Why 0.5 and 5.0 micron particles are measured in pharmaceuticals during non viable particle monitoring ?

Particles measurement is very important in classified area. Different guidelines like ISO, EU GMP has defined certain limits for these particles for a particular class in classified area. Particle counters are used for monitoring of non viable particles. If number of these particles increases in particular area then they may cause product contamination, so control over particles in classified area is very important. But why all guidelines talk about 0.5 and 5.0 micron particles. Why these particles particles are so important to measure and what about the particles smaller than 0.5 micron and particles greater than 5.0 micron, aren't they important? 

In classified area non viable particles are generated in the environment through system, process and movement but how viable particles are generated in the classified area? Personnels are the main source of viable particle contamination.Viable particles are carried by the personnels working inside the classified area.  Millions of bacteria are shed by the body of personnels on continous basis. But how microbes move in classified area, are they freely movable or require any source for movement? Microbes can't move on its own, first they require a source for attachment and this source is particle. Non viable particles are main source for movement of viable particles because microbes first attach to the non viable particles and after attaching to these particles they are carried from one area to another area through air movement. During non viable particle monitoring 0.5 and 5.0 micron particles are measured because most commonly found bacteria range is in between 0.5 to 5.0 micron so this range of particles are the main source of product contamination and can be carried from one area to another area very easily with the air movement. Size less than 0.5 micron is not very critical because microbes are less likely to attach to this size of particles and greater than 5.0 micron particles become heavy as size of these particles increases and can be easily settled down as compared to 0.5 and 5.0 micron particles. That's why 0.5 and 5.0 micron partilces are measured in pharmaceuticals.

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