The cumulative efficiency of a filter describes how effective a filter is in retaining solid particles. According to ISO 4572 it is defined by
ε x = filter cumulative efficiency for particles larger than x μm
Nu = number of particles > x μm upstream of the filter
Nd = number of particles > x μm downstream of the filter
The filter cumulative efficiency is normally stated in %. It is related to the value βx from the multipass test.