We report the application of an optical fiber-based humidity sensor to the problem of breathing diagnostics. The sensor is fabricated by molecularly self-assembling selected polymers and functionalized inorganic nanoclusters into multilayered optical thin films on the cleaved and polished flat end of a singlemode optical fiber. Prior work has studied the synthesis process and the fundamental mechanisms responsible for the change in optical reflection from such a multicomponent film that occurs as a function of humidity. We briefly review that prior work as a way to introduce more recent developments. We then discuss the application of these humidity sensors to the analysis of air flow associated with breathing. We have designed the sensor thin film materials to enable the detection of relative humidity over a wide range, from approximately 5 to 95%, and for response times as short as several microseconds. This fast response time allows the near real-time analysis of air flow and water vapor transport during a single breath, with the advantage of very small size. The use of multiple sensors spaced a known distance apart allows the measurement of flow velocity.