Closed circuit anesthesia must be considered as a primary contributor enabling to reduce the damaging effect of anesthetic agents on the environment. Halogenated vapors and nitrous oxide damage both the macro-environment, i.e. the earth's atmosphere layer and the micro-environment, i.e. the operating room ambience. As nitrous oxide is one of the most toxic agents, and as pure oxygen is harmful to the lungs, a technique of closed circuit anesthesia has been developed which employs oxygen/air (0.35:0.35 l/min) as a carrier of isoflurane. The safety of this oxygen-air technique of closed circuit anesthesia has been demonstrated by the greater stability of oxygen concentrations in the circuit in comparison to the more extensively used oxygen-nitrous oxide technique.