Differences might arise between the design and the real load of ventilation systems during operation.Consequently, the fresh air demand usually varies over a wide range. In normal usage, different types of gaseous contaminants enter the air that might be hazardous to health. The constant inhalation of contaminated indoor air might lead to discomfort or to harmful physiological effects.The difference between the design and the real load can be diminished by augmenting the ventilation system with appropriate components and controls. The proposed additional control system tracks the actual load and supplies the necessary supplemental fresh air without disturbing the air supply to other spaces. The benefit of this system is that the required air quantity is supplied to follow the load but no energy is wasted for the heating and cooling of spaces where the systems fresh air demand is below the design load.