Airborne Transmission of COVID-19: New Insights
May 7, 2021 | Article No. 51
Airborne Transmission of COVID-19: New Insights
May 7, 2021 | Article No. 79
Ayesha Siddiqua MSc, PhD
Mohit Bhandari MD, PhD
- Droplet and fomite transmission have arguably received the most attention for explaining the spread of SARS-CoV-2.
- There is lack of consensus on the definitions for “droplet,” “droplet nuclei”, “airborne,” and “aerosol”.
- There is growing evidence that airborne transmission is a significant contributor to the spread of SARS-CoV-2.
- The length of time a particle remains suspended in the air depends on a wide range of factors in addition to size – recent data shows large particles can remain suspended in the air for long periods of time and do not always fall to the ground within 2 meters from source of exhalation.
- Mask wearing and social distancing are still important for preventing the spread of SARS-CoV-2, but greater investment is needed to improve indoor air ventilation to reduce airborne transmission of this virus.
“As the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) pandemic rages on, so does the debate over what fraction of transmission occurs by aerosol exposure, as opposed to direct or indirect transmission by droplets and fomites.”
“This is an old debate that has been reignited by the appearance of yet another respiratory viral pandemic.”
Tang et al (2021) (1)
“A year into the pandemic, the evidence is now clear. The coronavirus SARS-CoV-2 is transmitted predominantly through the air — by people talking and breathing out large droplets and small particles called aerosols. Catching the virus from surfaces — although plausible — seems to be rare.”
Nature (2021) (2)
“The research (new review article published in the Lancet) adds to a growing chorus of experts saying the evidence for airborne transmission is “overwhelming” and the sooner global health authorities admit this, the sooner more effective measures to better protect the public can be implemented.”
Haridy (2021) (3)
With the progression of the COVID-19 pandemic, updates regarding several modes of transmission have emerged. It has been proposed that SARS-CoV-2 spreads through contact and droplet transmission, fomite transmission, as well as airborne transmission. Among these different modes of transmission, airborne transmission received the least amount of attention – largely due to lack of evidence during the early months of the pandemic as well as the predominant understanding of virus transmission for respiratory diseases. Until now, most public health measures have emphasized practices to limit droplet and fomite transmission of SARS-CoV-2, including wearing masks, maintaining social distancing of 2 meters, and routine sanitization of surfaces. While there is undeniable value in implementing all these practices, there is growing evidence suggesting that these practices alone will not be sufficient to limit the spread of SARS-CoV-2. Scientists have revisited the physical, epidemiological and virological principles of how respiratory aerosols are produced and spread, the process through which secondary cases of infection is identified and the challenges inherent in this process, as well as examined many instances where the droplet or fomite transmission hypotheses are simply not adequate to explain the spread of COVID-19. All these reconsiderations have renewed interest in the airborne transmission hypothesis, which is crucial for informing public health strategies as we progress through the pandemic in the months to come, and potentially prepare for the seasonal occurrence of COVID-19 in the future.
“The confusion (about the role and importance of aerosol transmission for SARS-CoV-2) has emanated from traditional terminology introduced during the last century. This created poorly defined divisions between “droplet,” “airborne,” and “droplet nuclei” transmission, leading to misunderstandings over the physical behaviour of these particles. Essentially, if you can inhale particles—regardless of their size or name—you are breathing in aerosols. Although this can happen at long range, it is more likely when close to someone, as the aerosols between two people are much more concentrated at short range, rather like being close to someone who is smoking.”
“People infected with SARS-CoV-2 produce many small respiratory particles laden with virus as they exhale. Some of these will be inhaled almost immediately by those within a typical conversational “short range” distance (<1 m), while the remainder disperse over longer distances to be inhaled by others further away (>2 m). Traditionalists will refer to the larger short range particles as droplets and the smaller long range particles as droplet nuclei, but they are all aerosols because they can be inhaled directly from the air.”
Tang et al (2021) (4)
“The flawed assumption that transmission through close proximity implies large respiratory droplets or fomites was historically used for decades to deny the airborne transmission of tuberculosis and measles. This became medical dogma, ignoring direct measurements of aerosols and droplets which reveal flaws such as the overwhelming number of aerosols produced in respiratory activities and the arbitrary boundary in particle size of 5 μm between aerosols and droplets, instead of the correct boundary of 100 μm.”
Grennhalgh et al (2021) (5)
Different Definitions, Different Conclusions
There are differences in understanding of the definition and appropriate application of terms such as droplets, droplet nuclei, aerosols, and particles, all of which are discussed in the context of the viral spread of respiratory diseases (Exhibit 1) (1).
|Term||Clinicians||Aerosol Scientists||General Public|
|Airborne||Capable of long-distance transmission
Requires N95/FFP2/FFP3 respirator or equivalent for controlling infection
|Anything that can remain suspended in the air||Anything in the air|
|Aerosol||Particle size <5 μm, which facilitates airborne transmission
These particles are produced during aerosol generating procedures
Requires N95 respirator
|Collection of solid and liquid particles of any size that can remain suspended in a gas||Produced from many personal care/cleaning products|
|Droplet||Particle size >5 μm, which falls to the ground rapidly within 1-2 meters from the source of exhalation
Requires a surgical mask
|Liquid particle||Substance excreted from droppers (e.g., eye dropper)|
|Droplet nuclei||Residue of a droplet which evaporates to size <5 μm
Used synonymously with aerosol
|Use the term “cloud condensation nuclei”
Refers to small particles where water condenses to form cloud droplets
|Unaware of this|
|Particle||Virion||Very small solid or liquid ‘blob’ in the air||Likened to soot or ash|
Current public health measures are largely based on a limited definition of the airborne nature of SARS-CoV-2. Under the dominant droplet transmission hypothesis, it is proposed that SARS-CoV-2 is transmitted through larger particles (droplets) which fall to the ground within a meter or so from the source of exhalation (4). However, this is challenged by recent findings that show long-range transmission of SARS-CoV-2 between people in adjacent rooms in quarantine hotels (5). Furthermore, particles as large as 100 µm have been found to remain suspended in the air for a long period of time (5). Taken together, the need for broadening our current understanding of SARS-CoV-2 and its transmission is clear, as the particle size cut off to determine its ability to remain suspended in the air appears arbitrary at best.
“Transmission of SARS-CoV-2 is higher indoors than outdoors and is substantially reduced by indoor ventilation. Both observations support a predominantly airborne route of transmission.”
“SARS-CoV-2 has been identified in air filters and building ducts in hospitals with COVID-19 patients; such locations could be reached only by aerosols.”
Greenhalgh et al (2021) (5)
“Respiratory droplets with a wide range of diameters can remain suspended in the air and be considered airborne. The sizes of exhaled particles cover a continuum. One cannot definitively specify a cut-off for the diameter of airborne particles because the ability of a particle to remain suspended depends on many factors other than size, including the momentum with which they are expelled, and characteristics of the surrounding air flow (speed, turbulence, direction, temperature and relative humidity).”
“Whether or not transmission occurs over longer ranges (beyond the social distancing range of 1–2 m) depends on several parameters. These include the quantity of airborne virions produced by the source; distribution of virions carried by different particle sizes; airflow patterns in the local environment; decay rate of virus infectivity; infectious dose needed to cause an infection in an individual; dilution of the inoculum at a distance; and timely removal by fresh air, ventilation or air cleaning.”
Tang et al (2021) (1)
Busting Virus Transmission Myths
Scientists have now taken a multidisciplinary approach to understand the dynamics of SARS-CoV-2 transmission from a more nuanced lens. Notably, they have identified several prominent myths and explained why these are not plausible by considering physical, epidemiological and virological principles, historical data, as well as current evidence (Exhibit 2). The key takeaway message from this analysis is that current preventative measures – i.e., mask wearing and social distancing – should not be abandoned in the context of airborne transmission of SARS-CoV-2. Rather, additional strategies must be implemented to ensure the unique risks that are imposed by the airborne transmission of the virus are effectively mitigated.
|Claim||Why claim is not plausible|
|Virus transmission is more likely to occur within short distances||Analyses of human behaviours, room sizes, ventilation, as well as other factors observed in diverse settings (e.g., concerts, cruise ships, care homes, correctional facilities) have shown long-range transmission and overdispersion of the basic reproduction number (R-naught) of the virus is consistent with airborne transmission of SARS-CoV-2.|
|Short range virus transmission can not be airborne||Findings from influenza studies show that airborne viruses of different sized particles are discharged by infected individuals over conversational distances (<1 meters).|
|Droplets are the primary mode of transmission||Notwithstanding the arbitrary particle size cut off distinguishing droplets from aerosols, the latter is responsible for a significant portion of virus transmission. Asymptomatic or presymptomatic individuals do not cough or sneeze, yet account for at least one third of all global transmissions. This is supported by airborne transmission as there is data showing speaking alone produces thousands of aerosol particles and only few large droplets. Furthermore, nosocomial infections have occurred in health care settings, where there were measures in place to protect against droplets but not aerosol exposure.|
|SARS-CoV-2 can not be airborne because it has a small R-naught (estimated to be 2.5)||R-naught does not indicate anything about mode of transmission – it simply indicates the average number of people that are infected after coming in contact with one single infected “index” case. It is also difficult to ascertain the number of secondary cases for influenza like diseases as a significant level of asymptomatic and pre-symptomatic transmission may occur, which makes contact tracing and follow up from single exposure events challenging.|
|If virus is airborne, surgical masks and cloth coverings will not work||Surgical masks can reduce exposure of incoming droplets and aerosols by an average of 6-fold (range 1.1- to 55-fold) (6,7). Cloth masks also reduce exposure of incoming particles up to 2-4-fold (i.e., approximately 50–75%) (8,9).|
“The way that evidence is being interpreted and applied differs between interested parties across the world. Baseline definitions of what constitutes sufficient evidence to support transmission by aerosols are many and varied. Without agreement, the debate will continue to drag on, confusing the issue, and placing more and more people at risk because the practical preventive interventions needed to control the virus are not adequately supported.”
Tang et al (2021) (1)
“Current evidence suggests that the virus spreads mainly between people who are in close contact with each other, typically within 1 metre (short-range). A person can be infected when aerosols or droplets containing the virus are inhaled or come directly into contact with the eyes, nose, or mouth. The virus can also spread in poorly ventilated and/or crowded indoor settings, where people tend to spend longer periods of time. This is because aerosols remain suspended in the air or travel farther than 1 metre (long-range).”
World Health Organization statement,
April 30, 2021 (10)
“If we accept that someone in an indoor environment can inhale enough virus to cause infection when more than 2 m away from the original source—even after the original source has left—then air replacement or air cleaning mechanisms become much more important. This means opening windows or installing or upgrading heating, ventilation, and air conditioning systems, as outlined in a recent WHO document. People are much more likely to become infected in a room with windows that can’t be opened or lacking any ventilation system.”
Tang et al (2021) (4)
Calling for Revised Guidelines and Practices
For a significant period of time during the pandemic, prominent public health governing bodies including the World Health Organization (WHO) and the Center for Disease Control (CDC) did not formally communicate the role of airborne transmission in spreading SARS-CoV-2. The WHO statement on April 30, 2021 has come a little too late, as hundreds of experts have urged the agency as early as in July 2020 to review the mounting evidence on airborne transmission of SARS-CoV-2 and update their advice accordingly (11). Yet, even as late as in October 2020, the WHO recommended sanitation practices for cleaning surfaces to reduce fomite transmission, even though there is limited evidence of the virus being transmitted through contaminated surfaces (12). Similarly, the CDC has also emphasized the need to limit fomite-based transmission of SARS-CoV-2.
The overwhelming focus on fomites have diverted attention and resources from the much bigger risk presented by airborne transmission of SARS-CoV-2. In addition to the continued enforcement of social distancing and wearing masks, what is direly needed right now is heightened investment in improving indoor air ventilation or installing air purifiers with proven effectiveness (2). While improving ventilation is not always an easy task, it has been widely touted that it will be well worth the investment given the future of the pandemic (3).
As the WHO now recommends, in order to reduce the spread of SARS-CoV-2, individuals should avoid the “Three C’s” (10):
- Crowded places
- Close contact settings where people have conversations within close proximity to each other
- Confined and enclosed places with poor ventilation
Albeit the delayed guidance, we are hopeful that public health governing bodies globally will now feel empowered to make the right decisions to protect the health of people living in different contexts with varying risk of infection from SARS-CoV-2, based on the evidence that is currently available and endorsed by the WHO.
1. Tang JW et al (2021). Dismantling myths on the airborne transmission of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). The Journal of Hospital Infection 110: 89-96. DOI: 10.1016/j.jhin.2020.12.022
2. Nature (2021). Coronavirus is in the air — there’s too much focus on surfaces. Nature 590 (7). DOI: 10.1038/d41586-021-00277-8
3. Haridy R (2021). Evidence of COVID-19 airborne transmission “overwhelming” say experts. Retrieved from https://newatlas.com/health-wellbeing/covid19-sars-cov-2-airborne-transmission-aerosol-evidence-study/
4. Tang JW et al (2021). Covid-19 has redefined airborne transmission. The BMJ 373. DOI: 10.1136/bmj.n913
5. Greenhalgh T et al (2021). Ten scientific reasons in support of airborne transmission of SARS-CoV-2. The Lancet 397(10285): 1603-1605. DOI: 10.1016/S0140-6736(21)00869-2
6. Health and Safety Executive UK (2008). RR619 Evaluating the protection afforded by surgical masks against influenza bioaerosols. Retrieved from https://www.hse.gov.uk/research/rrhtm/rr619.htm
7. Booth CM et al (2013). Effectiveness of surgical masks against influenza bioaerosols. The Journal of Hospital Infection 84(1): 22-26. DOI: 10.1016/j.jhin.2013.02.007
8. van der Sande M et al (2008). Professional and home-made face masks reduce exposure to respiratory infections among the general population. PLoS One 3(7): e2618. DOI: 10.1371%2Fjournal.pone.0002618
9. Davies A et al (2013). Testing the efficacy of homemade masks: would they protect in an influenza pandemic? Disaster Medicine and Public Health Preparedness 7(4): 413–418. DOI: 10.1017/dmp.2013.43
10. World Health Organization (2021). Coronavirus disease (COVID-19): How is it transmitted? Retrieved from https://www.who.int/news-room/q-a-detail/coronavirus-disease-covid-19-how-is-it-transmitted
11. Mandavilli A (2020). W.H.O. to review evidence of airborne transmission of coronavirus. Retrieved from https://www.nytimes.com/2020/07/07/health/coronavirus-aerosols-who.html
12. Lewis D (2021). COVID-19 rarely spreads through surfaces. So why are we still deep cleaning? Retrieved from https://www.nature.com/articles/d41586-021-00251-4
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