The effects of air temperature and background CO₂ concentration on human metabolic CO₂ emissions

The impact of temperature and air quality on human CO₂ emissions was studied in two experiments. In the first experiment, seven females and seven males were exposed to temperatures of 17, 20, 25, and 28 °C controlled using an air conditioner; the subjects were engaged in three activities: sitting, standing, and walking at 2 km/h, each taking 15 min. Their CO₂ emissions and respiratory physiological parameters were measured together with skin temperature and heart rate. They also rated their thermal sensation and comfort. CO₂ emissions did not change when sitting or standing but increased significantly for females walking at higher temperatures. Lower temperatures were rated as less comfortable. In the second experiment, five females and eleven males sat in a chamber at CO₂ levels of 800, 1400, and 3000 ppm with and without bio-effluents; the levels were adjusted by ventilation or adding pure CO₂. Higher CO₂ levels obtained either way caused CO₂ emissions and respiratory quotient to reduce significantly. CO₂ emission rates estimated from measurements of CO₂ in the chamber were about 10 % higher than those obtained from the respiratory monitors. Present experiments documented that CO₂ emission rates are affected by air quality levels and temperature, although in the latter case, the effect can be expected only at high activity. These observations imply that besides metabolic activity, environmental factors must be considered when estimating ventilation rates based on the measured (or estimated) levels of metabolic CO₂ in buildings. It would be useful to explore the physiological mechanisms behind these effects.