Why is achieving a higher air changes per hour (ACH) generally beneficial for contaminant control?

• A. It dilutes contaminants and reduces exposure.
• B. It increases noise and energy use with no benefit.
• C. It has no impact on exposure.
• D. It reduces ventilation requirements.

Answer: (A)

Increasing air changes per hour increases dilution of airborne contaminants, lowering the concentration people are exposed to. When air in a space is replaced more rapidly, contaminated air is removed and replaced with cleaner outdoor air more quickly. In a well-mixed room, this means the concentration of pollutants decays faster after a contaminant spike and, if contaminants are being generated continuously, the steady-state concentration drops as the ventilation rate rises (more air leaving and more clean air coming in). So the higher the ACH, the more effectively the space is kept at lower contaminant levels, improving exposure control. Trade-offs like increased energy use or noise exist, but the core benefit for contaminant control is the faster dilution and removal of pollutants.