In a system and method of controlling particulate count in a clean room having a number of compartments/zones in series, in response to a first level of energy consumption by speed adjustable fans supplying a first volume of air per unit of time to the clean room, differential air pressures are established in the clean room compartment/zone-by-compartment/zone in series from a first compartment/zone which has a requirement for the highest air pressure to a last compartment/zone which has a requirement for the lowest air pressure. In response to a second level of energy consumption by the speed adjustable fans supplying a second volume of air per unit of time to the clean room, the same differential air pressures are substantially maintained in the clean room.