Class 4 Secondary Enclosure is defined by which characteristic?

• A. The Building Is Not Airtight Enough to Economically Maintain Pressurization
• B. CBRN Filter Units Are Fully Integrated
• C. Transportable Filter Units Are Permanently Mounted
• D. A Temporary Airlock and Contamination Control Area Are Established

Answer: (A)

Classifying a secondary enclosure as Class 4 hinges on whether the building can be sealed tightly enough to maintain a pressurized environment in a cost-effective way. If the structure is not airtight enough, keeping a stable pressure differential would require excessive energy and costly sealing work, so it’s deemed Class 4 because pressurization isn’t economically feasible. In this scenario, containment and protection rely more on localized controls and procedures rather than relying on a tightly sealed whole-building envelope.

The other items describe specific features or components you might add—such as integrated or portable filtration units or establishing an airlock/contamination control area—but they don’t define the classification itself.