Importance of Occupants’ Distribution on CO2 Exposure in Ventilated Rooms

CO2 is used as a marker for exposure to indoor pollution. CO2 concentration measured in the air exhausted from the room or near the walls is used to estimate the CO2 exposure as well as to control supplied ventilation rate. However, research reveals that the CO2 concentration in the inhaled air can be more than 30% higher than the concentration measured in the air exhausted from the room. Previous research has identified that part of the own exhaled air is re-inhaled. This leads to increase of CO2 concentration in the inhaled air above the CO2 concentration in the surrounding room air. In rooms with total volume ventilation most of the inhaled air is from the convective boundary layer existing around human body. The convective flow entrains part of the own exhaled air with high CO2 concentration and thus contributes to the elevated CO2 exposure. The importance of the convective layer for the entrainment of air exhaled by the surrounding occupants, before it is mixed with the room air, on the CO2 exposure was studied. Experiments were performed with three persons and a breathing thermal manikin (BTM) in a room equipped with chilled ceiling, air supply from ceiling slot diffuser and heated window to simulate solar load. The CO2 concentration in the air exhaled by the manikin was increased to the level of an average person. The supplied outdoor flow rate, calculated based in the CO2 mass-balanced method to maintain 1000 ppm at the exhaust, was 29 L/s. The temperature of the supplied air was 18 C. The chilled ceiling temperature were controlled to maintain the room temperature at 26°C. CO2 concentration in the air inhaled by the BTM was 16% higher than at the exhaust. This was mainly due to re-inhalation of own exhaled air. The location of the neighbouring “occupants” had no impact on the inhaled CO2 concentration. The ventilation flow rate calculated by the mass balanced method and CO2 concentration measured in the exhaust air may not be accurate to assess occupants’ CO2 exposure.