Airborne disease transmission
Airborne disease transmission is a transmission mode where the disease is spread via droplets which are the result of sneezing, coughing or talking. While most of the droplets released tend to naturally drop on the floor, many of those particles lose a portion of their water content via evaporation and reach sizes of up to 5 microns and take the form of aerosol . As the smaller particles travel indoor they can disperse in different areas depending on the local air flow conditions. Controlling the air patterns inside a building and designing partitions between units can significantly reduce the airborne pathogen dispersion .
Airborne transmission is only one of the possible ways an infection can spread and typically happens indoor. It is possible to control it by tuning the design parameters of the ventilation systems inside the buildings. Computational Fluid Dynamics (CFD) simulations of airborne disease transmission are an effective tool for assessing and optimising a ventilation system.
Airborne transmission is also one of the ways the Covid-19 disease can spread, many recent studies and publications have studied the matter, see the Covid-19 airborne transmission section for reference.
To minimise the risk of airborne disease transmission many building design guidelines require a minimum number of ACH (Air Changes per Hours) [3-4] and prescribe the level of pressure to be maintained in some of the environments to force the air to flow in desired directions within the building.
Simulation of airborne disease transmission
CFD is a flow simulation tool capable of reliably predicting the internal flows within enclosed spaces and therefore is one of the main methods to assess the effect of different designs of the air conditioning system on the local flow inside a building. The main goal of different design choices is to reduce the local airborne disease pathogens concentration and therefore minimise the likelihood of an infection.
The results of a CFD simulation of airborne disease transmission carried out using the CFD software SimWorks are shown in the image above. The results show the different pathogen concentration levels in the room following the release of pathogen from a single patient lying on a bed. High concentration levels are shown in red while low concentrations are in blue inside a room. High pathogen concentration can be found above the patient’s bad and spread around the room along the side walls. The areas around the inlets on the celing, where clean air is injected show a very low level of pathogen.
A CFD simulation also takes into account the natural air movement due to the temperature differences within an enclosed environment. Specific simulations can take into account the air density variation with temperature and are capable of simulating the natural air convection resulting from differences in temperature and therefore density, this effect is called buoyancy.
With a CFD simulation it is therefore possible to predict in advance the main flow patterns in a room and this can help optimising the ventilation system design even before the building is completed and achieve specific targets of safety and user comfort.
It is also possible to simulate the effect of the airborne disease pathogens dispersed by an individual in an enclosed environment and assess how those can travel to the different areas within the building. In particular, it is assumed that an infected person is dispersing a defined amount of pathogens per minute and the pathogen concentration varies with the local flow characteristics within the building.
Assessment of the infection risks and Wells–Riley Equation
During the act of sneezing and coughing, an infected person releases droplets of different sizes that can directly infect nearby people. As reported by Wells in 1955  the main vehicle for airborne disease transmission are the droplet nuclei, which are what is left of the original droplets after they dried. Those particles are extremely small (below 5 microns in size) and can contain infectious pathogens. Due to their size, those particles can freely circulate within an enclosed environment. While the bigger droplets can be avoided by maintaining a minimum distance between people, the droplet nuclei particles should be controlled by means of designing a ventilation system capable of isolating the area where the particles are generated from the rest of the environments. One of the methods of assessing the infection probability relating to the local droplet nuclei concentration is to use the Well-Riley equation :
PI: Airborne infection probability of a person
C: Number of infection cases
S: Number of people in the environment considered
I: Number of infectors
p: Breathing rate of the person
q: Quantum generation rate by an infector
Q: Ventilation rate
The Wells-Riley equation is showing a direct correlation between the pathogen concentration and the probability of an infection, which reduces with the increase in the ventilation rate. The main goal of a ventilation system is to protect the building occupiers from pathogen dispersion by isolating the rooms where the pathogen can be generated (see article about the negative pressure room) and guaranteeing adequate fresh air changes in all the other areas of the building.
The simulation results shown in the image above, which are described in a SimWorks advanced tutorial, show the effect of increasing the ventilation rate from the case on the left to that to the right. On the latter, the simulation results show that areas where the pathogen concentration is high (red surfaces) are case confined close to the ceiling and the walls with most of the room showing little pathogen concentration compared to the case on the left where high concentrations of pathogens are present large areas of the room.
Design of a ventilation system: preliminary requirements
The starting parameter in sizing a building ventilation system is the calculation of the total air flow required by the project design requirements. The objective are to achieve a number Air Changes per Hour (ACH see  for reference) and a required pressure differences between different rooms. The required value of ACH changes according to the specific application and has a direct impact on the infection probability, so it will be higher for areas of the building where the pathogen concentration can be high like isolation rooms or intensive care areas.
Multiplying the internal air volume (in cubic meters m3) for the number of ACH required gives an estimate of the m3 of air required per hour. It is therefore possible to calculate directly the volume flow rate in m3 per minute required by the ventilation system. In real working conditions, the positions and sizes of the outlets and the exit doors affect the actual internal flow characteristics. For instance, there can be areas where the flow is recirculating and therefore the minimum ACH requirements are not met even if on average the ventilation system has been sized to achieve the design objective.
In many cases, in addition to the minimum ACH requirements, the project requires areas of the building to be kept at constant pressure (see the article about positive and negative pressure rooms). In this case, it is necessary to take into account the real working conditions of the building, like the size and position of the doors, air inlets and outlets. Therefore the best way to design a suitable ventilation system is to run a complete CFD simulation of the entire building and take into account the real working condition of the ventilation system itself. The results will help understand the flow patterns and suggest design changes, which in turn can be quickly assessed with further simulations until the optimum design is achieved.
Design of a ventilation system: suggestions
The first step in designing the ventilation system is to estimate the number of inlets required to guarantee the required average ACH. This will depend on their size and flow rate capacity of each inlet and the sum of all the inlet mass flow rates should match the required ACH. As explained previously, the actual mass flow rate needed could be higher than the nominal ACH value in order to compensate for local recirculation and stagnation zones.
The following step in the simulation will be to define the outlet positions. In this case the best practice is to introduce in the simulation a constant pressure outlet that simulates the effect of a natural flow exit, while the room is maintained at a slight over-pressure. This allows the simulation to calculate the distribution of mass flow at the different outlets and give an important feedback to the designer.
The position of inlets and outlets is as important as the mass flow rate of each component to achieve the desired design objectives and minimise the risk of infection. Outlets should be placed next to the generation of infectious pathogens, usually at the floor level of the room where the pathogens are generated, meanwhile inlets should be placed at an higher position, providing a continuous flow of clean air to the patients. Air inlets can also be used to isolate one room from the others by creating an “air curtain”. This is obtained placing an inlet directly above the entry door of a protected environment so that, when the door is opened, the external flow entering into the room is reduced and is diluted with fresh air. This is the same working principle of air conditioning systems used in HVAC to isolate refrigerated areas from an hotter external environment.
The diffusion of SARS-CoV-2 which leads to Covid-19 infection has been object of studies like the one in  and is broadly thought to follow similar principles to the cases presented above. Many studies show that one of the ways the Covid-19 infection is spreading is the airborne transmission where the dried droplets released by an infected person when sneezing, coughing or even when talking (which was the main objective of the study carried out in ). As mentioned at the beginning of this article the saliva droplets tend to lose a portion of their water for evaporation but not the viral content into them. As they become lighter because of the loss of water content they tend to become more persistent in the air and can travel with the airflow for long distances. The WHO has recently acknowledged that airborne transmission is one of the possible ways of transmission of the new virus and certainly is one of the transmission modes which has to be monitored.
Therefore, to guarantee the safety of an enclosed environments where contagion can take place, the room ventilation system should be to be designed with the principles presented previously. Clearly the ventilation system can only compound a series of additional safety measures to further reduce the likelihood of infection. We refer to the WHO website for further information.
Covid-19 superpreading events and impact of air ventilation
The Covid-19 transmission occurs sometimes in one of the so called superspreading events where a large number of individuals is infected in a single specific instance, the specific medical background of such events is beyond the scope of this article, but most of the times such events happen indoor and involve airborne disease transmission mechanism.
In particular a recent case study of a SARS-CoV-2 spreading inside a Japanese restaurant has been studied in  and in . In that case the arrangement of air conditioning units was creating a prevalent flow from one side to the other of the room, as one person was infected with Covid-19 a superspreading event took place where several people from the surrounding tables where infected. The infected people were all in the same recirculation area as the original infected person while other people next to the infected person were not affected. A CFD investigation of the event showed that the air conditioning unit on the wall was creating a recirculation region which correlated with the infection events.
This study is a good example of how critical the ventilation system design is in controlling the infection spreading. Introducing in the room external or filtered air in combination with the avoidance of recirculation regions can be a very effective way of minimising the chance of an airborne disease transmission event. This is supported by the evidence that people sitting next to the infected person but outside of the recirculation region next to the person were not affected.
Also it is important to note that as more studies about SARS-CoV-2 virus will become available we will know more about that particular virus.
The results and the description provided above where defined to describe physical behaviours under certain circumstances. They should not be considered a medical guidance and do not account for environmental variants such as humidity or wind.
1 – Sung, M. (2015) – HVAC system and contamination control in hospital relating MERS. – Mag. Soc. Air-Cond. Refrig.
2 – Chang Heon Cheong ,Seonhye Lee (2018) – Case Study of Airborne Pathogen Dispersion Patterns in Emergency Departments with Different Ventilation and Partition Conditions – International Journal of Environmental Research and Public Health
3 – American Institute of Architects (AIA) – 1996–1997 Guidelines for Design and Construction of Hospitals and Health Care Facilities – The American Institute of Architect Press: Washington, DC, USA, 1996.
5 – Wells WF (1955) – Airborne contagion and air hygiene – Cambridge Harvard University Press
6 – Valentyn Stadnytskyi, Christina E. Bax, Adriaan Bax, and Philip Anfinrud (2020) – The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission – Edited by Axel T. Brunger, Stanford University, Stanford, CA
 – Lu J, Gu J, Li K, Xu C, Su W, Lai Z, et al. COVID-19 Outbreak Associated with Air Conditioning in Restaurant, Guangzhou, China, 2020. Emerg Infect Dis. 2020;26(7):1628-1631.
 – Yuguo Li, Ph.D.; Hua Qian, Ph.D.; Jian Hang, Ph.D.; Xuguang Chen, M.Sc.; Ling Hong, Ph.D.; Peng Liang, M.Sc.; Jiansen Li, M.Sc.; Shenglan Xiao, Ph.D.; Jianjian Wei, Ph.D.; Li Liu, Ph.D.; and Min Kang, M.Sc – Evidence for probable aerosol transmission of SARS-CoV-2 in a poorly ventilated restaurant
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