Which is correct about the exhaust duct location for Class 4 air?

• A. It must be positively pressurized and can extend through ducts
• B. It must be negatively pressurized and cannot extend through ducts or plenums in positively pressurized spaces
• C. It may be unpressurized if filtered
• D. It must be connected to a centralized duct system

Answer: (B)

Class 4 air requires containment by maintaining the exhaust path in a negatively pressurized, isolated condition. Keeping the exhaust space at a lower pressure than surrounding spaces makes air flow into the Class 4 area rather than out of it, helping to capture contaminants in the exhaust and prevent them from migrating to other spaces. The exhaust duct must not extend through ducts or plenums that are part of positively pressurized spaces, because sharing or routing through those ducts creates a path for contaminants to leak into, or be drawn into, spaces that are intended to stay positive. This isolation protects other areas from cross-contamination. The other options would either imply positive pressure, or rely on unpressurized or central-duct configurations that do not guarantee the required containment.