Establishing a program to monitor waste anesthetic gas (WAG) in order to limit personnel exposure requires measuring the levels of WAG emitted and determining the effectiveness of scavenging methods to reduce such levels. In this study, the authors used infrared spectroscopy to measure levels of WAG emitted while anesthetizing mice with isoflurane for 15 min. They evaluated four different WAG scavenging conditions during induction and maintenance anesthesia: two conditions that used passive techniques and two that used active techniques. Isoflurane concentrations were measured at three different locations: in the operator's vicinity, at the mouse-facemask interface and in the room environment. Passive scavenging of WAG improved when chambers were purged with oxygen after induction and when a diaphragm-sealed facemask delivered a reduced anesthetic flow rate during maintenance anesthesia. Active scavenging of WAG improved when a relief intake opening was provided in the induction chamber's vacuum line, vacuum draw after induction was regulated and the anesthetic flow rate and vacuum scavenging draw were balanced during maintenance anesthesia using a facemask that separated the breathing space from the scavenging zone. Additionally, time-weighted average isoflurane WAG levels detected by personal dosimeters correlated with real-time measurements made using infrared spectroscopy. These observations contribute to the development of a substantiated program for monitoring WAG air quality.