The transmission of coronaviruses, such as the severe acute respiratory syndrome coronavirus (SARS-CoV), the Middle East respiratory syndrome coronavirus (MERS), and SARS-CoV-2, occurs through exposure to aerosol particles formed when an infected person coughs or sneezes. Currently, the rapid spread of SARS-CoV-2 across the world has persisted in the current coronavirus disease 2019 (COVID-19) pandemic for almost two years.
Study: Using portable air purifiers to reduce airborne transmission of infectious respiratory viruses - a computational fluid dynamics study. Image Credit: VectorManZone / Shutterstock.com
*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
How far can aerosols travel?
It is imperative to understand every aspect of viral transmission, including how far and fast aerosols can travel in indoor settings. Such information would help develop effective mitigation strategies to control the further spread of the disease.
Typically, the larger the size of a droplet, the lesser distance it would travel. Generally, in indoor settings, large droplets travel a distance of fewer than two meters. In contrast, small-sized droplets, also known as fine aerosols, could remain airborne for a longer duration and travel a longer distance.
Fine aerosols play a vital role in the transmission of airborne viral infection. In the current pandemic, scientists have stated that airborne mode of transmission played a vital role in the worldwide transmission of SARS-CoV-2.
Among the preventive measures encouraged by governments, one has been the use of air disinfectants. Aerosol dispersion strongly follows convective air flows; therefore, a proper ventilation system with high air exchange rates and the introduction of flow-directing geometries could stop the further spread of the disease.
However, redesigning entire heating, ventilation, and air conditioning (HVAC) systems could be extremely expensive. As a result, scientists and policymakers have encouraged the use of portable air purifiers.
Can air purifiers prevent the spread of airborne viral infection?
Air purifiers comprising high-efficiency particulate absorbing (HEPA) filters can filter more than 99% of particles that are larger than 200 nanometers (nm). This technology is cost-effective, portable, capable of filtering out many viral particles and is easy to operate as a result of its user-friendly control interfaces. Although many documents are available that support the fact that air purifiers can disinfect polluted air, scant evidence is available to show its effective usage.
Two of the essential questions regarding the use of air purifiers to experience the best results are include the best position to place an air purifier in a room and whether it necessary to use maximal flow rates in the purifiers for spaces with or without an HVAC system. The solution to these questions could enhance the function of air purifiers.
Previous studies have shown that higher flow rates can filter out more particles; however, a stronger outflow jet can hamper the airflow and may cause unwanted mixing of air. This system also creates more noise.
Keeping these aspects in mind, a new computational study published on the preprint server medRxiv* focuses on the performance of portable air purifiers based on Computational Fluid Dynamics (CFD) simulations.
CFD simulations and air purifiers
Many studies have applied CFD to assess aerosol-generating activities and ventilation in hospitals. The new study has used the geometry and setup of a consulting room as the starting point of simulations. This is because, in the present situation, the clearance time between consultations plays a determining role that affects the patient turnover rate.
In this study, the CFD modeling simulation showed a decrease in virus transmission after the implementation of portable air purifiers. The researchers not only characterized the effectiveness of the air purifiers but also provided a practical guide to experience maximum results.
Here, the researchers used a fixed number of aerosol (normalized mass) for the initial injection for all simulations. Various situations were considered, such as higher production of aerosol particles by the doctors, as they would be talking more, the position of the air conditioner, and ventilation, to name a few. As the breathing velocities of human models typically follow sinusoidal curves, the researchers observed distinct fluctuations in the local spherical region.
The simulation study showed large air purifiers placed on the floor could effectively reduce the relative aerosol numbers at a steady state in the entire room. More specifically, air purifiers could reduce the presence of aerosols on the floor by 77 %, 60% at 0.5 meters above the floor, and 68% at 1 meter above the floor. Smaller air purifiers were also found to be effective in reducing relative numbers of aerosol by 50% when placed on the doctor’s desk.
The application of two air purifiers, a large one in the slightly far-field empty space and a small one closer to the sitting area, further reduced the number of aerosols by 62%. Scientists found that the inlet of suction must be lifted above the floor to obtain a higher reduction of 40% in steady-state aerosol concentration.
These results show that portable air purifiers could be used as a mitigation strategy to prevent further transmission of respiratory viruses. Although the use of multiple air purifiers could be more effective, their relative positions play a vital role to obtain the best result.
Conclusions
In the current study, the researchers have used CFD modeling to study the efficiency of portable air purifiers in reducing the prevalence of fine aerosols. The current study showed that air purifiers can effectively reduce aerosols in hospital settings; however, their positions are important for obtaining the best result.
This study provides practical guidance that can be easily implemented to prevent the airborne transmission of respiratory infections in hospitals.
*Important notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
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