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Is it by droplets, aerosols, & airborne, or particulates: concerns of COVID-19 pandemic?

Featured Article , A_IR 2021 , Issue 1 , Article 2

We are writing this article for public awareness based on peer-reviewed articles and our ongoing WHO-cited research on managing the COVID-19 pandemic using advanced & high-performance nano-systems, sensors, and therapeutics. The motivation is based on the present unprecedented situation where most countries worldwide are facing severe health and economic issues due to pandemic. Besides, generating awareness is also an essential aspect as careful practices can avoid severe problems. Considering this, we are presenting an element of clean air to understand and manage viral infections.  Part of this article is also associated with the role of Ajeet Kaushik as an Editor-in-Chief of AIR (Air Innovation Research) magazine.

For centuries, viral infections are affecting humankind at different times and levels of severity. There have been instances when these diseases have turned into pandemics, endemics, and/or both, and as a result, we have changed several practices to have a better and healthy tomorrow [1]. These practices involved managing the viral infection, educating humans and governments, and paying particular attention to intelligent healthcare [1-3]. We have also tried to control viral infections via understanding the nature of pathogenesis and investigating therapeutics of higher efficacy. Fortunately, these situations were manageable as the pandemics were associated with animals (H1N1) or insects (Ebola, Zika, etc.). Their transmission was controlled by following approved guidelines issued by the health agencies [1, 2]. 

For more than a year, we are struck by a new viral infection known as COVID-19 infection. The disease is caused by a new type of human Coronavirus, namely severe acute respiratory virus-2 (SARS-CoV-2). The COVID-19 pandemic has created an unprecedented situation due to easy human-to-human transmission and high mortality rate [1-5]. The rapid and severe infection (especially in the elderly population) causes serious respiratory disorder and affects vital organs such as the heart, kidney, eye, brain, and, in some cases, death [4].

In the 21^st century, with tremendous medical resources and capabilities, the concern is how this virus can be so severe! Why is this infection not under control even after every possible high-level effort made by top health agencies like WHO, CDC, NIH, etc.? The probable answer could be the nature of virus transmission, which is human-to-human. In the present global business scenario and travel opportunities, how human-to-human virus transmission can be controlled? This is a very critical question that requires some effort! 

Health agencies suggested approaches, such as mask, self-health monitoring, and social distancing, have achieved some success levels [1-5]. Recent developments in the direction of selective early-stage diagnostics and therapies of higher efficacy are under investigation [3,6,7]. Most of such technologies have received emergency use authorization from the FDA, and outcomes of these diagnostics and therapeutic systems are remarkably significant. However, SARS-CoV-2 transmission is still not entirely under control, and therefore, COVID-19 disease is still far from reaching a manageable condition [1-5]. 

The intervention of the governments alone can’t solve this problem, nor can the scientists have a magic wand. The entire humankind needs to be aware of the mode of virus transmission (means how viruses jump from one person to another) and adopt simple practices that will prevent the spread of the virus [4]. The National Institute for Occupational Safety & Health (NIOSH) has raised the demand for generating awareness about the mode of SARS-CoV-2 transmission to manage COVID-19 infection [8, 9]. 

It is imperative to understand the transmission mode of the virus to design and implement multiple prevention strategies (e.g., masking, ventilation, air purification, social distancing, etc.) to control the spread [10]. Scientists have explored various modes of SARS-CoV-2 transmissions, such as respiratory droplets, aerosols, particulates (PM_2.5), or airborne [10-14].  Whether SARS-CoV-2 transmission occurs through droplets, aerosols, PM, or airborne, or a combination of all, in every case, we need to know about the steps to be taken to stop human-to-human transmission and be safe. 

1. Viral transmission through droplets. Initially, droplets were investigated as a primary cause of COVID-19 infection and hence the strategy of social distancing. Droplets are micro-sized (> 10 µm) drops of respiratory fluids, mainly saliva. These droplets are large enough to carry respiratory pathogens such as SARS-CoV-2 [2, 5]. It has been found that the SARS-CoV-2 can easily survive in droplets for several hours in open space and on surfaces. These droplets can move three feet in distance and settle out of the air onto the surfaces within minutes.  Large droplets have a higher probability of carrying a virion than smaller droplets. Large droplets can convert into tiny droplets by evaporation and remain suspended for an extended period. Viruses can be transferred by touching the mucous membrane in the mouth, nose, and eyes with hands soiled with the virus by touching the contaminated surfaces (fomite transmission). Droplets of good enough size (10-100 µm) can carry many virus particles and be responsible for severe COVID-19 disease. Thus, it is crucial to take necessary precautions to stop droplet-based SARS-CoV-2 transmission. Fortunately, droplet-based transmission, due to large droplet size, can be controlled and avoided using an ordinary mask (not necessarily N-95) and sanitizing the contaminated surface using disinfectants. The coating of surfaces using anti-viral or anti-bacterial materials (Cu, Ag, TiO₂, etc.) is a good approach to eradicate SARS-CoV-2 effectively. 
2. Aerosolized viral transmission. The COVID-19 infection via aerosol is one of the critical and most widely investigated modes of transmission. These virus-containing aerosol droplets of respiratory fluids are smaller in size (< 5 µm) and can travel in the air to 6 feet [2, 5]. Aerosols are generated during expiratory events such as coughing and sneezing. In general, these two modes can eject thousands of tiny drops containing millions of virus particles into the air, which could cause severe infection if inhaled by a person in the close vicinity of an infected person. Several environmental factors impact the aerosol-based transmission of the virus. Under certain conditions, aerosols can travel up to 30 feet in distance. In addition, environmental temperature and humidity can enhance the viability of the aerosolized virus and make them survive for a longer duration of time.  A regular mask may not be sufficient to trap all the virus-containing aerosol. Thus, a better-quality mask (N-95 and nano-enabled masks) is needed to prevent the aerosol-transmitted virus infection.  
3. Airborne viral transmission. The aerosol size is a mystery, and it has been calculated as 0.2 µm at genome level in hospitals. Such small particles can be suspended in the air for a long time, increasing the chances of inhalation, especially in closed premises [8, 10, 13]. This type of situation is the main reason to declare COVID-19 viral infection as airborne transmission. Recent reports are more focused on how this type of transmission can be controlled under different environmental conditions. Airflow in the premises can make virus particles travel a longer distance. Humidity and temperature may also support the survival of virus particles in the room. Therefore, it is necessary to actively remove these particles from the infected air to prevent people from getting infected with this deadly virus. Now the biggest challenge is how to clean the air or how to make air virus-free? A potential solution to managing airborne viruses could be utilizing an efficient air purifier to capture and neutralize the virus. 
4. PM_2.5/10.0 – Assisted viral transmission. Particulate assisted virus transmission is also a concern expressed recently by many virologists, which is not surprising [10-14]. As viruses containing aerosols remain suspended in the air, suspended aerosols may get adsorbed onto particulate matter (PM_1.0 PM_2.5 and PM_10.0) present in the air [14]. These particulates, namely, inorganic dust particles, organic particles, and biopolymers such as hair, pet dander, dead skin cells, etc., serve as viable platforms for viruses where they can stay longer. However, a more detailed investigation is needed to explore the possibilities and consequences of this mode of transmission. However, maintaining clean and virus-free indoor air will help in defeating the pandemic. As discussed in airborne virus transmission, an air purifier is helpful to trap these particulates and simultaneously neutralize the virus protein. 

In summary, aerodynamic plays an essential role in SARS-CoV-2 transmission. Therefore, precautions and safety aspects must be predicted, optimized, and recommended based on premises and environmental factors. Based on careful and critical analysis, the indoor SARS-CoV-2 transmission is severe and challenging to control. The mode of transmission and the impact of environmental factors alongside airflow are also crucial factors that need to be considered to fight against dreaded COVID-19 (as illustrated in Figure 1).

In such situations, an efficient air purifier is the most recommended approach to clean air by trapping and destroying Coronavirus.

Besides, sanitization of premises, using masks, and self-hygiene is always a good practice to follow every day to be safe and healthy.

Acknowledgment: Dr. Kaushik acknowledges the Academy of Scientist for Industrial Research and Education (ASIRE) for the opportunity and Florida Polytechnic University to provide support and facilities. 

References

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