In the event of a terrorist attack or an accidental release of hazardous substances in a subway system, an understanding of the dispersal of a toxic agent across the system with and without train movements is key information to aid rescue and escape procedures. This information is only available with knowledge of the airflow conditions inside the system. Apart from the train movements or active ventilation, the natural airflow regime determines which parts of the system will be contaminated. Tests with the tracer gas sulfur hexafluoride (SF6) and climatic studies have clearly confirmed the expected complex airflow system inside subway systems. The assumption that climatic and topographic factors of track routing as well as architectural characteristics have an influence on the air flow regime have been verified. The results show that in many instances designated escape routes are not safe. The emergency response could be improved and lives could be saved by implementing dynamic escape routing systems based on knowledge of the natural air flow.