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Ans. Because paper is composed of a randomly felted layer of fibers,  the structure has a varying degree of porosity. Paper is a highly porous material and contains as much as 70% air. Paper for air filtering is used in numerous applications such as car air filter, vacuum cleaner air bag, home heating furnace filter etc. Most of the papers are porous and will allow air to flow, but opening are small enough to retain dust/dirt particles. Paper used for air filtering are made porous and as per the application porosity is controlled. Three important characteristics of filter paper are:

1. What maximum size particle it will allow to pass? (Determines filtering efficiency or capability)
2. How fast it will let the air pass? (Determines air flow rate  through filter)
3. How much resistance it will offer to air to pass? (Air pressure/vacuum required to force or suck the air)

Two most important factors to control paper porosity are fiber length and amount of refining/beating. Longer the fiber and/or lesser the refining/beating, more porous will be the paper. Good quality air filter papers have rag content. Rag fibers are long and provides high porosity and good strength.

For further info on paper porosity

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