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expert reaction to commentary about potential airborne transmission of the SARS-CoV-2 virus

A commentary piece, published in Clinical Infectious Diseases, discusses the potential airborne transmission of the SARS-CoV-2 virus.


Two comments below are from Prof Cath Noakes and Dr Julian Tang who have contributed to the commentary in question so are not commenting as independent third party experts but providing further comments on the subject:

(Contributed to the commentary) Prof Cath Noakes, Professor of Environmental Engineering for Buildings at the University of Leeds, said:

“The virus is transmitted through droplets that are generated when people breathe, talk, cough and sneeze. If a person is infected these droplets can contain a virus. Airborne transmission happens when these droplets are very small – below about 10 microns in diameter – and therefore can be carried between people by the air and then inhaled. 

“Transmission is thought to be mainly through direct exposure to droplets when people are in close proximity and through touching contaminated surfaces. However, transmission through small aerosols is likely in some circumstances, for example when there is a high rate of respiratory droplet generation coupled with a poorly ventilated environment. 

” COVID-19 doesn’t spread as easily as diseases like measles or tuberculosis, which are true airborne diseases and can spread by air over long distances. COVID-19 is more likely to be “opportunistically” airborne and therefore poses a risk to people who are in the same room for long periods of time. 

“It is important that people use a combination of measures to control transmission that recognise the different transmission routes. Face coverings, screens and social distancing can reduce the risk of direct droplet transmission, and hand hygiene and good cleaning of surfaces reduces the risk of transmission through contact with contaminated surfaces and objects. We can also control the risk of airborne transmission by using face coverings and ensuring that buildings have good ventilation, which can dilute and remove any virus carrying aerosols from the air.”


(Contributed to the commentary) Dr Julian Tang, Associate Professor of Respiratory Sciences at the University of Leicester, said:

What exactly is airborne transmission?

“This implies that viruses are transmitted (by direct inhalation) over short-range (1m or less) and longer-range (greater than 1 m) distances by fine aerosols that can stay suspended in the air for long periods of time

“This does not include transmission via direct contact via surfaces and hands, or by larger droplets that fall quickly under gravity to the ground within 1 m that may land on clothing or surfaces – or occasionally be inhaled directly if close enough.”

What is the state of evidence of how the virus is spread? What are the knowns and unknowns?

“We know that SARS-COV-2 RNA can be found in the air and on high level surfaces (like ventilation grills/blades). The only way this virus can get into the air or to high level surfaces is if an infected patient exhales them and the virus then stays suspended in the air (i.e. does not fall to the ground immediately under gravity) then gets carried upwards to high level surfaces by air movements.

“So far we have yet to show that these viral RNA counts from air-samples also correlate to viable viruses (by viral culture) – but this does not by itself exclude airborne transmission, as the WHO claims.

“With over 11 million COVID-19 cases globally, and yet only about a dozen studies that have failed to find airborne viable virus, this does not exclude the possibility of airborne transmission of SARS-CoV-2.

“This is a relatively fragile enveloped virus and there are many environmental and methodological reasons why it may fail to culture after air-sampling, e.g. the air-sampling is quite a violent process, so the virus may be damaged by shear stresses produce during the sampling. So it may, in fact, have been viable in the air, but the air-sampling process destroyed this.

“Other studies using different, gentler, air-sampling methods are underway to demonstrate this.

“But the real question to WHO is – can you prove that none of those 11 million+ cases of COVID-19 acquired their infection via aerosols? Clearly, they cannot prove this either.

Is there a scientific consensus? If not, where do the discrepancies lie and according to what evidence/ lack of evidence?

“Clearly there is not a consensus – otherwise there would be no need for this Letter. See also answers above.

“The WHO say that there is insufficient evidence to prove aerosol/airborne transmission of SARS-CoV-2 is happening. We are arguing that there is insufficient proof that aerosol/airborne transmission does not occur.

“This difference is subtle but quite major in terms of how you interpret the evidence – absence of evidence is not evidence of absence.

“So our stance is a more precautionary once than that of the WHO – and is an important difference as countries are now coming out of lockdown with new spikes of COVID-19 cases globally.

“Some of these new cases may be due to aerosol transmission, particularly if new cases are occurring despite people masking.

“Standard surgical masks and face coverings do not offer complete protection against aerosols, though surgical masks have been shown to reduce aerosol exposure up to 4-6- fold.”

Do you agree that the WHO is underplaying the role and risk of airborne spread?

“To some extent. Note that there is no evidence that SARS-CoV-2 spreads via the contact/droplet routes either – this has just been assumed and accepted – based on other similar viruses – but this is not proof.

“The difference in the level/quality of evidence the WHO will accept to prove each of these different routes of transmission of SARS-CoV-2 is frustrating, subjective and unscientific.

“They seem to accept contact/droplet transmission with virtually no evidence related directly to SARS-CoV-2, yet demand the highest level of evidence to confirm the aerosol/airborne transmission route for this same virus – but neither do they define exactly the nature of the proof that will be acceptable to them.

“Their resistance to acknowledge/accept airborne/aerosol transmission may be based partially on socioeconomic considerations, as this will then require enhanced ventilation, N95 masks, etc. for all countries, including LMIC (low and middle income countries) – which the WHO fears may not be affordable for those nations.

“Yet, we can work towards funding/supporting such countries with these interventions – but only if the recognition of SARS-CoV-2 as an airborne-transmitted virus is included in their WHO guidance.

“If it is not included, then ironically, there will be no airborne precautions taken and the virus will just keep spreading amongst some of these same countries where their healthcare systems can least cope with it.

Just because airborne transmission may occur, does that mean it’s a driver of transmission i.e. how concerned should we be about potential airborne transmission?

“We cannot know this for sure and it may differ between each exposure situation.

“So far the contact (hand-washing)/droplet precautions have not been working well, so lockdown was required – which will prevent all routes of transmission (as people staying in their houses will be protected by physical walls, doors and windows).

“Now that people are wearing face coverings/masks, this will mitigate some of the post-lockdown spread of the virus, but even these will not protect fully against aerosols – so social distancing will still be required to dilute the virus and reduce your exposure overall.

“We don’t want a repeat of the initial wave which was devastating for the elderly, vulnerable (with multiple comorbidities) again, so the added airborne precautions will further flatten the epidemic curve and reduce the number of COVID-19 cases needing to attend hospital.

Any other comments?

“Acknowledging that the virus is airborne would allow and drive various interventions: including enhancing our ventilation systems and keeping windows open – to remove/dilute any airborne virus to prevent its concentration building up indoors; being more vigilant with our PPE and to allow the wearing of N95 masks outside of aerosol-generating procedures (AGPS).

“We understand that all of this cannot and will not happen overnight (except perhaps the wearing of N95 masks in COVID-19 patient areas and the opening of windows).

“But having the airborne transmission risk acknowledged in the WHO guidelines will motivate and fund hospital teams, governments, NGOs, etc. to work towards these goals, which can also be applied to some forms of public transport – buses, coaches, trains, planes, cruise ships.

“Other options include the introduction of air-cleansing technologies, like ultra-violet (UV) light (as germicidal UV), reducing over-crowding in public places and on public transport.

“All of the interventions can be combined to reduce the overall build-up and concentration of airborne SARS-CoV-2 in the air to reduce the risk of infection.”


Comments from third party experts:

Prof Babak Javid, Principal Investigator, Tsinghua University School of Medicine, Beijing, and Consultant in Infectious Diseases at Cambridge University Hospitals, said:

“The precise mode of SARS-CoV-2 transmission is not fully understood.  It seems increasingly likely that transmission from surfaces (fomites) is uncommon, whereas transmission by close and prolonged contact in indoor spaces is responsible for the majority of transmission events.  Such transmission can either be via large or small droplets.  Whilst this may seem like hair-splitting, it has important practical considerations, since mitigation measures differ between modes of transmission.  For example, the emphasis on hand-washing is really only very effective if fomite transmission predominates.  Similarly, physical distancing is more relevant for large droplet transmission.  The matter is further complicated because classically, airborne/aerosol (microdroplet) transmission recalls transmission of e.g. measles virus, which can be transmitted over long distances and even after e.g. the contagious patient has left a physical space, because their transmitted aerosols stay airborne for long periods of time.  The evidence for this type of ‘long-range’ airborne transmission of SARS-CoV-2 is certainly lacking.  However, it is much harder to distinguish between close-range airborne and large droplet transmission. In this opinion piece, the authors cite one well-documented transmission cluster in a restaurant in China as evidence for airborne transmission.  The source patient was at one table, and diners at several other tables were infected: all of them downstream of an air-conditioner that was positioned in the room.  There was no physical contact between the parties.  This certainly supports transmission via the air conditioner airflow, be they small or large droplets.  However, it should be noted that diners on adjacent tables to the source patient, but who were not in the direction of airflow were not infected.  This argues strongly against airborne particles that can distribute throughout a room over a period of time being a source of infection.  Furthermore, we know from numerous studies of clearly documented transmission events that physical distance is protective, even indoors.  For example, in the well described cluster of the first European outbreak in Munich, one diner transmitted to another, and their only contact was a brief conversation to pass the salt.  No other diners in the room were infected.  Studying transmission among Beijing households (i.e. necessarily indoors, and for prolonged periods), physical distance of >1m from the source patient was the most consistent and protective factor.

“The document makes a number of practical suggestions.  They argue that they are easy to implement.  Avoiding crowded indoor spaces appears sensible, and increasing ventilation by opening windows also seems like an ‘easy solution’.  However, even these easy fixes are not completely straightforward.  In many countries where air pollution is a major health hazard,  even opening windows does have some consequences, and whilst it’s generally easy to ventilate indoor spaces in clement weather, it’s less practical (or costly both economically and environmentally) in extremes of heat and cold.  Does that mean children should not attend schools in winter, since no amount of physical distancing is safe if windows have to be closed?  Other suggested measures such as installation of highly efficient filtration systems or germicidal ultra-violet light in all indoor spaces are costly and not easy to implement on a planet-wide basis.  

“I agree with the authors that it’s possible and even likely that airborne transmission occurs for SARS-CoV-2 sometimes.  It’s not at all clear how common this is, and I agree this is an important and urgent matter to better understand.  Before invoking the precautionary principle one should acknowledge that no measures come without potential downsides.  Zero risk approaches to COVID-19 have major implications for other health and societally important factors (e.g. education).”


Prof Jose Vazquez-Boland, Chair of Infectious Diseases, University of Edinburgh, said:

“A problem here is the potential conflict between the technical notion of airborne transmission and the perception of the general public about this term.  

“A technical distinction is typically made between “transmission by respiratory droplets” expelled through coughing or sneezing, relatively heavy —thus reaching relatively short distances— and assumed by WHO and other official bodies to be the main transmission mechanism for SARS-CoV-2 upon contact with them; and “airborne transmission”, which involves smaller particles suspended in the air (aerosols).

“The latter may travel longer distances but are more exposed to desiccation and WHO and other public health bodies interpret that, except in certain circumstances (aerosol-generating clinical procedures during patient care), they play a lesser role in covid-19 transmission.

“However, this does not mean that under specific conditions of humidity, temperature and ventilation, aerosols formed by exhaled microdroplets could not be a major source of airborne transmission for covid-19, as highlighted by the experts in their commentary.

“For the public it may be difficult to differentiate between the different situations and technical definitions.

“Consequently there is clearly a danger of people misinterpreting WHO guidance regarding airborne transmission and underestimating the general risk of contracting COVID-19 through respiratory / mucosal exposure (and the importance of wearing protective masks).”



The commentary “It is Time to Address Airborne Transmission of 1 COVID-19” by Lidia Morawska and Donald K Milton was published in Clinical Infectious Diseases




All our previous output on this subject can be seen at this weblink:


Declared interests

Prof Cath Noakes and Dr Julian Tang have contributed to the commentary so are not commenting as independent third party experts but providing further comments on the subject

None others received.

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