How does air filter paper work?

Because paper is composed of randomly felt fiber layers, the structure has varying degrees of porosity. Paper is a highly porous material containing up to 70% air. Paper for air filtration is used in many applications, such as automotive air filters, vacuum cleaner air bags, household heating furnace filters, etc. Most paper is porous and will allow air to flow, but the opening is small enough to hold dust/dirt particles. The paper used for air filtration is made porous and the porosity is controlled according to the application. The three important characteristics of filter paper are:

What size particles are allowed to pass through? (Determine filtration efficiency or capacity)

How fast does it move the air through? (Determine the air flow rate through the filter)

How much resistance will it provide for the air to pass through? (Air pressure/vacuum required to force or suck air)

Two important factors that control the porosity of paper are fiber length and the amount of refining/beating. The longer the fiber and/or the less refined/beaten, the more porous the paper. Air filter paper has rag content. Rag fibers are long and provide high porosity and good strength.