Air exchange rate measures how frequently all air within a space is completely replaced with fresh air, expressed as air changes per hour (ACH). This metric directly influences indoor air quality, humidity control, and pollutant dilution. Typical commercial buildings operate at 4 to 8 ACH, while surgical suites and laboratories may require 12 to 25 ACH for contamination control.
Calculation and Standards
Air exchange rate equals total air volume supplied per hour divided by room volume. A 2,000-square-foot space with 9-foot ceilings (18,000 cubic feet) receiving 1,500 CFM achieves 5 ACH (1,500 CFM × 60 minutes = 90,000 cubic feet per hour divided by 18,000 cubic feet). ASHRAE standards specify minimum ACH requirements by occupancy type: general office spaces require 4 to 6 ACH, healthcare facilities need 6 to 12 ACH depending on area classification, and clean rooms require 15 to 25 ACH or higher.
Practical Significance
Higher air exchange rates improve odor removal, reduce airborne pathogen concentration, and limit contaminant accumulation. However, increasing ACH substantially raises energy consumption and equipment costs. Designers must balance air quality requirements against operational expenses. Demand-controlled ventilation systems modulate ACH based on actual occupancy and contaminant levels, optimizing performance without excessive energy waste.