Which statement about Class 4 exhaust is correct?

• A. It must be positively pressurized
• B. It must be isolated from indoor air
• C. It must be negatively pressurized and cannot pass through ducts or plenums in positively pressurized spaces
• D. It must be different from the main building exhaust by at least 6 feet

Answer: (C)

Class 4 exhaust is designed to contain potentially contaminating air by keeping the exhaust stream at a lower pressure than surrounding spaces and ensuring it does not flow through ducts or plenums that are positively pressurized. The negative pressure draws air from adjacent spaces into the exhaust path, preventing contaminants from migrating into other parts of the building. Keeping this exhaust out of positively pressurized ductwork stops backflow and cross-contamination through the building’s ducts. Positive pressurization would push air (and any contaminants) into other spaces, undermining containment. Merely isolating from indoor air doesn’t guarantee the necessary containment or airflow direction, and the six-foot separation from the main building exhaust is not the defining requirement for Class 4 exhaust.