What is the minimum outdoor ventilation air for a 1000 sq ft theater lobby? (in cfm)

• A. 1800
• B. 2400
• C. 3600
• D. 3000

Answer: (D)

Minimum outdoor ventilation air is determined by two parts: outdoor air per person and outdoor air per area. You estimate how many people will be in the space and how big the space is, then multiply each by its respective rate and add them together. For a theater lobby—a public assembly space—the per-area rate is relatively high because the space can accommodate many people and needs good dilution of contaminants across the floor area. A common design guideline for such spaces is about 3 cfm of outdoor air per square foot. So for a 1000 sq ft lobby, the area-based portion is 3 cfm/ft^2 × 1000 ft^2 = 3000 cfm. The occupancy-based portion would add additional cfm depending on how many people are expected, but in many lobby designs this area-based requirement already sets the practical minimum. That’s why 3000 cfm is the correct choice. Smaller values would imply lower per-area rates (e.g., 1.8 or 2.4 cfm/ft^2), and a higher value would require more aggressive ventilation (3.6 cfm/ft^2), which doesn’t align with the standard rate used for this space.