In small buildings and residences in Korea, unvented or improperly vented combustion appliances are used to heat rooms. These heaters are mostly gas-fired portable equipment used for auxiliary heating. Although their maintenance cost is very low, unvented gas-fired heaters emit air contaminants such as carbon dioxide and carbon monoxide. If there is inadequate ventilation and even a little fresh air with oxygen comes into the appliances, combustion occurs less efficiently and heaters generate more air contaminants. Natural ventilation is the best way to prevent air pollution in small buildings. Outdoor air has a lot of oxygen and can reduce air pollution. The objective and scope of this research is to predict the distribution of indoor air pollution, according to the results of field measurements and CFD (Computational Fluid Dynamics) simulations. In a room of 6.4m×3.1m×2.6m in size, air contaminants (CO2 and CO) and air temperature were measured in two instances--with natural ventilation and without natural ventilation. After comparing the results obtained with computer simulation and field measurements, passive control methods to improve indoor air quality in a room are proposed.