Characterization of trace amounts of odorants in air in an ICU room (ca. 257m²) was carried out by gas chromatography and atmospheric pressure ionization-mass spectrometry (API-MS). The concentrations and odor recognition threshold values of the detected odorants, acetaldehyde, ethanol, n-butyric acid, iso-valeric acid and n-valeric acid are as follows: 44.7ppb and 15ppb; 19710ppb and 6100ppb; 0.50ppb and 0.4ppb; 0.45ppb and 0.4ppb; 0.67ppb and 0.5ppb, respectively. The detected concentrations of these odorants were significantly higher than the odor recognition threshold values. The compounds may, therefore, be responsible for perception of such odors as mixed odors, body odor, and faint or recognizable alcoholic odor, disinfectant odor, and sour, pungent, and goaty odors.