FOREWORD Due to the frequent practice of prefiltering@ filters are often used in two-stage systems. Because the first filter removes some of the particles from the airstream@ the downstream filter sees fewer large particles and a greater percentage of smaller particles. This results in a different performance for the downstream (final) filter than would be expected based on loading with the same dust as the prefilter. Thus@ an optional test method has been developed to allow standard testing for two-stage systems. Note: In this addendum@ changes to the current standard are indicated in the text by underlining (for additions) and (for deletions) unless the instructions specifically mention some other means of indicating the changes.