Most cleanrooms are held in positive pressure (except when dealing with hazardous products, which must be held in negative pressure). This means that the air flows out of the room instead of into it, thus preventing unfiltered air or air particulates from entering the cleanroom.
The HVAC system is responsible for creating pressure differentials to keep pressure cascades between the divisions of the cleanroom and the outside of the cleanroom. The presence of a fume hood or a biosafety cabinet (BSC) must be taken into account in the design of the HVAC system, as some BSCs exhaust the air to the outside (hazardous drugs or other chemicals), which unbalances room pressure. To maintain the correct pressure in the cleanroom, the air output must be compensated with fresh air.
CE will help determine the correct pressure differential for your application.