Skip to main content
  • Letter to the Editor
  • Open access
  • Published:

Scientific evidence supports aerosol transmission of SARS-COV-2

We question the evidence cited by Conly et al. [1] to justify recommending masks for routine care of COVID-19 patients. As evidence, the authors cite the R0 and include several references that are not primary research, and only two primary studies. One of these is a study of hospital contamination which found evidence of surface contamination within a hospital but was negative for air samples [2]. This same study is used as evidence supporting contact and fomite transmission, but other studies which did find virus in air samples were disregarded [3,4,5,6] Ong et al. found evidence of virus on hospital air vents, but this is disregarded, and the meaning of finding viral RNA in air samples is questioned by Conly et al. This represents shifting goalposts for proving airborne transmission of SARS-COV-2, which was initially denied altogether, then changed to questioning the infectious potential of air in which viral RNA is found, to later questioning the infectious dose required in air, after viable virus was demonstrated in the air [6]. In fact, viable SARS-COV-2 has been found in the air in hospital rooms in the absence of aerosol generating procedures [6].

The other cited evidence is lack of transmission on an aircraft while the index case was symptomatic [7]. The majority of transmission occurs in the 48 h prior to symptom onset and in the first 6 h of symptoms, with a declining infectious function thereafter, so if the cited case was already symptomatic, he was likely less infectious while flying [8]. Further, the lack of transmission on board this aircraft could equally be used to “disprove” droplet transmission, given other passengers would have been seated within 2 m of the patient, so this is not credible evidence. In fact, there have been other airplane outbreaks, as well as outbreaks on buses that support aerosol transmission [9, 10]. Long range faecal aerosol transmission of SARS-COV-2 in an apartment block has also been documented [11].

The authors incorrectly cite the R0 of SARS-COV-2 as evidence to support droplet transmission. The R0 is not, and has never been a criterion for defining the mode of transmission. R0 is a function of the pathogen, the host and the environment, and varies for any given pathogen by factors such as population density and environment. As such, it is not a scientific measure of transmission mode, and cannot be used selectively to support droplet transmission of SARS-COV-2. Accepted estimates for the R0 of SARS-COV-2 are between 2 and 4 [12], but as high as 6 in New York State and Wuhan [13, 14]. Given that over 80% of cases are mild, and there is substantial asymptomatic infection, the official case counts upon which the R0 is calculated are likely a vast under-estimate, and R0 is likely higher.

Tuberculosis, which is accepted as airborne, has a R0 range which lower than that of SARS-COV-2 [15]. Influenza, too, has been shown repeatedly to be capable of aerosol transmission and found in the air hours after an infectious patient has left the room [16, 17], despite having a lower R0 range than SARS-COV-2 [18]. Pertussis is as infectious as measles, with a R0 range up to 18, but is classified as droplet transmitted [19], again highlighting that R0 is not a valid measure of mode of transmission. One reason is the infectious dose, which differs for different airborne pathogens, and explains why tuberculosis has a much lower R0 than measles—the difference is explained by a much lower infectious dose of measles. This complexity also illustrates why R0 cannot be used to determine transmission mode. Further, the infectious dose of SARS-COV-2 is still unknown, and this uncertainty in itself should warrant a precautionary approach.

The authors correctly cite measles and tuberculosis as airborne infections, but neither has ever been isolated as viable pathogens from air samples [20, 21], whereas SARS CoV2 has [6]. Airborne transmission has been subject to a much higher burden of proof than droplet or contact transmission of SARS-COV-2, and also to a higher evidence standard compared to other pathogens. This is the opposite of a precautionary approach in the face of uncertainty.

The cost of persevering with an argument based on selective evidence is the safety and lives of health workers, who are being denied airborne precautions by health authorities all over the world. Many guidelines still advocate the surgical mask which is not actually designed or approved for respiratory protection. The best available data on beta coronaviruses show superior protection offered by N95 respirators compared to surgical masks [22]. Finally, transmission mode is only one consideration in making guidelines for PPE. Other criteria include work health and safety obligations, the presence of scientific uncertainty, immunity, the availability of drugs and vaccines for the disease and the seriousness of the infection [23]. Getting it wrong for a serious infection such as COVID-19 matters much more than getting it wrong for a trivial infection. The same dogmatic arguments about droplet versus airborne precautions occurred in Toronto in 2003 during SARS, with a failure to provide airborne precautions for health workers, a subsequent outbreak and health worker deaths. The SARS Commission in Toronto recommended the use of the precautionary principle to protect health workers during a serious emerging infectious threat [24]. The case for the acceptance of the science around airborne transmission coupled with precautionary control measures has never been more compelling than during the COVID-19 pandemic.


  1. Conly J, Seto WH, Pittet D, Holmes A, Chu M, Hunter PR. Use of medical face masks versus particulate respirators as a component of personal protective equipment for health care workers in the context of the COVID-19 pandemic. Antimicrob Resist Infect Control. 2020;9(1):126.

    Article  Google Scholar 

  2. Ong SWT, Y.K., Chia, P.Y., , et al. Air, surface environmental, and personal protective equipment contamination by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from a symptomatic patient. JAMA. 2020;323:1610–2.

    Article  CAS  Google Scholar 

  3. Guo ZD, Wang ZY, Zhang SF, Li X, Li L, Li C, Cui Y, Fu RB, Dong YZ, Chi XY, Zhang MY. Aerosol and surface distribution of severe acute respiratory syndrome coronavirus 2 in hospital wards, Wuhan, China, 2020. Emerg Infect Dis. 2020.

    Article  PubMed  PubMed Central  Google Scholar 

  4. Santarpia JL, Rivera DN, Herrera VL, Morwitzer MJ, Creager HM, Santarpia GW, et al. Aerosol and surface contamination of SARS-CoV-2 observed in quarantine and isolation care. Nat Sci Rep. 2020;10(1):12732.

    Article  CAS  Google Scholar 

  5. Chia PY, Coleman KK, Tan YK, et al. Detection of air and surface contamination by SARS-CoV-2 in hospital rooms of infected patients. Nat Commun. 2020;11:2800.

    Article  CAS  Google Scholar 

  6. Lednicky JA, Lauzardo M, Fan ZH, Jutla AS, Tilly TB, Gangwar M, et al. Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients. Int J Infect Dis. 2020;100:476–82.

    Article  CAS  Google Scholar 

  7. Glauser W. Communication, transparency key as Canada faces new coronavirus threat. Can Med Assoc J. 2020;192(7):E171.

    Article  Google Scholar 

  8. He X, Lau EHY, Wu P, Deng X, Wang J, Hao X, et al. Temporal dynamics in viral shedding and transmissibility of COVID-19. Nat Med. 2020;26:672–5.

    Article  CAS  Google Scholar 

  9. Khanh NC, Thai PQ, Quach HL, Thi NH, Dinh PC, Duong TN, et al. Transmission of severe acute respiratory syndrome coronavirus 2 during long flight. Emerg Infect Dis. 2020;26(11):2617–24.

    Article  Google Scholar 

  10. Shen Y, Li C, Dong H, Wang Z, Martinez L, Sun Z, et al. Community outbreak investigation of SARS-CoV-2 transmission among bus riders in Eastern China. JAMA Intern Med. 2020;180:1665–71.

    Article  Google Scholar 

  11. Kang M, Wei J, Yuan J, Guo J, Zhang Y, Hang J, et al. Probable evidence of fecal aerosol transmission of SARS-CoV-2 in a high-rise building. Ann Intern Med. 2020;Sep 1:M20-0928.

    Article  Google Scholar 

  12. Centres of Disease Control and Prevention 2020. Accessed at CDC at

  13. Ives AR, Bozzuto C. State-by-State estimates of R0 at the start of COVID-19 outbreaks in the USA. medRxiv. 2020:2020.05.17.20104653.

  14. Sanche S, Lin YT, Xu C, Romero-Severson E, Hengartner N, Ke R. High contagiousness and rapid spread of severe acute respiratory syndrome coronavirus 2. Emerg Infect Dis. 2020;26(7):1470–7.

    Article  CAS  Google Scholar 

  15. Ma Y, Horsburgh CR, White LF, Jenkins HE. Quantifying TB transmission: a systematic review of reproduction number and serial interval estimates for tuberculosis. Epidemiol Infect. 2018;146(12):1478–94.

    Article  CAS  Google Scholar 

  16. Blachere FM, Lindsley WG, Pearce TA, Anderson SE, Fisher M, Khakoo R, et al. Measurement of airborne influenza virus in a hospital emergency department. Clin Infect Dis. 2009;48(4):438–40.

    Article  Google Scholar 

  17. Yan J, Grantham M, Pantelic J, Bueno de Mesquita PJ, Albert B, Liu F, et al. Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community. Proc Natl Acad Sci. 2018;115(5):1081.

    Article  CAS  Google Scholar 

  18. Nikbakht R, Baneshi MR, Bahrampour A. Estimation of the basic reproduction number and vaccination coverage of influenza in the United States (2017–18). J Res Health Sci. 2018;18(4):e00427-e.

    Google Scholar 

  19. Anderson RMM, May RM. Directly transmitted infections diseases: control by vaccination. Science. 1982;215:1053–60.

    Article  CAS  Google Scholar 

  20. Bischoff WE, McNall RJ, Blevins MW, Turner J, Lopareva EN, Rota PA, et al. Detection of measles virus RNA in air and surface specimens in a hospital setting. J Infect Dis. 2016;213(4):600–3.

    Article  CAS  Google Scholar 

  21. Yates TA, Khan PY, Knight GM, Taylor JG, McHugh TD, Lipman M, et al. The transmission of Mycobacterium tuberculosis in high burden settings. Lancet Infect Dis. 2016;16(2):227–38.

    Article  Google Scholar 

  22. Chu DK, Akl EA, Duda S, Solo K, Yaacoub S, Schünemann HJ, et al. Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis. Lancet.

  23. MacIntyre CRC, Chughtai AA, Seale H, Richards GA, Davidson PM. Respiratory protection for healthcare workers treating Ebola virus disease (EVD): are facemasks sufficient to meet occupational health and safety obligations? Int J Nurs Stud. 2014;50(11):1421–6.

    Article  Google Scholar 

  24. Campell A. SARS commission final report: spring of fear. Toronto: Government of Ontario; 2006.

    Google Scholar 

Download references





Author information

Authors and Affiliations



CRM conceived the manuscript; both authors contributed to the writing and finalized it. All authors read and approved the final manuscript.

Corresponding author

Correspondence to C. Raina MacIntyre.

Ethics declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Yes on behalf of both authors.

Availability of data and material

Not applicable.

Competing interests

Nil to declare.

Additional information

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit The Creative Commons Public Domain Dedication waiver ( applies to the data made available in this article, unless otherwise stated in a credit line to the data.

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

MacIntyre, C.R., Ananda-Rajah, M.R. Scientific evidence supports aerosol transmission of SARS-COV-2. Antimicrob Resist Infect Control 9, 202 (2020).

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI: