The CO ₂ emissions were measured in environmental rooms in 15 animal operation studies, including studies of dairy cow, steer, swine, turkey, laying hen and broiler. The objectives of this paper are to summarize CO ₂ emission data in these 15 studies;, to investigate the possible diet effect; and to evaluate the performance of the CO ₂ balance approach to estimate ventilation rate (VR) of animal houses. Lower CO ₂ emissions were observed from steers fed diets containing 60% DDGs as compared to that from steers fed the control diets (0% DDGs). Significant differences in CO₂ emissions were observed among different studies for each species of broiler, laying hen, swine, and steer, which could be the result of different management practices, different stages of production, or different weather conditions. In 10 out of the 15 studies, the measured CO₂ emissions were significantly larger than the estimated CO₂ productions, which could represent the corresponding amount of CO₂ generation that was not accounted in the metabolic CO ₂ production as well as uncertainties in measurements of CO₂ emissions. The overall R² was 0.97 when treating each of the 15 studies as one data point. The relative differences between measured and estimated VR were in the range of 6.3% to 20.5%. Uncertainties in estimated VR in dairy cow studies were relatively large because the relatively large variations in measured CO₂ emissions in these studies were not well represented by the relatively constant estimated CO₂ production values.