The air quality in Delhi during Diwali celebrations on Thursday was recorded in the “severe” category, data from the Central Pollution Control Board showed.

At 10 pm, the central pollution board recorded the air quality at 409 in Delhi. A reading between the 401 to 500 range means that the air quality is severe.

According to the agency’s air quality index, or AQI, any reading above 100 on a scale of 500 is progressively unsafe for health.

Earlier on Thursday, the national Capital recorded a 24-hour average air quality index of 314, 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 was 386 in Delhi at 10 pm on Thursday.

Delhi’s 24-hour average air quality index was 341 on Wednesday, 303 on Tuesday and 281 on Monday.

At Safdarjung and the Indira Gandhi International Airport, poor visibility in the range of 600 to 800 metres was recorded on Thursday morning, The Indian Express reported. Through the day, visibility remained low in the range of 800 to 900 metres.

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.

Levels of the most dangerous particles, called PM2.5, was on Thursday 170 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.

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