Filter performance is often expressed in terms of percent efficiency, defined as the ratio of upstream particle concentration compared to the downstream concentration that has passed through a filter body (multiplied by 100).
Percent Efficiency = 1 - Downstream Concentration/Upstream Concentration x 100
The efficiency of a filter varies for different particles sizes and flow rates. For this reason, it is meaningless to specify the efficiency of a filter without also stating the pertinent particle size and flow.