Because paper is composed of a randomly felted layer of fiber, it follows that the structure has a varying degree of porosity. Thus, the ability of fluids, both liquid and gaseous, to penetrate the structure of paper becomes a property that is both highly significant to the use of paper. Paper is a highly porous material and contains as such as 70% air. Porosity is a highly critical factor in Printing Papers Laminating Paper, Filter Paper, Cigarette Paper. Bag Paper, Antitarnish Paper and Label Paper.

Porosity is the measurement of the total connecting air voids, both vertical and horizontal, that exists in a sheet. Porosity of sheet is an indication of absorptivity or the ability of the sheets to accept ink or water. Porosity can also be a factor in a vacuum feeding operation on a printing press.