A dense layer of smog lingered over Delhi on Saturday as the city’s air quality dropped to the “severe category”, fed by smoke from agricultural fires in neighbouring states, data from the Central Pollution Control Board showed.

According to the agency’s air quality index or AQI, any reading above 100 on the scale of 500 is progressively unsafe for health. At 6 pm, the AQI was 426 in Delhi, indicating “severe” conditions in the Capital.

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 456 at 6 pm. Levels of the most dangerous particles, called PM2.5, climbed to around 305 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.

An AQI between 0 and 50 is considered “good”, 51 and 100 “satisfactory”, 101 and 200 “moderate”, 201 and 300 “poor”, 301 and 400 “very poor”, and 401 and 500 is considered “severe”.

SAFAR said the city’s air quality is likely to remain “severe” on Diwali too, which is on November 14. It said the AQI is likely to remain in the “upper end of ‘very poor’ category on November 13 and ‘severe’ category on November 14”.

VK Soni, head of India Meteorological Department’s environment monitoring research centre, told PTI that “a very large number of farm fires over Punjab is the primary reason for the severe air quality in the region”. SAFAR said the share of stubble burning in Delhi’s PM2.5 pollution was estimated at 21% on Friday. It was 42% on Thursday, the maximum so far this season.

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

This year’s haze comes as the Capital battles a new surge in coronavirus infections, and health experts fear that people with chronic medical conditions could become more vulnerable to the disease. On Friday, the health ministry informed a parliamentary committee that air pollution may lead to faster spread of the coronavirus infection, as it causes coughing and sneezing

Last month, the Indian Council of Medical Research had also said, citing international studies, that air pollution levels can lead to rise in coronavirus mortality. Delhi has been reporting over 6,000 new coronavirus cases for the last few days.

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