The city witnessed a thick layer of haze on Tuesday morning, which affected visibility. Experts warned that episodes of smog will begin in a couple of days.

"It is haze since the Ozone is at a good level," IMD Head K.J. Ramesh told IANS. Haze is the reflection of sunlight from particulate matter (PM). Smog, which is far more dense, is a mixture of moisture and PM, and allows for limited visibility.

At 3 p.m., the Air Quality Index (AQI) of Delhi, which includes 36 regions, was 401 considered severe. The volume of particulate matters was well above "severe" levels by 9 a.m.

According to the Central Pollution Control Board (CPCB), the air quality is set to deteriorate further from November 1, and the Diwali this year is likely to be more polluted.

On Tuesday, almost all places in the NCR saw a sharp rise in the major pollutants -- PM2.5 and PM10 -- particles in the air with diameters less than 2.5mm and 10mm, respectively.

Across 35 active pollution monitored area in Delhi, the average concentration of PM2.5 and PM10 was 273 microgrammes per cubic meters and 487 units by 3 p.m. The same PM2.5 and PM10 concentration across 48 monitored area of National Capital Region (NCR) was 263 units and 470 units.

Air quality is considered "severe" when either AQI is between 401 and 500 or PM2.5 is between 250 and 300 or the PM10 levels are between 430 and 500, according to CPCB.

The safe limit for PM2.5 is 60 units as per national standards and 25 units according to the international standards. For PM10, up to 100 units is safe from national standards and 50 units international standards.

At 3 p.m., the AQI was 425 at Gurugram, 412 in Faridabad, 410 at Sector-125 Noida, and 449 at Ghaziabad, all considered severe. At Greater Noida, AQI was 395, on the verge of being severe.