Author: Wang Yihan
It’s been three years since we first heard this draining word: COVID-19. The dominant role of indoor transmission [1] in the spread of this seemingly endless pandemic, is meant to undoubtedly bring the topic of prevention and control of indoor transmission of infectious diseases back into the public spotlight. Probably especially in hospitals, which played a significant role in amplifying the spread [2]. Please picture the hospital scene in your mind: the limited physical distance between patients and healthcare workers, the complex mobility of people imposed by visitors, they all lead to the risk of nosocomial transmission [3]. The vital role and connectivity of hospitals in society, and the particular susceptibility of their occupants make any possibility of nosocomial transmission highly worrying.
Significance of nosocomial infections
Indeed, nosocomial infections have significantly contributed to the overall transmission of various notorious respiratory infectious diseases these years. It was estimated that up to 20% of infections of SARS-CoV-2 (the responsible virus of COVID-19) in inpatients and 73% in healthcare workers (HCWs) may be due to nosocomial transmission in the UK [4]. A huge fraction of Middle East respiratory syndrome (MERS) cases was linked to the healthcare setting, ranging from 43.5% for the nosocomial outbreak in Jeddah, Saudi Arabia, in 2014 to 100% for both the outbreak in AL-Hasa, Saudi Arabia, in 2013 and that in South Korea in 2015 [2]. A well-known nosocomial outbreak of severe acute respiratory syndrome (SARS) that originated from ward 8A at the Prince of Wales Hospital in Hong Kong eventually infected 69 HCWs, 16 medical students, 53 patients, and a number of visitors [5]. Other cases include chickenpox, seasonal flu, Methicillin-resistant Staphylococcus Aureus (MRSA), and many more.
Infectious respiratory disease transmission route
How are these annoying, and sometimes deadly diseases transmitted within hospitals? Well, just as other indoor environments, multiple transmission routes can lead to nosocomial infections. As for infectious respiratory diseases, they can be classified into three major routes based on the difference in exposure mechanisms: droplet spray routes, short- or long-range airborne routes, and fomite routes [6][7] as illustrated in the following figure [7]. Various sizes of pathogen-carrying droplets can be released by respiratory activities (e.g., talking, coughing, sneezing, normal breathing, etc), ranging from submicron to millimeters [8]. After that, they disperse and deposit differently mediated dominantly by the size difference, with fine droplets being able to remain suspended in indoor environments for a sufficient period for long-distance spread [9]. Droplet spray transmission happens when medium or large droplets are deposited directly on the mucous membranes of the susceptible host, whereas airborne transmission is caused by direct inhalation of pathogen-carrying small droplets or aerosols (< 10 μm) [7]. Fomite transmission happens when infectious agents are left on inanimate objects like doorknobs, light switches, mobile phones, and so on, and new hosts come in contact with bodily secretions or receptor areas of the body.
Compared to other transmission routes, airborne transmission is relatively poorly understood and less acknowledged. For instance, in early 2020 when COVID-19 reached a pandemic status, the World Health Organization (WHO) initially dismissed the role of airborne transmission and focused on transmission through contaminated surfaces, i.e. fomite transmission [10]. That had a reason indeed. In fact, fomite transmission is also considered responsible for the majority of hospital-acquired infections (HAI) for diseases in general [11], and thus, the hospital attaches great importance to it. Doctors and other healthcare workers are required to thoroughly disinfect skin and equipment, wash hands frequently, and sanitize surfaces regularly [12]. Furthermore, large droplets can be easily visualized by high-speed cameras and were also well-acknowledged for their role in the transmission of COVID-19 in early 2020. Therefore, the maintenance of a safe distance and hand sanitation were considered the main precautions against the infection, while high-grade protection equipment (e.g., N95 masks) was only used in so-called aerosol-generating health-care procedures [13]. However, it proved wrong. Fine droplets have been proven as important carriers of influenza viruses [14], and, recently, of SARS-CoV-2 [15], and there is now overwhelming evidence that indoor airborne transmission associated with relatively small, micron-scale aerosol droplets plays a dominant role in the spread of COVID-19 [16].
Ventilation, the savior of airborne transmission infection?
As the importance of indoor airborne transmission becomes clearer, corresponding targeted protective measures are also highlighted. Ventilation is considered as a key factor influencing the transmission of airborne diseases. First of all, increasing the ventilation rate is believed to reduce the cross-infection of airborne transmitted diseases by removing or diluting pathogen-laden aerosols. For instance, Jiang et al. found that larger ventilation windows were correlated with lower infection risk based on the 2003 SARS outbreak in Guangdong [17]. Due to the inherent tension between ventilation and energy expenses, guidelines are recommending the lowest required ventilation rate for hospitals trying to make the balance between reducing the cross-infection risk and saving energy. The recommended minimum ventilation rate for airborne infection isolation rooms is 12 air changes per hour (ACH) in most guidelines (CDC, AIA, ASHRAE, etc.), which originated from 6 ACH in Center for Disease Control and Prevention (CDC) guidelines and then doubled after the 2003 SARS. However, according to Li et al.’s systemic review [18], these recommendations may lack strong scientific evidence for infection control.
The impact of ventilation rate on the cross infection of airborne transmitted disease can be described by the well-known Wells-Riley equation [19], and it successfully predicted a measles outbreak in a suburban school in the USA [19]. This equation and its improvements have been widely used to predict outbreaks of airborne infections and indicate that ventilation rate can reduce the infection risk significantly [20]. However, this great equation is based on a well-mixed space assumption, and that is far from what we expect from an airborne transmitted disease isolation room. Flow direction and airflow pattern also greatly impact the effect of ventilation, and for ventilation systems designed with no respect to these, a higher ventilation rate might be rather a disaster.
Flow direction can control contaminants transport among wards with different functions and the control is achieved through the pressure difference. The protective environment (PE) isolation room utilizes a positive pressure difference to resist incoming contaminated air, whereas the airborne transmission disease isolation rooms utilize a negative pressure difference to prevent the droplet nuclei generated by infected patients from spreading outside [20]. However, it is worth noting that the design goal is not always guaranteed to be met, and the intended negative pressure could often be broken mainly due to inadequate reliability of pressure monitoring and controlling devices, strong diffuser flow directed at the door, interaction with other exhaust ventilation systems and poor airtightness of the suspended false ceiling [21].
Airflow pattern also plays an important role in airborne transmission control. Three kinds of ventilation systems are widely used, i.e., mixing ventilation, downward ventilation, and displacement ventilation, and the respective airflow pattern is shown in the following picture. The above-mentioned well-known SARS nosocomial outbreak place, 8A wards in Prince of Wales Hospital in Hong Kong, utilized mixing ventilation, which aimed to make temperature and pollutants as well distribute uniformly. The predicted spatial risk distribution based on CFD airflow pattern analysis agreed well with the spatial infection distribution of actual SARS cases [22]. Downward ventilation and personalized ventilation are currently under active research to be improved for airborne infection control within hospital wards [20].
(A) Downward ventilation (B) Displacement ventilation (C) Mixing ventilation
One further point to mention about the mechanical ventilation systems in hospitals is that they need to be well maintained to achieve their infection-preventing goals. Clogged filters, leaking or even contaminated ducts may lead to the accumulation of the infectious agents they were designed to remove, and end up as a source of nosocomial airborne infection [11].
So, before the next major outbreak of infectious disease – which none of us want to happen, but which has to be prevented before it happens – hospital ventilation systems and WHO ventilation guidebooks may need to be systematically studied, examined and updated.
Reference
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