I work for an analytical laboratory in food microbiology, and we do daily
testing of air quality. We don't neccessarily test per cubic meter but we
test busy areas by placing a petri dish with a non-specific, nutrient agar
in it for a set amount of time.
Our default is one microbial organism per minute. I don't know what your
specs would be for a biomedical cleanroon but I would guess it would be
much tighter than this. I hope I have been at least a little bit of help
in steering you in some sort of direction. Good luck.