In case of a fire or the release of an airborne toxin by a terrorist attack in a subway station or tunnel, information on the dispersion of the hazardous substance is important for the rescue operation because it helps to decide which parts of the subway system can be expected to be contaminated. The goal of the ongoing OrGaMIR project is to provide the operating company and rescue forces with this information. In order to predict the dispersion, airflow and thermal conditions inside selected stations in different subway systems are monitored. A number of SF6 tracer gas experiments were conducted to investigate the spatial and time dependent contamination of the underground air by smoke or toxins. The paper describes the main results of three tracer gas experiments in different subway systems which are characterized by different building structures and ventilation influences.