A filter's efficiency is defined by the ratio of the number of particles it traps to the total number found in the upstream air; a count of the number of downstream particles is often used to determine how many have been trapped. Filter efficiency can either be based on a specific range of particle sizes or defined for particles of all sizes. Mechanical filter media depend on four primary mechanisms: sieving, impaction, interception; diffusion. Each of these is responsible for filtering a different range of sizes; for instance, sieving is known to be the mechanism for trapping the largest particles and diffusion for trapping the smallest.
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