Air Changes per Hour (ACH) measures how many times the entire cleanroom air volume is replaced with filtered air per hour. Typical values: ISO Class 5 cleanrooms: 300-600 ACH. ISO Class 7: 60-90 ACH. Class 100: often 400+ ACH. Higher cleanliness requires more air changes. Calculation: ACH = Airflow rate (CFM) x 60 / Room volume (cubic feet). Purpose: Dilute and remove airborne particles. More changes = faster particle removal and better cleanliness. Design factors: Particle generation rate (people, equipment), cleanliness class requirement, room volume, ceiling coverage. Energy impact: Very high ACH is expensive - more fan power, more conditioning of makeup air. Balance cleanliness vs cost. Comparison to other environments: Homes: 0.5 ACH. Offices: 6-10 ACH. Operating rooms: 20-25 ACH. Semiconductor fabs: 300-600+ ACH. Measurement: Calculate from supply air volume flow rate measured at diffusers or FFUs. Uniformity: ACH should be relatively uniform across the room. Dead spots with low flow accumulate particles.