Clean Room Validation Recovery test

How a recovery test is done during validation of AHU/clean room in pharmaceutical manufacturing?

Recovery test in AHU or clean room validation is a test where the ability of the clean room or AHU to maintain the level of cleanliness or the particle count below the rated classification limit.

In the process clean room is sprayed with known number of particles. Aerosol of particles of known concentration and known size is sprayed in the clean room, and the time taken by clean room to filter out and clean the sprayed particles to the average count of the clean room is checked.

This is a kind of challenge test, like we do challenge test for empty blister rejection on tablet packing machine or on checkeweigher.

A known amount of particle aerosol is sprayed in the clean room and time taken to clear the particles to the base line particle count is noted.

This data is used to assign the cleanup time of the clean room, which helps pharmaceutical manufacturing pharmacist for planning for the actual work in the clean room.

How much concentation of particles should be taken for recovery test?

Guidelines suggest that the particles sprayed in the recovery test should not be unnecessarily high. There is no point in keeping higher concentration. Since the aerosol it self may lead to untoward effect on the cleaness. Therefore even the ISO standards mention this. Particle count taken for recovery test should be aroun 100th of desired cleaness level.
But this some times is not recommended.
Arosol concentation should be taken 1.5 of the baseline particle count obtained in notmal working of Clean Room. For an example the clean room is grade A and particle count for grade A qualification is 3500 particles of not mor than 0.5 microns.
And the observed mean count on day today basis is 2000 particles. This is the baseline count of the clean room. And 1.5x 2000 number of particles should be sprayed in the room to study the recovery. Then 3000 number of particles will be loaded in the clean room during the validation process, or recovery test, this will not create risk associated with higher number particle recovery test