With the increasing air pollution each day, it is very important to keep a check at the air quality we are breathing in. This can be made possible by designing an environmental sensor that is low-cost, robust, readily available, and easy to manufacture. In this research work, we propose a 12-bit smart chipless radio frequency identification tag having a radius of 7.4 mm. The miniaturized chipless RFID multi-sensor is based on humidity sensitive Kapton®HN substrate with Copper as a radiator. These materials selection enable the vast adaptability of this sensor in such areas where the use of a commercial environmental sensor is not economically supportive. The frequency response of the tag contains twelve uniquely identifiable frequency resonances in a 4–18 GHz frequency band. The gas sensing characteristic is incorporated by deploying multi-wall carbon nanotubes over the center of the tag design. The changes in the frequency response of the central slot due to CO2 concentrations are demonstrated in a radar cross-section graph. The proposed tag shows a reliable performance under changing atmospheric conditions.