The term "fogging" refers to the condensation of water vapor on a plastic film's surface. This results in the formation of water droplets. In food packaging films the droplets makes the content less visible and may decrease the quality of the packed product. In agricultural applications as greenhouses, fogging will affect the transmittance of light, causes burns of the crops by the lens effect of the droplets, and a continuous water drip.
Typical Polyolefins are very hydrophobic. Water on a film surface will cause the formation of droplets (high contact angle: no wetting or spreading of the water over the surface).
The addition of (internal) anti-fogging additive makes the surface more polar. They have a controlled incompatibility with the polymer. This causes a migration to the surface of a plastic film. There it is responsible for a polar layer at the surface. This increases the surface energy. The difference between the surface energy of the plastic surface and the water reduces, thus the contact angle reduces. The water spreads out as a thin film, instead of forming thick drops
The optical properties remain less affected.