All the particle counter can do is give a reading of the number of particles per cubic foot at any one point at one time. It doesn’t indicate what the particles are, or where they come from. the stepwise checkout of a clean room should be first to run the DOP test to find the significant leaks and eliminate them. If the subsequent introduction of personnel and equipment causes generation of particulate matter which may be of concern to the successful operation of the room one more step to be taken.A sample of the room air should be taken with a millipore-type filter after which the particles collected may be sized and counted, but and this is the most important- they may now be identified. This identification will enable the room operator to establish the source of contamination is harmful to the work being done. No particle counter in existence can do this for you. As a matter of fact, to quote from paragraph 60 FED, STD, 209A referring to the DOP test,” …. in most instances, this type of facility check will be more meaningful than any attempt to make an actual particle count, due to the extremely low particle concentrations expected, plus the difficulty of taking a representative sample in a laminar air flow stream”.