It has been recommended that ventilation of health care facilities should be monitored regularly to reduce the risk of nosocomial transmission of tuberculosis. We developed a simple method to measure air-change rates and direction of airflow in patient care areas. Pure carbon dioxide (CO2) was released at 13.5 L/min for 5 min, then measured for 30 min within the room and outside in the hallway. Smoke tubes were also used to measure direction of airflow. Doors and windows (if openable) were manipulated. This protocol, when conducted in five offices in 30 patients care areas in two hospitals, provided good mixing and reproducible decay curves, with less than 15% coefficient of variation for repeated measures over a wide range of air-change rates. Manipulation of door and/or window produced significant changes in air-change rates and airflow direction, although calculated air-change rates were more variable. Smoke tube measurements were inconsistent, agreed poorly with evidence of CO2 movement from room to hall, and were strongly affected by room to hallway temperature differentials. CO2 release and measurement proved to be a simple, yet reliable, method to measure air-change rates and the effect of door or window manipulation. Smoke tube measurements were not reliable to characterize direction of airflow.