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Table of Contents
Year : 2022  |  Volume : 8  |  Issue : 1  |  Page : 9-13

Horizontal unidirectional airflow for reducing cross-infection of COVID-19: A narrative review

Department of Community and Family Medicine, All India Institute of Medical Sciences, Raipur, Chhattisgarh, India

Date of Submission08-Oct-2021
Date of Decision12-May-2022
Date of Acceptance13-May-2022
Date of Web Publication30-Jun-2022

Correspondence Address:
Hunny Sharma
Department of Community and Family Medicine, Gate No-5, Medical College Building, Great Eastern Rd., Opposite Gurudwara, All India Institute of Medical Sciences Campus, Tatibandh, Raipur - 492 099, Chhattisgarh
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ijcfm.ijcfm_81_21

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Recent decades have witnessed the emergence of many airborne diseases such as severe acute respiratory syndrome, Middle East respiratory syndrome, and COVID-19, which have highlighted the importance of effective ventilation in residential, work, or hospital premises. Ventilation which plays an essential role in reducing or diluting the airborne contaminants. However, it is not always easy to achieve by natural ventilation as it depends on many other factors such as temperature and climatic conditions. (wind velocity, wind direction, and housing pattern/design). Horizontal unidirectional airflow (HUAF) is one such method that can be achieved at low cost and can reduce cross-infection of COVID-19 to much extent. Hence, this narrative review aims to bring some insight into what is HUAF, how it can be achieved, and what are its possible implications in preventing COVID-19 transmission.

Keywords: Airborne diseases, coronavirus infections, COVID-19, cross-infection, ventilation

How to cite this article:
Sharma H, Ruikar M. Horizontal unidirectional airflow for reducing cross-infection of COVID-19: A narrative review. Indian J Community Fam Med 2022;8:9-13

How to cite this URL:
Sharma H, Ruikar M. Horizontal unidirectional airflow for reducing cross-infection of COVID-19: A narrative review. Indian J Community Fam Med [serial online] 2022 [cited 2023 May 28];8:9-13. Available from: https://www.ijcfm.org/text.asp?2022/8/1/9/349390

  Introduction Top

COCID-19 has affected many countries around the globe with ongoing emergence of new variants. The disease which was thought to spread because of droplets or fomites generated by infected persons is now being considered airborne.[1],[2]

With the current development in research and recent in Lancet, evidence is strong enough for aerosols as a major mode of COVID transmission, mostly occurring in indoor premises.[3] Several preprints have also demonstrated the presence of RNA of severe acute respiratory syndrome-CoV-2 in the air of various indoor settings through the air-sampling technique. This evidence has raised concern among several doctors, scientists, and policymakers regarding preventing the spread of this disease.[4],[5],[6],[7]

There are various methods to keep indoor air clean and protecting the occupants from indoor pathogens, i.e., dilution, filtration, ultraviolet germicidal irradiation, photocatalytic oxidation, desiccant rotor, plasma cluster ions, essential oils, and silver nanoparticles. These methods are effective in cleaning indoor air, but each has its advantage and disadvantage with major concerns regarding their affordability in many scenarios.[8]

Keeping indoor environment virus-free plays a key part in reducing or slowing the transmission of various airborne infections. Ventilation which is a key factor in keeping the indoor environment virus-free may not be easier to achieve in all settings but still is currently of prime importance. There are different types of ventilation such as natural, mechanical, or hybrid ventilation. Natural ventilation depends on factors such as temperature and climatic conditions such as wind velocity, wind direction, and housing pattern/design which makes it difficult to achieve in many scenarios and settings, thus giving rise to the need for mechanical ventilation.[9],[10],[11]

The horizontal unidirectional airflow (HUAF) is one such method of mechanical ventilation that can be utilized in where natural ventilation is not possible. Thus, this narrative review aims to bring some insight into what is HUAF, how it can be achieved, and what are its possible implications in preventing COVID-19 transmission.

  Airborne Transmission Top

Airborne transmission in terms of COVID-19 is the spread of virus particles caused by the circulation of respiratory droplets or nuclei (aerosols) that remain infectious and suspended in the air over long distances and time.[12],[13]

This type of transmission generally occurs in indoor places or poorly ventilated spaces such as lifts, closed rooms, and work premises. The virus particles are spread by the infected person, and travel from one person to other with the help of turbulence or air vortex created by ceiling fans and indoor exhaust with closed windows.[14],[15]

  Factors Affecting Viral Load in Residential or Work Premises Top

According to a report published in Lancet by Greenhalgh et al. in 2021, the viral load and risk of infection for COVID-19 increase drastically when the one who is infected exhales, speaks, shouts, sings, sneezes, or coughs [Figure 1].[3],[16]
Figure 1: Factors with viral load in residential or work premises depends

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According to a featured news topic on COVID-19 published in May 2020 by the Center for Infectious Disease Research and Policy, the viral load in any public building can be reduced by effective utilization of engineering techniques such as proper ventilation, prevent overcrowding, regular air filtration and disinfection of air [Figure 1].[17]

  Criteria of Effective Ventilation Top

According to the Occupational Safety and Health Administration guidelines of the United States, proper ventilation improves and maintains the quality of air in the occupational work environment. In a simpler term, it is a method by which we can control proper airflow in environment.[18]

The ventilation of any premises can be determined based on the three criteria, namely ventilation rate, airflow direction, and airflow distribution or pattern.[19]

“Ventilation rate” depends on quality and quantity of the air being delivered to the premises, whereas “airflow direction” deals with the direction of air being delivered which should ideally be from a fresh zone to a contaminated zone. “Air distribution or pattern” deals with how effectively the air is being delivered, and how effectively it eliminates the contaminants from the contaminated zone. A balance between these three creates effective ventilation.[19]

  Role of Effective Ventilation in Cross-Infection Top

Effective ventilation is believed to decrease the risk of cross-infection (i.e., any infection which a patient contracts in a health-care institution) of any airborne viral or bacterial disease, by either removing the contaminants completely or by diluting the suspended airborne contaminates, or infected droplet nuclei to an extent where it fails to reach a threshold level to initiate disease. A higher ventilation rate can dilute the contaminated air inside the space more rapidly and decrease the risk of cross-infection. A higher rate and more organized ventilation reduced the risk of cross-infection to a large extent which can be well explained by Wells–Riley equation.[20]

According to Wells–Riley equation: P=C/S=1−exp (−Iqpt/Q)[20],[21]

where P = Risk of cross-infection

C = Number of cases to develop infection

S = Number of the susceptible

I = Number of infectors

p = Pulmonary ventilation rate of each susceptible (m3/h)

Q = Room airflow rate (m3/h)

q = Quanta produced by one infector (quanta/h)

t = Duration of exposure (h).

  Role of Horizontal Unidirectional Airflow in Effective Ventilation Top

Unidirectional airflow in residential or work premises ensures that the airflow is only in one particular direction. In unidirectional airflow, the air in the room moves in parallel streamlines with uniform velocity over its cross-section in the entire room. This parallel stream of air prevents turbulence in the air in the room. Air enters from one side and leaves the room from the other side, thus reducing the chance of viral particles jumping around and landing in areas where they are not supposed to land. Thus, HUAF can control contaminant transport among different places.

The entire unidirectional airflow method is based on the concept that air should make a single pass through the room, removing as much contaminated air as possible. When this concept is applied in regular residential or work premises, air enters through one wall and exits through the opposite wall. It can be referred to as HUAF.

  Achieving Horizontal Unidirectional Airflow Top

An effective HUAF between fresh and contaminated zones can be achieved by pressure difference. This pressure difference can be achieved by utilizing exhaust fans of different volumes and velocities in opposing walls, such as using low-volume and low-velocity exhaust fans to throw air in the inward direction, whereas high-volume and high-velocity fans remove air in the outward direction from opposing wall. This imbalance of airflow rate thus helps to create HUAF with the added advantage of preventing air turbulence.[20]

  Implementation of Horizontal Unidirectional Airflow in Real-Life Settings Top

While most hospital premises are constructed keeping in mind the effective ventilation to reduce air contamination, this may not be true for other residential premises, work premises, retail shops, schools, offices, restaurants, or movie theaters.

In circumstances of unaffordability for costly equipment and its maintenance to filter indoor air, HUAF may prove to be useful in reducing cross-contamination. During current COVID-19 pandemic, HUAF can also be utilized in newly converted exhibition centers and stadiums as COVID care centers and residential premises.

  Limitations of Horizontal Unidirectional Airflow Top

HUAF in real-life settings has some limitations, such as it can only be implemented in settings with enough room for single row sitting arrangement for the employees, students, or other individuals. This method can also increase the power consumption but not to an extent of professionally designed ventilation systems.

  Recommendations Top

  1. HUAF can be achieved using different volume and velocity exhaust fans in opposing walls depending on the type and volume of rooms [Figure 2].
  2. While achieving HUAF, ceiling fans or other sources of air should be avoided as they may create air turbulence and disruption of HUAF, thus spreading contaminants [Figure 3].
  3. A single row of individuals should sit or work in a room preferable at 90° to prevent cross-infection. Any addition of rows will expose one or the other to the contaminant zone in HUAF [Figure 2].
  4. The windows or any other opening for natural air ventilation in adjoining walls should be closed to prevent disturbance in HUAF.
  5. The outward exhaust should be placed such that to face open space or in direction of a nearby building with 12-feet distance.
Figure 2: Concept of HUAF in residential or work premises. HUAF: Horizontal unidirectional airflow

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Figure 3: Air vortex circulating virus particles from an infected person to a healthy person

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  Conclusion Top

It should be realized that natural ventilation or engineered ventilation system cannot be practiced in all settings due to different designs of buildings or premises, and the affordability of the residents or individuals. Simpler methods of ventilation should be developed considering the laws of physics and microbiology. HUAF, although not full proof, can reduce the spread of COVID-19 in indoor premises to much extent at a relatively low cost and with ease of implementation. Further research should be carried out to test such cost-effective models in real-life scenarios.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

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  [Figure 1], [Figure 2], [Figure 3]


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