The ventilation system removes pollutants effectively, and the resultant vertical temperature difference in the room greatly affects the indoor air distribution. A reasonable air distribution system is essential to provide a satisfying indoor air quality (IAQ) for the occupants, of which air quality in the breathing microenvironment plays a major role in occupant health, as they are exposed to this region directly. With a view to ultimately optimize the air quality in the breathing microenvironment, indoor air stability, namely, the background temperature effect, integrated with a displacement ventilation system, is analyzed in this study. Experiments were conducted in a full-scale test room with a test subject performing normal breathing activity. The concentration of the carbon dioxide (CO2) was measured as a proxy for the pollutant concentrations in the breathing microenvironment. The results indicate that the vertical movement of the exhaled air is inhibited in a displacement ventilation system combined with a stable condition, so the inefficient dispersion leads to a severely polluted breathing microenvironment, increasing the infection risk of the occupants. It is also shown that with the same displacement ventilation system, the unstable condition causes the indoor air to undergo significant mixing, which to some extent destroys the stratification resulting from the displacement ventilation. A much greater irregularity appears in the exhaled airflow, so the pollutants travel widely beyond the breathing microenvironment, contributing to a less polluted region. It is concluded that indoor air stability will affect the ventilation systems performance and the unstable condition is advisable for minimizing the amount of pollutants in the breathing microenvironment, reducing the risk of infection and providing a better air quality for the occupants.