Abstract: A monitoring device system and a method for controlling ventilation in an enclosed or confined environment by regulating the effective ventilation rate, air exchange rate, mixing factors, stratification of air within the confined area, the percentage of outside air which is introduced (when possible), the percentage of recirculated air, mean air life, air half-life are described. The device may be used for controlling an HVAC system so that air quality conforms to predetermined air quality standards. The monitoring device consists of a multiport sample collection system and one or more sensors to detect determinants of air quality. The method of control uses a CPU to regulate fans, dampers, and other systems to maintain the air quality within predetermined ranges, based upon the monitoring data. A tracer gas, preferably CO.sub.2, is used to determine system parameters.