The air quality in Delhi dropped to “very poor” category on Tuesday, according to data shared by the Ministry of Earth Sciences’ SAFAR, or System of Air Quality and Weather Forecasting And Research.

Heavy fog shrouded large parts of the city as lower temperature trapped pollutants to push the air quality index in the national capital to 359 as of 12.20 pm, according to real-time figures shared by SAFAR.

An air quality index between zero 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 “severe”. The AQI in the “very poor” category can lead to respiratory illness on prolonged exposure.

Meanwhile, the PM10 index, which measures the concentration of particulate matter of 10 microns diameter or less in the air, reached 299, according to SAFAR.

Levels of PM2.5 climbed to around 196 micrograms per cubic meter. This falls under the “very poor” category.

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.

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 to be considered safe.

On Monday, Commission for Air Quality Management had decided not to invoke curbs under Stage III of the anti-air pollution action plan despite the situation in the national capital worsening to the “severe” category, reported Mint.

The Graded Response Action Plan devised by the Central government’s air quality panel says that if the air quality is projected to reach the “severe” category, the restrictive actions under Stage III are to be invoked at least three days in advance.

Some of the curbs under the Stage III include a ban on non-essential construction and demolition and closure of stone crushers and mining activities in Delhi and national capital region.

“Since the forecast does not predict any deterioration and the overall AQI of Delhi is predicted to further improve from tonight/ tomorrow, the GRAP sub-committee has unanimously decided not to invoke restrictions under Stage III,” the air quality panel said in its Monday order.