IMPACTION PRINCIPLE in microbial air sampling

For active microbial air sampling based on the impaction principle, a predefined volume of air is drawn through a perforated sieve. The air and the particles in it are accelerated and directed onto an agar plate positioned below the sieve.

Due to inertia, particles above a certain size are impacted onto the agar medium. After appropriate incubation, microbe-bearing particles become visible as colony forming units (CFU), can be counted and reported as CFU/m³. The particle size at which 50% of the particles impact and the other half remain in the air stream is called the cut off diameter d₅₀ (um). 

Great emphasis is placed on precision in the manufacture of MBV perforated sieves. This ensures that the cut off diameter is identical in all holes and that microbial air sampling is repeatable.