Both physical and biological collection efficiencies are considered during the validation process because the impact velocity of the air hitting the media is a design compromise in each sampling device. Impact velocity must be sufficiently high enough to allow particles as small as 1 um to be captured; however, the impact velocity must also be sufficiently low enough to ensure viability of microbiological particles by avoiding mechanical damage or break up of clumps of bacteria on impact. In other words, what is good for increasing physical collection efficiency is not good for high biological collection efficiency. Given the wide range of air flow rates seen amongst the many commercially available microbial samplers, it is the responsibility of manufacturers to evaluate their designs and flow geometries for both physical and biological collection efficiency.