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Outcome of the struggle over how COVID-19 spreads


According to conventional medical opinion, nearly all respiratory infections transmit as droplets through coughs or sneezes: Whenever a sick person coughs, bacteria and viruses spray out in droplets, quickly falling and sticking to any surface within a radius of 1 to 2 metres. If these droplets land on a nose or mouth (or on a hand that then touches the face), they can cause an infection. Until recently, only a few diseases were thought to be transmitted differently. Measles and tuberculosis transmit in an airborne manner inside aerosols, which are microscopic particles that can stay suspended for hours and travel longer distances. They can spread when contagious people simply breathe.

The distinction between droplet and airborne transmission has enormous consequences. To combat droplets, a leading precaution is to wash hands frequently with soap and water. To fight infectious aerosols, the air itself is the enemy. In hospitals, that means expensive isolation wards and N95 masks for all medical staff. It seems likely that the daunting challenge that this presented to public health officials made it difficult for them to accept the reality of airborne transmission.

Airborne vs. droplet transmission

Wells (1934) analyzed air samples and plotted a curve showing how the opposing forces of gravity and evaporation acted on respiratory particles. The calculations made it possible to predict the time it would take a particle of a given size to travel from someone’s mouth to the ground. He found that particles larger than 100 microns sank within seconds, while smaller particles stayed in the air. Most viruses can embed in particles of any size and infect cells all along the respiratory tract. However, tuberculosis, the subject of Wells’ studies, can only invade a subset of human cells in the deepest reaches of the lungs, and only particles smaller than 5 microns can do this. Wells chose the tuberculosis bacterium because it is hardy and could be aerosolized.

As chronicled by Molteni (2021), it appears that after Wells died, scientists inside the United States Centres for Disease Control and Prevention (CDC) took the 5 micron size of the particle that transmits tuberculosis out of context, generalising it as a definition of airborne spread, whereas in reality airborne particles as large as 100 microns, including the SARS-CoV-2 virus, can travel large distances and infect people with COVID-19 disease.

Struggle between atmospheric physicists and public health officials over mode of transmission

Even before the appearance of SARS-CoV-2, scientists were becoming aware of the role of airborne transmission of the influenza virus (Marr et al., 2019). Since the appearance of SARS-CoV-2, these experts have been trying to convince public health authorities that aerosols, not droplets, are the main cause of transmission of COVID-19 (Allen and Marr, 2020; Morawska and Cao, 2020; Tang et al., 2021). They cite a growing list of superspreading events in restaurants, call centers, cruise ships and a choir rehearsal; instances where people got sick even when they were across the room from a contagious person. The incidents contradicted the World Health Organisation’s (WHO’s) main safety guidelines of keeping 1 to 2 metres of distance between people and frequent handwashing. If SARS-CoV-2 traveled only in large droplets that immediately fell to the ground, as the WHO was saying, then the distancing and handwashing should have prevented such outbreaks. The WHO’s experts appeared to be initially unmoved by these arguments, and wanted more direct evidence that the virus was abundant in the air before being prepared to call SARS-CoV-2 an airborne virus.

In July 2020, Dr Lidia Morawska of the Queensland University of Technology (QUT) and 237 other scientists and physicians worldwide signed an open letter (Morawska and Milton, 2020) addressed to public health authorities, including the WHO. They warned that without stronger recommendations for masking and ventilation, airborne spread of SARS-CoV-2 would undermine even the most vigorous testing, tracing and social distancing efforts.

This news made headlines and provoked a strong backlash from prominent public health officials who rushed to defend the WHO. Days later, the WHO released an updated scientific brief, acknowledging that aerosols could not be ruled out, especially in poorly ventilated places. However, the WHO kept the 1- to 2-metre rule, advising people to wear masks indoors only if they could not keep that distance.

Later in July 2020, Linsey Marr and Kimberly Prather sent slides to Anthony Fauci, director of the United States National Institutes of Allergy and Infectious Diseases. One of them showed the trajectory of a 5-micron particle released from the height of the average person’s mouth. It went much farther than 2 metres – a hundred metres farther. A few weeks later, Fauci admitted that the 5-micron distinction was wrong, and had been for years, conceding that there is much more aerosol transmission than had been thought.

Still, the droplet theory prevailed. In early October 2020, a group of scientists and doctors (Prather et al. (2020)) published a letter in Science, urging formation of a consensus on how infectious particles move, and abandonment of the 5-micron threshold. They argued that only then could they provide clear and effective advice to the public. The same day, the CDC surreptitiously updated its guidance to acknowledge that SARS-CoV-2 can spread through long-lingering aerosols.

In December 2020, the WHO also began to talk more publicly about aerosols and finally recommended that everyone always wear a mask indoors wherever there is a risk of COVID-19 spreading. In an interview, the WHO’s Maria Van Kerkhove said that the change reflected the organisation’s commitment to modifying its guidance when the scientific evidence compels a change. She stated that the reason they were promoting ventilation was that this virus can be airborne but, because that term has a specific meaning in the medical community, she tended to avoid it, instead emphasizing the types of settings that pose the largest risks. Yet she admitted that it may be time to rethink the droplet-airborne dichotomy, and said that the WHO would formally review its definitions for describing disease transmission in 2021. 

Tang et al. (2021a) published an editorial in the BMJ entitled “COVID-19 has redefined airborne transmission” and posted their paper on the origins of the 5-micron error to a public preprint server. On Friday April 30, 2021, the WHO surreptitiously updated a page on its website. In a section on how the coronavirus gets transmitted, the text now states that the virus can spread via aerosols as well as larger droplets. This significant modification of their position passed with no news conference or public declaration. In early May 2021, the CDC made similar changes to its COVID-19 guidance, now placing the inhalation of aerosols at the top of its list of how the disease spreads, but without a news conference or press release.

Conclusions and Implications

The SARS-CoV-2 virus, like the cause of many respiratory diseases, is airborne, but it is not very contagious. It is not like measles, which is so contagious that it infects 90 percent of susceptible people exposed to someone with the virus. There is little evidence that the SARS-CoV-2 virus often infects people over long distances or in well-ventilated spaces. The virus spreads most effectively in the immediate vicinity of a contagious person, so it resembles a droplet-based pathogen, giving rise to the ambiguity in its transmission mechanism which has now been resolved. 

This new understanding of how COVID-19 spreads implies that public health authorities should spend most of their COVID-19 resources on upgrading the ventilation in buildings and public transport rather than on things such as mass COVID-19 testing of students. Influenza infects millions of people each year, killing between 300,000 and 650,000 globally. It seems likely that implementation of this new understanding could reduce the death toll of future influenza and other coronavirus pandemics.

Risk modelling should further be used to collect further predictions. 


Allen J.G., and Marr L.C. (2020). Recognizing and controlling airborne transmission of SARS-CoV-2 in indoor environments. Indoor Air. 2020;30:557–558. [PMC free article] [PubMed] [Google Scholar]

Marr, Linsey C., Julian W. Tang, Jennifer Van Mullekom and Seema S. Lakdawala (2019). Mechanistic insights into the effect of humidity on airborne influenza virus survival, transmission and incidence. Journal of the Royal Society Interface. Published:16 January 2019.

Molteni, Megan (2021). The 60-year-old scientific screwup that helped COVID kill. Slate Backchannel 13 May 2021.

Morawska, Lidia and Junji Cao (2020). Airborne transmission of SARS-CoV-2: The world should face the reality. Environment International 139 (2020) 105730. Clin Infect Dis. 2020 Dec 3;71(9):2311-2313. doi: 10.1093/cid/ciaa939. PMID: 32628269; PMCID: PMC7454469.

Morawska, Lidia and Donald K Milton (2020). It is time to address airborne transmission of Coronavirus disease 2019 (COVID-19), Clinical Infectious Diseases, Volume 71, Issue 9, 1 November 2020, Pages 2311–2313,

Prather, K.A. Linsey C. Marr, Robert T. Schooley, Melissa A. McDiarmid, Mary E. Wilson, and Donald K. Milton (2020). Airborne transmission of SARS-CoV-2. Science  16 Oct 2020: Vol. 370, Issue 6514, pp. 303-304. DOI: 10.1126/science.abf0521

Tang, J.W., W.P. Bahnfleth, P.M. Bluyssen, G. Buonanno, J.L. Jimenez, J. Kurnitski, Y. Li, S. Miller, C. Sekhar, L. Morawska, L.C. Marr, A.K. Melikov, W.W. Nazaroff, P.V. Nielsen, R. Tellier, P. Wargocki, l. and S.J. Dancer (2021a). Dismantling myths on the airborne transmission of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). J. Hosp. Infect2021 Apr; 110: 89–96. Published online 2021 Jan 13. doi: 10.1016/j.jhin.2020.12.022.

Tang, Julian W., Linsey C Marr, Yuguo Li, and Stephanie J Dancer (2021b). COVID-19 has redefined airborne transmission. BMJ 2021;373:n913,, Published: 14 April 2021

Wells, W.F. (1934). On air-borne infection. Study II. Droplets and droplet nuclei. Am. J. Epidemiol. 20, 611-618 (doi:10.1093/oxfordjournals.aje.a118097.

About the author/s
Paul Somerville
Chief Geoscientist at Risk Frontiers | Other Posts

Paul is Chief Geoscientist at Risk Frontiers. He has a PhD in Geophysics, and has 45 years experience as an engineering seismologist, including 15 years with Risk Frontiers. He has had first hand experience of damaging earthquakes in California, Japan, Taiwan and New Zealand. He works on the development of QuakeAUS and QuakeNZ.

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