EQc5 Negative Exhaust Rates

I'm trying to figure out the exhaust airflow rates required for chemical pollutant source rooms in a hospital (e.g. janitor's closets, copy rooms, etc.). The LEED credit requirements state: "The pressure differential with the surrounding spaces must be at least 5 Pascals (Pa) on average and 1 Pa at a minimum when the doors to the rooms are closed." This is a bit ambiguous, but I take this to mean that the average exhaust rate of these rooms must meet at least a 5 Pa pressure differential over their entire operating range but the instantaneous pressure differential can get as low as 1 Pa. This implies that LEED is giving allowance for spaces where the chemical use is variable and the exhaust fans have multi-speed control where they ramp up only when chemicals are in use. Most of the exhaust air systems I work with for these types of spaces are simply equipped with constant volume fans however. So if the exhaust air volumes don't change, then it sounds like we must meet a 5 Pa pressure differential to meet the credit requirements. Is my interpretation correct? Has anyone had experience or LEED comments related to this specific issue? Thanks,