Delhi's air quality deteriorated to "severe" due to a dust storm in North West India which led to a sudden spike in the pollution level in the national capital, the Centre-run SAFAR said.
The System of Air Quality and Weather Forecasting and Research (SAFAR) recorded the air quality index at 408 which falls in severe category, while the Central Pollution Control Board showed the AQI in very poor category at 347.
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 "severe".
The SAFAR also gives a list of precautions that people should take under the severe air quality condition, which includes avoiding all physical activity outdoors.
"Give a miss to walk today. Stop any activity level if you experience any unusual coughing, chest discomfort, wheezing, breathing difficulty, or fatigue and consult doctor. If the room has windows, close them. If the air conditioner provides a fresh air intake option, close it," the SAFAR said.
"Avoid burning anything, such as wood, candles or even incense. Keep the room clean -- do not vacuum. Do wet mopping frequently. Masks known as N-95 or P-100 respirators may only help if you go out. Do not rely on dust masks for protection," the SAFAR said.
The SAFAR said the whole of North West Indian region is under the grip of blowing dust and the AQI is predicted to remain in severe to upper side of very poor range on Thursday and Friday.
"Further deterioration of air quality is expected for today despite high ventilation with lead pollutant changing from PM 2.5 to PM 10. In Delhi, air quality is predicted to touch higher end of the very poor category," the SAFAR said in its daily forecast.
"After May 10, Delhi air quality will improve due to expected western disturbance and gusty thunder storm but occasional episodes of sudden peaking of dust on isolated places in north west India, including Delhi will be seen," the SAFAR said.
The level of PM2.5 (particles in the air with a diameter of less than 2.5 micrometres) was recorded at 125 and the PM10 level 439, it said.