The easiest way to assure proper space pressurization control system operation is to monitor it with sensitive equipment which has been properly calibrated and to provide alarms to alert personnel when conditions are outside specifications. A simple qualitative measure of space pressurization control system effectiveness is smell. If you can smell the chemicals which are used in the laboratory when you are in the corridor this may be an indication that your space pressurization controls are ineffective. It may also mean that contaminated air from the laboratory is be reingested back into the building air supply due to inadequate stack design. In the case of a clean space, excessive particulate counts may indicate a space pressure problem causing infiltration. Another simple experiment which can be done quickly and easily is the foot-in-the-door test. Open the lab door and place your foot in the doorway next to the jamb and allow the door to close against it. Next, feel the airflow through this opening with your hand, or use a smoke tube to determine its direction. It’s not quantitative, but it will sure tell you if your positive when you should be negative (or visa versa) or if there is an excessive pressure differential. If either of these are detected then a more quantitative approach should be used to diagnose the magnitude and cause of the problem. During the design of the laboratory environmental control system, a careful analysis of the failure modes of each component and the effect of a failure on the operation of the system should be undertaken. There are many methods to do this, most of which are beyond the scope of this article. One of the more popular methods is called a fault tree. This method may be used on entire systems, subsystems, and individual components. For example, if attempting to do a fault tree analysis on a room pressure control system, choose the component you wish to analyze such as a through-the-wall velocity sensor. List this at the top of the tree. Next consider all of its failure modes and list them underneath. Next, for the most serious failure modes consider all the affects on controlled components and other coupled and decoupled systems and list them underneath that mode. Repeat this process until all the paths are complete. It may be necessary to do all the branches of the tree to discover which paths represent the worst scenarios. When all the failure modes of all the components, subsystems and systems have been completed, you can make some strategic design modifications to eliminate or ameliorate the most serious scenarios by installing more reliable components in key locations or installing redundant controls or systems to provide backup and thus truncate the tree.