To filter or not to filter

Recently, I had a customer call about positive pressuring their new clean assembly room. This customer rebuilds high-speed high precision spindles so it was critical to have the incoming air be dust and particle free.  The concept was to always have air flowing from the cleanroom back to the disassembly area.  
At first, I considered using a couple Keep The Heat Air Exchangers.  Since the air exchangers run under slight negative pressure and we could run a couple of fresh airdrops into the cleanroom, this could work just fine.  However, the incoming air would not be filtered, leaving the potential for a small amount of particulate to enter the cleanroom.
Instead, we decided to use a HEPA industrial air cleaner.  This unit will deliver 1875 cfm of 99.97% dust and particulate-free air.  We placed the unit in the “dirty” disassembly area and ducted the filtered air through two other parts rooms to reach the new clean assembly room.  From there we dropped the air into ceiling diffusers for a clean finished look.  
In the end, the customer was happy with the solution.  The cleanroom should be up and running in a few days and I should have some positive feedback.