In addition, settle plates are not as prone to variation among
different vendors as are active samplers (5). However, it is not clear whether all the advantages cited for pas- sive sampling apply in areas of laminar air flow at the rates used for modern clean rooms. In addition, settle plates may be particularly susceptible to handling, transport, and lab contamination. However you view their usefulness, current regulatory expectation for air monitoring includes the use of settle plates and the justification of sampling sites. A prudent measure is to use the same number of sampling sites for settle plates as used for the active viable and non-viable sampling programs. These will not be the same sites, but similar in number.