Carbon dioxide is a colorless, odorless gas formed in all processes in which combustion of substances containin carbon occur. In non-industrial indoor environments the main sources are human respiration and smoking; although CO2 levels can also be increased by the existence of other combustion (cooking and heating) or by the traffic vehicles in the neibourhood. The concentration of carbon dioxide in an indoor environment can provide information on different aspects of the building: odor problems, ventilation, correlation with helth problems of the occupants, etc. To measure the concentration of carbon dioxide in a building or industrial enviroment today, the most reliable technology are the NDIR sensors.