The capnography market is growing at an annual rate of more than 15%, driven by an increased awareness among clinicians of the importance of early detection of respiratory disorder. So far, respiratory monitoring has been dominated byCO2, but we propose to expand this to also monitor oxygen, which gives a more complete assessment of the respiratory and metabolic functions. Our invention is a small and lightweightFaraday rotation oxygen sensor (FOx) for continuous in-airway monitoring of oxygen at high sampling rate. The sensor will have dimensions of<100x80x80 mm, weigh less than 500 g, and use a very small sampling volume to directly probe the exhaled air in mainstream or sidestream configuration. The sensor will be compatible with capnographic systems from leading medical equipment manufacturers, have a measurement precision of <1% O2 with a time resolution of 10ms, and be capable of fully resolving the oxygen concentration within a single human breath cycle.