In laboratories containing extremely toxic or infectious agents such as Biosafety Level 3 or 4 laboratories, it may be prudent to utilize both a ÆP and a ÆV system to assure that an adequate differential pressure is maintained at all times. The most common way of doing this is to design a basic ÆV system as previously described, and add a ÆP sensor and controller which is used to reset the ÆV system offset. Here, the long time delays required to produce an accurate average do not affect the speed of response of the system. The ÆP system can dynamically calculate an appropriate offset. As the characteristics of the room change, such as duct leakage and envelope tightness, the offset will change (it usually grows) to maintain the desired laboratory space pressure. Once the offset has grown to a predetermined value it may be necessary to recalibrate flow measuring instruments, seal ductwork, or seal bypasses in the laboratory envelope to bring the system back into specification. Monitoring the offset in a hybrid system of this type is a good way to monitor the integrity of the total duct/control/envelope system.