Filter efficiency is the ratio of particles trapped by a filter over the total number of particles found in the air upstream of the filter. A count of the downstream particles is often used to determine the number of particles trapped by the filter. Filter efficiency can either be based on specific particle size ranges or based on the total number of particles of all sizes. There are numerous testing procedures utilized for determining filter efficiency.
Particles Trapped / Particles Upstream = Filter Efficiency
(Particles Upstream - Particles Downstream) / Particles Upstream = Filter Efficiency