Seventeen years ago, the ventilation rates established by ANSI/ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality, increased substantially over those previously required by the 1981 version of the standard. Fifteen years later, Standard 62.1-2004 (Standard 62.1) prescribed new minimum breathing zone ventilation rates and a new calculation procedure to find the minimum intake airflow needed for different ventilation systems. Described in previous articles, these new rates and procedures must be used to find the design or “worst-case” outdoor air intake flow, which establishes the required capacity of mechanical system equipment. In this article, we turn from ventilation system design to operation.