Abstract: A passive, non-invasive, non-contacting apparatus and method for monitoring the respiration of a subject within a monitored environment is disclosed. The apparatus generally comprises a pair of sensors which detect changes in infrared energy. The first sensor detects changes in infrared energy which signifies and corresponds to changes in the monitored environments of a component to be monitored and generates a first signal. The second sensor detects changes in infrared energy which signifies reference infrared energy in the monitored environment and generates a second signal. A processing system converts the first and second signals into a third signal which signifies the concentration of the monitored component in the monitored environment. The monitored components may be CO.sub.2 H.sub.2 O or a constituent of exhaled breath such as a ketone, amino acid, insulin or pintane. In another embodiment of the invention changes in blood pH may be monitored by adding an additional sensor.