The air quality in Delhi on Saturday deteriorated to “severe” category due to stubble burning and bursting of firecrackers, the Central Pollution Control Board said.

According to the agency’s air quality index or AQI, any reading above 100 on a scale of 500 is progressively unsafe for health. At 5 pm, the AQI in Delhi was at 423, indicating “severe” conditions that pose a risk of respiratory problems. Haze shrouded the city again by Saturday evening, reducing visibility significantly.

While the pollution control board uses 24-hour average data, the Ministry of Earth Sciences’ SAFAR, or System of Air Quality and Weather Forecasting And Research reports real-time figures. The government-run monitoring agency said the overall AQI struck 421 in Delhi at 6.45 pm.

Meanwhile, the PM10 index, which measures the concentration of particulate matter of 10 microns diameter or less in the air, hit 408, SAFAR said. This is coarse particulate matter and mostly dust, which attaches to toxic material from other emissions. A level of 500 is considered “hazardous” and people are usually advised to remain indoors.

Levels of the most dangerous particles, called PM2.5, climbed to around 271 micrograms per cubic meter, which is considered hazardous to breathe. Particulate matter smaller than 2.5 microns (or about a 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 to be less than 60 micrograms per cubic metre of air in any given 24-hour period.

“Even a small increase in local additional emissions is likely to have significant deterioration impact on Sunday and Monday,” SAFAR said, according to PTI. It added that stubble burning accounted for 32% of the city’s PM2.5 pollution.

Earlier in the day, it said that the concentration of PM2.5 in Delhi on Diwali could be the lowest in the last four years if no firecrackers are burnt.

Delhi’s 24-hour average AQI was 414 on Saturday, 339 on Friday and 314 on Thursday, PTI reported.

Kuldeep Srivastava, the head of the Indian Meteorological Department’s regional forecasting centre, told PTI that the air quality was likely to improve after Diwali due to an increase in the wind speed.

Earlier this week, the National Green Tribunal had imposed a blanket ban on the sale of firecrackers in Delhi and the National Capital Region till November 30.

Gurugram also imposed a complete ban on firecrackers, three days after the Haryana government permitted their sale and use for two hours on festivals such as Diwali, citing impact on small traders.

Delhi’s air pollution typically worsens in October and November due to farmers burning stubble in neighbouring states, unfavourable wind speed and local emission of traffic fumes in the city. Firecrackers ignited for Diwali add to the problem.