Originally Posted by Mike7
I see with positive pressure it would be like a vacuum cleaner and quickly fill with dust.
Not if like I said the inlet fans have filters. This way ALL the air going in has already been filtered. With a negative pressure, a portion of the air going in through any cracks, seams or holes has not been filtered.
This is how clean rooms work. Highly filtered air is blown into a room at slightly higher than atmospheric pressure. The slight positive pressure prevents doors etc from admitting contaminants into the room. It is also how biological contaminants are isolated, but the concept is reversed by putting a slight negative pressure in the room which isolates contaminants to that room. The exhaust system has UV light, scrubbers and other devices which kill or remove biologicals.