Athosanswered grow question 5 years ago This only matters if you have a carbon, filter. What you want is negative pressure, what does that means? It means that your exhaust airflow (CFM / mΒ³h) is greater than your intake airflow. If it were the other way around and the intake is greater than the exhaust (positive pressure), then the excess air would escape by any orifice and crevice it can find, bypassing the carbon filter.