The present invention is a system and method of conditioning the sample and surrounding environment of an exhaled breath sensor. It includes (i) using a particular molecular sieve for a triple purpose: (a) maintaining the sensor at a constant relative humidity during storage, (b) equilibrating the incoming breath stream to the same relative humidity as the sensor, and (c) eliminating the analyte from the environment around the sensor during storage. The present invention also includes removal of interfering components from the exhaled breath, such as carbon dioxide, as well as a thermal management technique.