remain within ‘upper end of very poor’ or ‘lower end of severe’ for next three days as fire counts are increasing significantly with favorable strong upper level wind flow,” SAFAR said.

Air quality deteriorates sharply in the winter months in Delhi, which is often ranked the world’s most polluted capital. Stubble burning in Punjab and Haryana, along with falling temperatures, decreased wind speed and emissions from industries and coal-fired plants contribute to air pollution.

On Saturday, the Commission for Air Quality Management directed authorities in the National Capital Region to immediately implement restrictions under stage III of the Graded Response Action Plan – a a step-by-step emergency measures to fight the pollution.

Under stage III, restrictions include a ban on construction and demolition activities, except for essential projects concerning national security, defence, railways and metro rail, among others.