The air quality in Delhi on Tuesday entered the “very poor” category for the first time this season, data from the Central Pollution Control Board showed.

According to the agency’s air quality index or AQI, any reading above 100 on a scale of 500 is progressively unsafe for health. The national Capital recorded a 24-hour average air quality index of 303, indicating “very poor” conditions that pose a risk of respiratory problems on prolonged exposure.

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 air quality index struck 309 in Delhi at 10.55 am on Wednesday.

Delhi’s 24-hour average air quality index was 218 on Monday, 289 on Sunday and 268 on Saturday.

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

In September, the Delhi Pollution Control Committee had banned the sale and bursting of firecrackers up to January 1, 2022.

“The stubble burning induced impact in AQI is expected to increase significantly from November 4 onwards due to transport level winds reaching Delhi from North-Westerly where large stubble burning hot spots are expected,” SAFAR said.

Levels of the most dangerous particles, called PM2.5, was on Wednesday morning 131 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.

On Tuesday, stubble burning accounted for 6% of PM2.5 pollution in Delhi, according to PTI. The rest of the pollution was because of local sources, said Gufran Beig, the founder project director of SAFAR.

In October, Delhi did not record even a single day of “very poor” or “severe” air quality due to an extended monsoon season, PTI reported.