All air systems must have the capability to provide outdoor air to the building in order to

• A. To recirculate air only
• B. To humidify ambient air
• C. To filter air without outdoor air exchange
• D. The ability to provide outdoor air to replace air and dilute contaminants

Answer: (D)

Ventilation aims to keep indoor air quality by introducing outdoor air to replace and dilute indoor contaminants. Having the ability to supply outdoor air ensures that pollutants like carbon dioxide, odors, and VOCs are reduced through dilution and replacement, not just recirculation. This is the fundamental reason for requiring outdoor-air capability in air systems.

Recirculating air alone wouldn’t lower contaminant concentrations since it reuses indoor air without bringing in fresh air. Humidifying ambient air is a separate function and doesn’t address contaminant dilution. Filtering without outdoor air exchange cleans what’s inside but doesn’t reduce indoor pollutant levels by bringing in fresh air.