SARS-CoV-2 is an Airborne Virus?

I thought I would say something more about airborne viruses. They have been a hobby of mine for some time.  The first time I heard about how easily the current coronavirus was spreading I thought “this is an airborne virus”. But then there were the usual protests. Some scientists came out and said that coronavirus requires “droplet” precautions rather than “airborne” precautions. The common definition is that droplets are large enough settle out quickly. They may touch you directly if you are close enough but a lot of what droplets do is settle on surfaces. That leads to surface contamination and spread of the virus.  Airborne viruses can travel longer distances within smaller droplets. An even more unique characteristic is that airborne viruses are released with normal expiration. No coughing, sneezing, or shouting is necessary. Just normal breathing can release airborne respiratory viruses into the air in the smallest droplet fraction.

An easy way to conceptualize airborne viral spread is to look at the distribution of particles occurring with every breath. They can range in size from 200 µm on the large end to less than 5 µm on the lower end. Experiments can be done to show how aerosols of these particle sizes spread through a room. Naturalistic experiments can also look at how these are all spread through hospitals, airplanes, and other natural settings. By looking at those patterns and the distance involved as well as any other paths that could lead to transmission – a determination can be made about whether or not a viruses “airborne” or not.

The second issue is whether or not live infectious virus is spread by these droplets. In many cases that has been limitation in the experiments. Direct evidence of viral spread at a distance requires culture of the virus or a bioassay that shows the samples have a cytopathic (killing) effect on live cells. Indirect evidence is typically by PCR where the available nucleic acid is amplified and assayed.  In researching this topic, I found early literature that documented numerous respiratory viruses in ventilation systems of buildings. That research was possible because nobody knew how to capture and culture the virus samples until that time. The other technical issue is air sampling. Certain air sampling devices do not allow for adequate virus detection. There is a technical report recently that showed of a total of four commonly used air sampling devices only one allowed virus to be identified or cultured. These technical constraints have led to a lot of confusion about whether a particular virus is airborne or not.

A practical definition of an airborne virus is one that is exhaled during normal breathing can travel a distance in small droplets (less than 5 µm) and create infection by landing on mucosal surfaces, facial services, or the lining of the nasopharynx, oropharynx, or lungs.  In that case you don’t have to be in the presence of anyone coughing or sneezing. You don’t have to touch any contaminated surfaces. You don’t have to touch your face. You just have to breathe contaminated air. It is entirely possible that you can walk through a cloud of airborne virus and not be aware of how it got there. A common example is walking down the hallway after a crowd of people have walked through some of them carrying the virus. Once you know about airborne viruses it gives you an entirely different perspective on social distancing.  Airborne particles can easily transverse the longest dimension of typical rooms. The distance of 6 feet suggested in typical social distancing guidelines is not nearly far enough.

In a previous post I highlighted an experiment by Donald K. Milton where he designed a machine to sample flu viruses in a natural setting. Research subjects breathed normally and their expired air was sampled for influenza virus. Influenza virus was recovered from 89% of the nasopharyngeal swabs and 39% of the fine aerosol sample defined as droplets less than or equal to 5 µm and greater than 0.05 µm. There was no coughing or sneezing during the collection.. That is proof that influenza virus is airborne.

A recent experiment carried out at the University of Nebraska Medical Center (3) in their bio containment and quarantine units showed that in addition to abundant surface contamination by SARS-CoV-2 the virus was also present in hallway air samples, in areas where only aerosol deposits could reach, and personal air samplers carried by staff.  The virus was detected by PCR analysis as well as culture in some cases. The authors concluded that this was definite evidence of an airborne virus and that facilities treating patients with SARS-CoV-2 should use airborne precautions. This paper is currently undergoing peer review and is on a preprint server.

The technical aspects of aerosol fractionation and dispersion is a subject best addressed in the engineering literature. The best paper I could find on the subject (4) had a goal of studying the dispersion characteristics of exhaled droplets in a ventilated room. The engineering goals are to find out what physical parameters affect the dispersion rate and hopefully lead to better indoor environments. The experimental paradigm looked at a room that was 5 x 4 x 3 m (L x W x H) and ventilated by a downward airflow from the ceiling. Aerosols were injected into this room and the following distance and trajectory along with trajectory time was calculated for the fractional droplets. As expected droplets with larger diameters (110-115 µm) fell at a faster rate and traveled a much shorter distance than the smaller diameter fraction. Droplet dispersion was studied under a number of physical conditions including different airflow, different temperatures, different relative humidity, and the original exhaled initial velocity.

The 0.1, 1.0, and 10 µm droplets covered horizontal distance of 5 m or the entire length of the room. Droplets in the range of 50 to 200 µm cover distances ranging from 0.5 to 4 m.  This study has numerous excellent graphs about varying conditions and how droplets in different fractions are dispersed. The reader is referred to the original reference to review all those conditions. The bottom line for me was it is clear that airborne droplets in the 0.1 to 10 µm range are commonly seen in exhaled air and easily covered distance in this experiment of 5 m or (for Americans) 16.4 feet. That is well beyond social distancing recommendations it explains why this virus continues to spread. Within the past week I have seen clips of New Yorkers in congested subway cars.  Any asymptomatic SARS-CoV-2 carriers in those cars are expelling virus into the air just by tidal volume breathing.  Coughing and sneezing is not required. It would be very good if you covered your cough or sneeze but when your mouth is not covered and you are a symptomatic or asymptomatic carrier you are exhaling viral particles.

The relevance of this virus being airborne cannot be underestimated. It is a substantial part of the reason behind social distancing and stay-at-home orders. It is the reason for the new recommendation to wear masks in public and the various explanations being given for those masks. The absolute best approach is to not have any close social contact until this pandemic is over. I am very concerned about my colleagues and fellow healthcare workers who have inadequate personal protective equipment (PPE). The expectation is that they will continue to take care of the very ill patients with COVID-19. Every effort must be made to make sure they have adequate protection. 

In my particular specialty, all psychiatrists should have access to electronic interviewing at this point. As I pointed out in my earlier posts, I am used to talking people for 30 to 60 minutes at time. Some of those evaluations are 90 minutes in duration. During that time the patient may be coughing or sneezing but for the most part they are breathing and engaged in normal interview conversation. My office is smaller than the room described in the droplet dispersion experiment. By the end of one interview, the smallest fraction of exhaled droplets is dispersed throughout the room. Any viral particles in that fraction can lead to infection.

All health care workers in these environments need immediate protection by the implementation of airborne precautions. I am hopeful that as more people become aware of the airborne route of transmission that it will lead to more caution on the part of the public and the social distancing and staying at home and away from this kind of transmission will make more sense. In the future we need to design indoor environments that can minimize this type of transmission. There is too little innovation in the role of the physical environment in airborne transmission of infectious illness.

I hope the added cost will be looked at in terms of the total cost of illnesses caused by respiratory viruses every year with the recognition that they can still cause devastating pandemics.

George Dawson, MD, DFAPA


References:


1: Lei H, Li Y, Xiao S, Lin CH, Norris SL, Wei D, Hu Z, Ji S. Routes of transmission of influenza A H1N1, SARS CoV, and norovirus in air cabin: Comparative analyses. Indoor Air. 2018 May;28(3):394-403. doi: 10.1111/ina.12445. Epub 2018 Jan 6. PubMed PMID: 29244221. 

2: Yu IT, Li Y, Wong TW, Tam W, Chan AT, Lee JH, Leung DY, Ho T. Evidence of airborne transmission of the severe acute respiratory syndrome virus. N Engl J Med. 2004 Apr 22;350(17):1731-9. PubMed PMID: 15102999.

3: Santarpia et al. (2020) Transmission potential of SARS-CoV-2 in viral shedding observed at the University of Nebraska Medical Center. Retrieved from https://www.medrxiv.org/content/10.1101/2020.03.23.20039446v2.

4: Chen C, Zhao B. Some questions on dispersion of human exhaled droplets in ventilation room: answers from numerical investigation. Indoor Air. 2010 Apr;20(2):95-111. doi: 10.1111/j.1600-0668.2009.00626.x. Epub 2009 Sep 24. PubMed PMID: 20002792.

5: Li, Y., Huang, X., Yu, I.T.S., Wong, T.W. and Qian, H. (2005), Role of air distribution in SARS transmission during the largest nosocomial outbreak in Hong Kong. Indoor Air, 15: 83-95. doi:10.1111/j.1600-0668.2004.00317.x

6: Carl Heneghan, Jon Brassey, Tom Jefferson. SARS-CoV-2 viral load and the severity of COVID-19.  March 26, 2020.  Link

7:  National Research Council 2020. Rapid Expert Consultation on the Possibility of Bioaerosol Spread of SARS-CoV-2 for the COVID-19 Pandemic (April 1, 2020). Washington, DC: The National Academies Press. https://doi.org/10.17226/25769.

"While the current SARS-CoV-2 specific research is limited, the results of available studies are consistent with aerosolization of virus from normal breathing."

8: Pan M, Bonny TS, Loeb J, Jiang X, Lednicky JA, Eiguren-Fernandez A, Hering S, Fan ZH, Wu CY. Collection of Viable Aerosolized Influenza Virus and Other Respiratory Viruses in a Student Health Care Center through Water-Based Condensation Growth. mSphere. 2017 Oct 11;2(5). pii: e00251-17. doi: 10.1128/mSphere.00251-17. eCollection 2017 Sep-Oct. PubMed PMID: 29034325.

9: 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 Jul [date cited]. https://doi.org/10.3201/eid2607.200764

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