Residence Time Distribution Analysis of Patient-Emitted Air in a Mock Hospital Isolation Room: A CFD Study

Air changes per hour (ACH) is the primary metric specifying ventilation in hospital isolation rooms, yet presumes the room behaves as a perfectly mixed reactor. However, short-circuiting pathways and recirculation (dead) zones make the removal of patient-discharged air strongly dependent on airflow topology. This study applies residence time distribution (RTD) analysis to quantify that dependence. Computational fluid dynamics (CFD) simulations of a mock hospital isolation room were performed at 6 and 12 ACH in two exhaust configurations: a normal design (NMD), with the exhaust displaced from the patient, and a short-circuiting design (SCD), preserving a direct patient-to-exhaust pathway that acts as source capture. A passive tracer pulse released at the patient mouth was tracked to the exhaust, giving the breakthrough time, the 10–90% removal spread, and the mean residence time, each normalized by the theoretical mean residence time. At 6 ACH both configurations stayed close to the perfectly mixed prediction. At 12 ACH they diverged sharply: SCD reduced normalized breakthrough time from 0.311 to 0.0283, narrowed normalized spread from 2.29 to 1.64, shortened mean residence time from 6.23 to 2.64 min, and cut long-term healthcare worker exposure from 1.27 to 0.42 times the perfectly mixed value. Doubling ACH reduced that exposure by 75% under SCD but only 39% under NMD, and shortened early exposure only where the direct pathway was preserved. ACH specifies how much air is supplied, not where it goes; RTD metrics capture that difference and are obtainable from CFD or physical tracer testing.