How is COVID-19 Transmitted? What is the Right Mask for COVID-19? A Physician-Scientist’s Perspective.

August 2, 2020

Harsha Moole, M.D., MBBS

1 Comment

How is COVID-19 Transmitted? What is the Right Mask for COVID-19? A Physician-Scientist’s Perspective.

Disclaimer: This is a fact check blog post from a purely scientific perspective. Research studies have flaws, and we have to take that into account while using them to create healthcare policies or suggestions.

Learn about the accuracy of COVID-19 test on a separate blog post: How Accurate is COVID-19 Test? COVID-19 Test Sensitivity is Unclear.

Learn about KN95 vs. N95 Masks on a separate blog post: KN95 vs. N95 Masks – It Is Time to Use Common Sense



What PPE Should I Use In Various Scenarios?


I am a health care worker (HCW) in a hospital/clinic directly taking care of COVID-19 patients or Patients Under Investigation (PUI): 

  • Inside the hospital/clinic (hallways / offices / cafeterias / workstations, etc.), HCWs should wear an N-95 mask, head covers, and eye protection gear.
  • While going into the patient’s room, it is appropriate wear a full body suit with an N95 and eye protection gear / PAPR is appropriate. Scrubs should be changed at the end of each shift.

I am a HCW in a hospital/clinic who is not actively taking care of COVID-19 patients or PUI:

  • Inside the hospital/clinic (hallways / offices / cafeterias / workstations, etc.), HCWs should wear a N-95 mask, head cover, and eye protection gear.

I am an asymptomatic person in the community:

  • In the ongoing pandemic, everyone should wear a surgical mask for source control (to prevent the infected wearer from breathing out the virus particles into the open).
  • Many COVID-19 patients are not symptomatic. Hence, the masks can reduce the spread. If surgical masks are not available, yes, one can use a cloth mask / bandana. Cloth masks / bandanas are only 1/3rd as effective as surgical masks.
  • Bear in mind that homemade masks should only be considered as a last resort, but it would be better than no protection.
  • Common perception: Wearing a mask / bandana will prevent the virus from infecting/getting to the wearer – This is false.
  • Reality: Wearing a surgical mask/bandana will prevent the spread of the virus into open air if the wearer has the infection. It is primarily used for source control.

I am a Patient under investigation (PUI) / COVID-19 positive patient in the hospital:

  • This population should wear an N-95 mask if possible (or a surgical mask if unable to tolerate an N95 mask). Airborne precautions should be followed. Negative air flow rooms, or properly ventilated rooms should be used to hold these patients / PUIs.

I am a Patient under investigation (PUI) / COVID-19 positive patient in the community:

  • This population should wear an N-95 mask if possible (or a surgical mask if unable to tolerate an N95 mask).
  • Self-isolation until the infection is resolved (duration of this is unclear currently as more data is coming from South Korea regarding concerns for re-activation of dormant infection vs re-infection)

What is the scientific rationale of the above-mentioned suggestions? Let’s get into the meat of it!

How is COVID-19 Transmitted? 



  • COVID-19 has a diameter of about 0.06 to 0.140 microns as seen on an electron microscope1. The virus particle is almost always transmitted by piggybacking on respiratory secretions (of infected patients) that are in the range of <0.6 to 1,000micron size2.
  • The virus is transmitted directly via respiratory secretions of an infected patient, and indirectly via contact of a contaminated surface on which these infected secretions land.
  • There is a lot of debate whether the respiratory secretions of COVID-19 are transmitted via aerosols, and if so, by how much? The answer is – Yes, it definitely aerosolizes but we do not know much. Here is why:

Old facts – Dichotomous concept

  • Back in the day, there were a few studies from 1940s and 1950s that defined an arbitrary concept that a respiratory secretions with particle size of >5 microns are called a droplet, and <5 microns is aerosol3.
  • Larger droplets may travel only a few feet to up to 26 feet if propelled by a sneeze—before falling to the ground or onto another surface, such as someone’s skin or clothes3.
  • Some old studies suggest that majority of the large droplets gravitate and settle down within 6 feet, based on which public health policies (i.e 6ft physical distancing) are conceptualized. This could not be anymore irrelevant in the current COVID-19 situation in the health care settings.
  • There is in-conclusive evidence that the COVID-19 laden droplets emitted from an infected patient travel only 6ft and not farther. We are unsure if the virus laden particles travel for 6ft or 26ft or if they can travel further. On the other hand, aerosols are smaller size, thought to be suspended in air, flow with air currents, and do not gravitate downwards. Aerosols do not have a limit on how far they can travel. Therefore, aerosols have a much higher risk of spreading to farther distances for longer periods of time.

New facts – Non-binary concept

  • Recently, there are newer studies which postulate that the direct transmission of the virus is more of a high-momentum cloud carrying the particles long distances (turbulent puff cloud dynamic model)3,4.
  • These studies show that when a person coughs, speaks, sneezes, or even breathes – the respiratory secretions (virus laden) are emitted in all sizes5. Some are large, while others are small.
  • The large droplets fall quickly to the ground, while the small droplets can dehydrate and linger as “droplet nuclei” in the air, where they behave like an aerosol and thereby remain suspended in the air and can travel far in the air currents6.
  • In regard to COVID-19 transmission, there are no cut-offs of 26ft travel distance, or 5micron respiratory secretion particle size, rather it is more like a spectrum.
  • The newer studies are guiding us to change our understanding of the previous dichotomous/binary concepts. There is no hard – fast rule that says the respiratory secretions from a COVID-19 positive patient gravitates only within 6ft or 26ft.
  • Yes, high pressure procedures like nebulization, intubation etc. increase the chance of aerosolization (smaller particle size). However, newer research data suggests that just the act of breathing and talking can also produce adequate numbers of aerosols that are <0.6micron size7.
  • Inhaled droplets and aerosol particles have different sites of deposition in the recipient. Inhaled droplets are deposited in the upper regions of the respiratory tract, from which they may be removed by natural body defenses. In contrast, inhaled aerosolized particles can escape these defenses and penetrate to the depths of the lungs, where they may be deposited in the alveoli8

How far can the virus travel?

  • In early 2020, several COVID-19 patients were treated in the University of Nebraska Medical Center. The virus was found in the air inside the patient rooms, and outside the patient rooms in the hallways. This is a strikingly important finding noted in their report9.
  • COVID-19 was found in the ventilation systems of patient rooms in hospitals in China that were treating COVID-19 patients.
  • The previous research that the virus laden droplets settle down on surface within 6ft, and that virus laden aerosols are produced only with high pressure procedures (intubation, nebulization etc.) does not hold true anymore.
  • We can now confidently say that the virus laden respiratory secretions are capable of travelling more than 6ft and are aerosolized even with routine breathing and talking.
  • If you are a healthcare worker working in the ICU and not taking care of COVID-19 patients, but have COVID-19 patients in the ICU, it means that you are being exposed to COVID-19 in the hallway air, and you are breathing it in.

How long can virus laden aerosols stay in the air?

  • Although the larger respiratory secretions gravitate and settle down immediately after they are expelled, smaller particles do not. This is due to their lighter weight, ability to stay suspended in air, form light mist, and their ability travel with air currents.
  • There is a common perception that the virus laden aerosols can stay suspended in the air for 1-3 hours. A recent experimental study debunked this perception – experimentally produced COVID-19 aerosols were present at significant numbers even at 3hours mark when the experiment ended. This obviously means that COVID-19 is viable and infectious in the air for much longer than 3 hours. We just do not how long because this research study ended at 3hr mark10.


  • COVID-19 can be transmitted via small respiratory secretions much more than previous estimates, in addition to large respiratory secretions.
  • COVID-19 can be aerosolized when an infected patient breaths or talks, it can travel with the air currents for much longer than 6ft, and it can stay active in air much longer than 3 hours.
  • This might partially explain why this virus is highly infective compared to other viruses.

How is COVID-19 Transmitted? Debate on COVID-19 Droplet vs. Airborne Precautions:

  • Based on the above-mentioned information, it is a no-brainer that COVID-19 should be approached with air-borne precautions in health care settings.
  • This should be the case until further convincing evidence surfaces, proving that the virus does not aerosolize as much as the current studies suggest that it does.
  • Scientists and clinicians from reputed institutions opine that definitive research on airborne transmission of COVID-19 may take years and waiting that long to be convinced will only increase the spread of infection11.
  • Logic says: When in doubt, use the safer approach. You can always downgrade at a later date, but you cannot undo the harm this is already done.

World Health Organization’s (WHO) Stance on COVID-19 Transmission:

  • As of mid-April 2020, WHO is recommending droplet and contact precautions for COVID-19 patient care in health care settings12.
  • They are primarily basing this decision on a single study that came out from China which states that there was no airborne transmission of the virus. One may question the validity of the single study coming from China. Instead we probably require independent studies from multiple nations to validate these claims.
  • World Health Organization (WHO) 2014 statement13 recommended that any novel acute respiratory infection (ARI) should be dealt with in airborne precautions until strong evidence is available regarding the viral transmission of the virus.
  • Based on the limited research that is currently available, the COVID-19 transmission seems to aerosolize more than previously anticipated. Hence, it is not appropriate to consider that COVID-19 transmission is similar to the well-known Influenza virus or previous coronaviruses.

How is COVID-19 Transmitted?

Source: World Health Organization (WHO) Guidelines – Infection prevention and control of epidemic- and pandemic-prone acute respiratory infections in health care 
  • WHO has determined that COVID-19 can be managed with droplet precautions with literally no definitive in-vivo studies to support their claim.
  • WHO mentioned in their recent statement that the new studies that support airborne transmission of COVID-19 are either in vitro (experiments performed in a laboratory as opposed to real life data) or have flaws.
  • A recent in vivo University of Nebraska Medical Center (UNMC) report suggests that COVID-19 transmission is likely airborne.
  • We do agree that we need large RCTs which can give us a better idea. However, until then, COVID-19 should be treated as a novel ARI (as recommended by WHO in 2014), and follow protocols for a novel ARI, rather than classifying it as a well-known entity similar to prior influenzas/coronaviruses.
  • It is all the more important to do so due to the high mortality, and high level of infectivity with COVID-19.
  • During the SARS outbreak in 2002-2003, aerosols drove severe bouts of viral spread in health care settings14.
  • We don’t know enough about COVID-19 transmission. Until then airborne precautions are needed in healthcare settings where COVID-19 patients are being managed.

COVID-19 and Masks:


Why is it important to talk about masks?

  • A research article from 2011 summarizing high-quality studies found that among all physical interventions used against respiratory viruses—including handwashing, gloves, and social distancing—masks performed best, although a combination of strategies was still optimal2.


Types of masks:


Regular surgical masks:

  • These masks are three-ply (three layers). This three-ply material is made up of a melt-blown polymer, most commonly polypropylene, placed between non-woven fabric. They are not designed to protect the wearer from inhaling airborne bacteria or virus particles and are less effective than respirators, such as N95 or FFP masks, which provide better protection due to their material, shape and tight seal15.
  • There is a very common and dangerous misconception that surgical masks can prevent the non-infected wearer from getting infected. This is incorrect.
  • With regards to respiratory viral illnesses like COVID-19, these surgical masks are primarily used for ‘source control’ – they are intended to prevent the infected wearer from spreading the disease – masks stop the infected virus particles from going out into the open.
  • A high-quality surgical mask made per US regulations (there are many masks that do not meet the US certification standards), can filter 99% filtration of particles <0.1micron size. However, it is the poor fitting and air leak that make these masks inefficient.
  • However, these masks are not effective in preventing a non-infected wearer from getting infected. When worn appropriately, surgical masks have air-leaks all around it. (80-100% of the times)16.
  • COVID-19 laden respiratory secretions have a high likelihood to leak through your surgical mask, and you will inhale them.
  • Trying to prevent the non-infected surgical-mask wearer from getting infected with COVID-19 is like – trying to get across a turbulent ocean with a commercial manufactured kayak. Is it better than nothing? Yes, it is. Is it going to fail you? Probably, and most likely.

Cloth masks / Homemade masks:

  • As mentioned above, these are intended for source control, but they are about 1/3rd as effective as regular surgical masks in doing so (preventing an infected wearer from spreading the virus17.
  • These masks are obviously much less effective than surgical masks in preventing a non-infected wearer from contracting the infection (i.e. air leaks, and the ability for more particles to pass through the cloth).
  • Trying to prevent the non-infected cloth mask wearer from getting infected with COVID-19 is like – trying to get across a turbulent ocean with a home-made wooden kayak. Is it better than nothing? Yes, it is. Is it going to fail you? Probably, most likely, and may be definitely.

N95 and equivalent masks (KN95, FFP2, etc.):

  • These are often called respirators. They are made with superior materials than can filter out >95% of the particles that are >0.3micron. These masks usually fit well with minimal air leaks, which make them extremely effective.
  • Non infected health care workers (HCWs) wear these masks to prevent inhaling the COVID-19 aerosols / small particles in the air that are released from COVID-19 patients.
  • Fit testing performed by institutions ensures that the wearers have minimal air leak. Currently in the US, we do not have fit tested N95 equivalent masks readily available. The N95 equivalent masks are made by different brands, in different shapes, with a non-reliable availability and supply chain.
  • It is impossible to fit test every healthcare worker with every mask type. In this current dire situation that we are experiencing, one should think about the next best alternative to the fit tested N95 equivalent, which is a non-fit tested N95 equivalent mask. Healthcare workers should be able to trial the available models and use the N95 equivalent mask that fits them the best.   
  • A randomized clinical trial (RCT) of 1,441 health care workers in 15 Beijing hospitals was performed during the 2008/2009 winter. Non-fit-tested N95 respirators were significantly more protective than medical masks against clinical respiratory illness. Rates of infection in the medical mask group were double that in the N95 group18.

How is COVID-19 Transmitted?

Source19: Centers for Disease Control and Prevention (CDC) – Understanding the Difference

Closing Thoughts:

  • The current state of COVID-19 in USA has created a wide range of “scientific” opinions based on in-adequately powered research studies, and research studies taken out of context. These opinions have become fodder for several political and national entities to spin the conclusions of these research studies in a way that fits their personal agenda. When political and national entities start to consider these ‘opinions’ as meaningful data, they wrongly influence the public health care policies that effect millions of people throughout the country.
  • Although there is a lot of ‘action’ in response to COVID-19 pandemic, many key topics are being mis-represented, and this is affecting millions of people. It has been a truly frustrating last few months for Physician-Scientists like me.
  • The scientist part of me understands the research and scientific facts; the clinician in me working with COVID-19 patients is noticing a massive discord in what is needed to be done versus what is currently being done.
  • We know with a fair level of confidence that science is presenting us certain definitive facts, however the political and regulatory authorities have been spinning the facts, and altering them. This is in turn resulting in inadequate public health care policy.
  • Overall, we do not know enough about this beast. Follow the science and focus on the facts. As a scientist, I am trained to do that. Yes, there may be several “guidelines” from your local / national governing bodies, but their recommendations may have agendas that might not be in your best interest.
  • When you sit back, cut out the noise, and listen to the facts – you will know the right thing to do.
  • Overall, we need further thorough research studies that are bias-free, that are adequately powered, specific to COVID-19, and performed in vivo, as randomized trials.

Learn about the accuracy of COVID-19 test on a separate blog post: How Accurate is COVID-19 Test? COVID-19 Test Sensitivity is Unclear.

Learn about KN95 vs. N95 Masks on a separate blog post: KN95 vs. N95 Masks – It Is Time to Use Common Sense

1. The New England Journal of Medicine (NEJM) – A Novel Coronavirus from Patients with Pneumonia in China, 2019
2. Wired – It’s Time to Face Facts, America: Masks Work
3. JAMA Network – Potential Implications for Reducing Transmission of COVID-19
4. The New England Journal of Medicine (NEJM) – Visualizing Speech-Generated Oral Fluid Droplets with Laser Light Scattering
5. Chao CYH, Wan MP, Morawska L, et al. Characterization of expiration air jets and droplet size distributions immediately at the mouth opening. J Aerosol Sci 2009;40:122-133
6. Marr LC, Tang JW, Van Mullekom J, Lakdawala SS. Mechanistic insights into the effect of humidity on airborne influenza virus survival, transmission and incidence. J R Soc Interface 2019;16(150)
7. The size distribution of droplets in the exhaled breath of healthy human subjects. Papineni RS, Rosenthal FS. J Aerosol Med. 1997 Summer; 10(2):105-16
8. The New England Journal of Medicine (NEJM) – Droplets and Aerosols in the Transmission of SARS-CoV-2
9. MedRXiv – Transmission Potential of SARS-CoV-2 in Viral Shedding Observed at the University of Nebraska Medical Center
10. The New England Journal of Medicine (NEJM) – Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1
11. The Scientist – The COVID-19 Coronavirus May Travel in Aerosols
12. World Health Organization (WHO) – Modes of transmission of virus causing COVID-19: implications for IPC precaution recommendations
13. World Health Organization (WHO) Guidelines – Infection prevention and control of epidemic- and pandemic-prone acute respiratory infections in health care
14. Live Science – New coronavirus may spread as an airborne aerosol, like SARS
15. Wikipedia – Surgical mask
16. Oberg, T., and L. M. Brosseau. 2008. Surgical mask filter and fit performance. American Journal of Infection Control 36, (4) (May): 276-82
17. Cambridge University Press – Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?
18. MacIntyre  CR, Wang  Q, Cauchemez  S,  et al.  A cluster randomized clinical trial comparing fit-tested and non-fit-tested N95 respirators to medical masks to prevent respiratory virus infection in health care workers.  Influenza Other Respir Viruses. 2011;5(3):170-179
19. Centers for Disease Control and Prevention (CDC) – Understanding the Difference

How is COVID-19 Transmitted?

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Authored by Harsha Moole, M.D., MBBS

Hey there! I hope you enjoyed reading this blog. PhysicianEstate is my brain child and passion project. I run this platform to empower entrepreneurially motivated physicians to make financially educated investment decisions and discuss asset protection strategies. Lots of important but free content here and here! If you have any questions or if you are interested in partnering with me, let’s connect!

1 Comment


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William papke

“When you sit back, cut out the noise, and listen to the facts – you will know the right thing to do. ” I wish it was that easy. I know very few people who have more than a fuzzy idea what their meds or have even read the insert. I do have one question that I have been unable to pin down and may be a good topic for your blog, restoration of electrostatic charge to a N95/99 or equivalent.

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