A level up to 100 is considered "good" or "satisfactory", an index value between 101 and 200 is "moderate", between 201 and 300 is "poor", 301 to 400 is categorised "very poor", and a higher value is "severe".
Stringent measures, including minimisation of private vehicles and shutdown of coal and biomass industries, are recommended by the Central Pollution Control Board (CPCB)-led task force between 1 and 10 November to deal with the alarming increase of pollution in the national capital.
It also recommended intensifying efforts of transport department to check polluting vehicles and controlling traffic congestion.
Friday's maximum temperature settled at 30.4 degrees Celsius, while the minimum temperature was recorded at 15.8 degrees Celsius, both one notch below the season's average.
Alarmed at the situation, the Central Pollution Control Board (CPCB) has recommended that people avoid heavy workouts, try to get themselves less exposed to toxic air, and cut down on the use of private cars.
The government-run System of Air Quality Forecasting and Research also reported an AQI in the "very poor" category.
The PM2.5 (presence of particles in the air with a diameter of less than 2.5 micrometres) was recorded at 225, the highest of this season.More news: Amgen, Regeneron appear most at risk from Trump's latest pricing plan
Air pollution continued to worsen in the Indian capital Saturday, in spite of a sunny morning, prompting the environment authorities to advise people to stay indoors ahead of Diwali, the festival of lights.
The PM2.5, also called "fine particulates", can be a matter of more serious health concern than PM10.
"Pollution would reduce in the next couple of days if the weather condition remains same. Unless there are strong winds in the category of at least 20kmh, this pollution won't go away", said Mr Mahesh Palawat, vice-president of meteorology and climate change at Skymet Weather Services.
Fugitive dust from construction and demolition (C&D) work is also said to be a major reason for air pollution in Delhi-NCR.
Government authorities have issued warning for Delhiites, advising to keep windows shut, wear masks, prefer short walks, minimise use of private vehicles to battle against the risky levels of air pollution in the city.
"The prevailing meteorological conditions are not very favourable for dispersal of pollutants for next two days due to very low ventilation index and low wind speed", the IITM warned.