The Indo-Gangetic Plain (IGP), home to over 900 million people, has experienced a marked rise in air pollution as well as winter fog episodes, the World Meteorological Organization (WMO) said on Friday.

Although fog is a seasonal occurrence, the WMO said its growing frequency and duration are increasingly linked to ever-increasing human activities and regional environmental changes.

Fog forms when air near the ground cools to the dew point, allowing moisture to condense on certain airborne particles [known as `fog condensation nuclei` (FCN)] and form tiny droplets. This process is heavily influenced by fine particles (PM2.5) in the atmosphere, released from sources such as tail-pipe emission, industrial flue gas and burning of crop residue, fossil fuel and domestic biomass.