Ideally you would have a carbon filter attached to a fan that would suck air through the filter and back into the room, therefore continuously scrubbing the air. A separate extraction fan would then take care of the ventilation. However, oftentimes monetary factors prevent people from buying two fans.

Depending on the size of your grow there could be traces of smell left in the air if it is only scrubbed once, and then evacuated. The best way to to employ the system I described above: two fans, one w/ scrubber, and one for extraction.