Thatiparti Deepthi Sharan, Ghia Urmila, Mead Kenneth R
Department of Mechanical Engineering, University of Cincinnati, 2851 Woodside Dr., Cincinnati, OH 45221, USA.
Centers for Disease Control and Prevention (CDC), National Institute for Occupational Safety and Health (NIOSH), Division of Applied Research and Technology (DART), Cincinnati, OH, USA.
Sci Technol Built Environ. 2016;23(2):355-366. doi: 10.1080/23744731.2016.1222212. Epub 2016 Sep 19.
When infectious epidemics occur, they can be perpetuated within health care settings, potentially resulting in severe health care workforce absenteeism, morbidity, mortality, and economic losses. The ventilation system configuration of an airborne infection isolation room is one factor that can play a role in protecting health care workers from infectious patient bioaerosols. Though commonly associated with airborne infectious diseases, the airborne infection isolation room design can also impact other transmission routes such as short-range airborne as well as fomite and contact transmission routes that are impacted by contagion concentration and recirculation. This article presents a computational fluid dynamics study on the influence of the ventilation configuration on the possible flow path of bioaerosol dispersal behavior in a mock airborne infection isolation room. At first, a mock airborne infection isolation room was modeled that has the room geometry and layout, ventilation parameters, and pressurization corresponding to that of a traditional ceiling-mounted ventilation arrangement observed in existing hospitals. An alternate ventilation configuration was then modeled to retain the linear supply diffuser in the original mock airborne infection isolation room but interchanging the square supply and exhaust locations to place the exhaust closer to the patient source and allow clean air from supply vents to flow in clean-to-dirty flow paths, originating in uncontaminated parts of the room prior to entering the contaminated patient's air space. The modeled alternate airborne infection isolation room ventilation rate was 12 air changes per hour. Two human breathing models were used to simulate a source patient and a receiving health care worker. A patient cough cycle was introduced into the simulation, and the airborne infection dispersal was tracked in time using a multi-phase flow simulation approach. The results from the alternate configuration revealed that the cough aerosols were pulled by the exhaust vent without encountering the health care worker by 0.93 s after patient coughs and the particles were controlled as the aerosols' flow path was uninterrupted by an air particle streamline from patient to the ceiling exhaust venting out cough aerosols. However, not all the aerosols were vented out of the room. The remaining cough aerosols entered the health care worker's breathing zone by 0.98 s. This resulted in one of the critical stages in terms of the health care worker's exposure to airborne virus and presented the opportunity for the health care worker to suffer adverse health effects from the inhalation of cough aerosols. Within 2 s, the cough aerosols reentered and recirculated within the patient and health care worker's surroundings resulting in pockets of old contaminated air. By this time, coalescence losses decreased as the aerosol were no longer in very close proximity and their movement was primarily influenced by the airborne infection isolation room airflow patterns. In the patient and health care worker's area away from the supply, the fresh air supply failed to reach this part of the room to quickly dilute the cough aerosol concentration. The exhaust was also found to have minimal effect upon cough aerosol removal, except for those areas with high exhaust velocities, very close to the exhaust grill. Within 5-20 s after a patient's cough, the aerosols tended to break up to form smaller sized aerosols of less than one micron diameter. They remained airborne and entrained back into the supply air stream, spreading into the entire room. The suspended aerosols resulted in the floating time of more than 21 s in the room due to one cough cycle. The duration of airborne contagion in the room and its prolonged exposure to the health care worker is likely to happen due to successive coughing cycles. Hence, the evaluated alternate airborne infection isolation room is not effective in removing at least 38% particles exposed to health care worker within the first second of a patient's cough.
当传染病流行发生时，它们可能在医疗机构内持续存在，这有可能导致医护人员严重缺勤、发病、死亡以及经济损失。空气传播感染隔离病房的通风系统配置是保护医护人员免受感染患者生物气溶胶影响的一个因素。虽然空气传播感染隔离病房的设计通常与空气传播传染病相关，但它也会影响其他传播途径，如短程空气传播以及受传染物浓度和再循环影响的飞沫和接触传播途径。本文介绍了一项关于通风配置对模拟空气传播感染隔离病房中生物气溶胶扩散行为可能流动路径影响的计算流体动力学研究。首先，对一个模拟空气传播感染隔离病房进行建模，其房间几何形状和布局、通风参数以及加压情况与现有医院中观察到的传统天花板式通风布置相对应。然后对一种替代通风配置进行建模，在原始模拟空气传播感染隔离病房中保留线性送风扩散器，但互换方形送风口和排风口的位置，使排风口更靠近患者源头，并让来自送风通风口的清洁空气以从清洁到污染的流动路径流动，从房间未受污染的部分进入受污染患者的空气空间之前。模拟的替代空气传播感染隔离病房通风率为每小时12次换气。使用两个人体呼吸模型来模拟源患者和接收医护人员。将患者咳嗽周期引入模拟中，并使用多相流模拟方法及时跟踪空气传播感染的扩散情况。替代配置的结果显示，咳嗽气溶胶在患者咳嗽后0.93秒被排风口吸走，没有接触到医护人员，并且由于气溶胶的流动路径未被从患者到天花板排风口排出咳嗽气溶胶的空气粒子流线中断，粒子得到了控制。然而，并非所有气溶胶都被排出房间。其余咳嗽气溶胶在0.98秒进入医护人员的呼吸区域。这就医护人员接触空气传播病毒而言，是关键阶段之一，也让医护人员有机会因吸入咳嗽气溶胶而遭受不良健康影响。在2秒内，咳嗽气溶胶重新进入并在患者和医护人员周围再循环，导致出现旧的污染空气聚集区。此时，由于气溶胶不再非常靠近，聚并损失减少，其运动主要受空气传播感染隔离病房气流模式的影响。在远离送风的患者和医护人员区域，新鲜空气供应未能到达房间的这部分区域，无法迅速稀释咳嗽气溶胶浓度。还发现排风口对去除咳嗽气溶胶的效果极小，除了那些靠近排风格栅、排气速度高的区域。在患者咳嗽后5 - 20秒内，气溶胶倾向于破碎形成直径小于1微米的更小尺寸气溶胶。它们保持悬浮状态并被卷入送风气流中，扩散到整个房间。由于一次咳嗽周期，悬浮气溶胶在房间内导致的漂浮时间超过21秒。由于连续咳嗽周期，房间内空气传播传染的持续时间及其对医护人员的长时间暴露很可能发生。因此，所评估的替代空气传播感染隔离病房在患者咳嗽的第一秒内无法有效去除至少38%暴露给医护人员的粒子。
