Viable bioaerosol sampling is based upon the principle of sieve impaction in which air is aspirated through the perforated head of a volumetric air sampler. The air stream, which contains viable particles, is directed onto the surface of an agar plate. After sample collection, the agar plate is incubated and the resultant colonies are counted and expressed as colony forming units (CFU) per cubic meter (m3) of air (CFU/m3). As the number of viable particles in the air increases, the probability of more than one particle entering the same hole (perforation) in the sampler head increases. Therefore, the number of CFUs that are counted requires a statistical correction. This correction is specific for each sampler and type of head and is referenced in the manufacturer’s user manual for the air sampler.
We recommend pore correction for all of our customers since USP <797> (current version and proposed revision) states that the manufacturers’ instructions must be followed when using air sampling equipment.
When customers provide the type of air sampler and the number of holes in the sampler head, U.S. Micro-Solutions will apply pore correction to the CFU count. The pore correction factor is the number that is added to the agar plate count to determine the total count for the sample. The pore correction factor typically comes into play at CFU counts greater than 15, so it will not affect the action levels for ISO 5 or ISO 7 areas.