A contractor discovers friable asbestos during renovation and must establish a negative pressure enclosure. Calculate the minimum air changes per hour required if the enclosure volume is 2,400 cubic feet and the exhaust system moves 600 CFM.
Correct Answer
A) 15 air changes per hour
Air changes per hour = (CFM × 60) ÷ Volume = (600 × 60) ÷ 2,400 = 36,000 ÷ 2,400 = 15 air changes per hour. Cal/OSHA requires minimum 4 air changes per hour for negative pressure enclosures.
Why This Is the Correct Answer
Air changes per hour (ACH) = (CFM × 60 minutes) ÷ enclosure volume in cubic feet. Substituting: (600 × 60) ÷ 2,400 = 36,000 ÷ 2,400 = 15 ACH. This meets and exceeds the Cal/OSHA minimum of 4 ACH for negative pressure asbestos enclosures.
Why the Other Options Are Wrong
Option B: 12 air changes per hour
12 ACH would result from a calculation error, such as using 480 minutes instead of 60, or dividing by the wrong number. The correct formula yields 15, not 12.
Option C: 20 air changes per hour
20 ACH would require (20 × 2,400) ÷ 60 = 800 CFM from the exhaust system. The given 600 CFM system only produces 15 ACH, not 20.
Option D: 18 air changes per hour
18 ACH would require (18 × 2,400) ÷ 60 = 720 CFM. With only 600 CFM available, the system achieves 15 ACH, not 18.
Memory Technique
Remember the formula as 'CFM times 60 over volume' — think of it as: CFM gives you per-minute flow, multiply by 60 to get hourly flow, then divide by the box size to get how many times the air turns over. Visualize the entire enclosure filling up 15 times each hour.
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