It is Time to Address Airborne Transmission ofCOVID-19

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Lidia Morawska1,*, Donald K. Milton2

1anInternational Laboratory for Air Quality and Heath, WHO Collaborating Centre, Queensland
University of Technology, 2 George Street, Brisbane, QLD 4001 Australia. Email:l.morawska@qut.edu.au2ed

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Institute for Applied Environmental Health, University of Maryland School of Public Health, 255
Campus Dr, College Park, Maryland, USA. Email: dmilton@umd.edu

AcceptCorresponding author: Lidia Morawska, Email: l.morawska@qut.edu.au

Keywords: airborne transmission; airborne infection spread; coronavirus; COVID-19; SARS-CoV-2virus.

© The Author(s) 2020. Published by Oxford University Press for the Infectious Diseases
Society of America. All rights reserved. For permissions, e-mail:journals.permissions@oup.com.

Commentary

We appeal to the medical community and to the relevant national and international bodies torecognize the potential for airborne spread of COVID-19. There is significant potential for inhalation
exposure to viruses in microscopic respiratory droplets (microdroplets) at short to medium distances(up to several meters, or room scale), and we are advocating for the use of preventive measures tocr
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mitigate this route of airborne transmission.

Studies by the signatories and other scientists have demonstrated beyond any reasonable doubt

usthat viruses are released during exhalation, talking, and coughing in microdroplets small enough toremain aloft in air and pose a risk of exposure at distances beyond 1 to 2 m from an infected

anindividual (see e.g. [1-4]). For example, at typical indoor air velocities [5], a 5 μm droplet will traveltens of meters, much greater than the scale of a typical room, while settling from a height of 1.5 m

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to the floor. Several retrospective studies conducted after the SARS-CoV-1 epidemic demonstrated

edthat airborne transmission was the most likely mechanism explaining the spatial pattern ofinfections e.g. [6]. Retrospective analysis has shown the same for SARS-CoV-2 [7-10]. In particular, acept

study in their review of records from a Chinese restaurant, observed no evidence of direct or indirectcontact between the three parties [10]. In their review of video records from the restaurant, theyobserved no evidence of direct or indirect contact between the three parties. Many studies

Acconducted on the spread of other viruses, including respiratory syncytial virus (RSV) [11], Middle
East Respiratory Syndrome coronavirus (MERS-CoV) [8], and influenza [2,4], show that viableairborne viruses can be exhaled [2] and/or detected in the indoor environment of infected patients[11-12]. This poses the risk that people sharing such environments can potentially inhale theseviruses, resulting in infection and disease. There is every reason to expect that SARS-CoV-2 behaves
similarly, and that transmission via airborne microdroplets [10,13] is an important pathway. Viral
RNA associated with droplets smaller than 5 μm has been detected in air [14], and the virus hasbeen shown to maintain infectivity in droplets of this size [9]. Other viruses have been shown tosurvive equally well, if not better, in aerosols compared to droplets on a surface [15].

The current guidance from numerous international and national bodies focuses on hand washing,
maintaining social distancing, and droplet precautions. Most public health organizations, including
the World Health Organization (WHO) [16], do not recognize airborne transmission except forcr
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aerosol-generating procedures performed in healthcare settings. Hand washing and social distancing
are appropriate, but in our view, insufficient to provide protection from virus-carrying respiratory
microdroplets released into the air by infected people. This problem is especially acute in indoor orus

enclosed environments, particularly those that are crowded and have inadequate ventilation [17]anrelative to the number of occupants and extended exposure periods (as graphically depicted inFigure 1). For example, airborne transmission appears to be the only plausible explanation forM

several superspreading events investigated which occurred under such conditions e.g. [10], anded

others where recommended precautions related to direct droplet transmissions were followed.

The evidence is admittedly incomplete for all the steps in COVID-19 microdroplet transmission, but it

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is similarly incomplete for the large droplet and fomite modes of transmission. The airborne
transmission mechanism operates in parallel with the large droplet and fomite routes, e.g. [16] thatare now the basis of guidance. Following the precautionary principle, we must address everyAcpotentially important pathway to slow the spread of COVID-19. The measures that should be taken
to mitigate airborne transmission risk include:Provide sufficient and effective ventilation (supply clean outdoor air, minimize
recirculating air) particularly in public buildings, workplace environments, schools,
hospitals, and aged care homes.

Supplement general ventilation with airborne infection controls such as local exhaust,
high efficiency air filtration, and germicidal ultraviolet lights.

Avoid overcrowding, particularly in public transport and public buildings.

Such measures are practical and often can be easily implemented; many are not costly. For example,simple steps such as opening both doors and windows can dramatically increase air flow rates inmany buildings. For mechanical systems, organizations such as ASHRAE (the American Society ofHeating, Ventilating, and Air-Conditioning Engineers) and REHVA (the Federation of European
Heating, Ventilation and Air Conditioning Associations) have already provided guidelines based on

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the existing evidence of airborne transmission. The measures we propose offer more benefits thanpotential downsides, even if they can only be partially implemented.

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Figure 1. Distribution of respiratory microdroplets in an indoor environment with (a) inadequate
ventilation and (b) adequate ventilation.

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It is understood that there is not as yet universal acceptance of airborne transmission of SARS-CoV2;
but in our collective assessment there is more than enough supporting evidence so that theed

precautionary principle should apply. In order to control the pandemic, pending the availability of ace
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vaccine, all routes of transmission must be interrupted.

We are concerned that the lack of recognition of the risk of airborne transmission of COVID-19 andthe lack of clear recommendations on the control measures against the airborne virus will haveAcsignificant consequences: people may think that they are fully protected by adhering to the current
recommendations, but in fact, additional airborne interventions are needed for further reduction ofinfection risk.

This matter is of heightened significance now, when countries are re-opening following lockdowns bringing people back to workplaces and students back to schools, colleges, and universities. We
hope that our statement will raise awareness that airborne transmission of COVID-19 is a real risk
and that control measures, as outlined above, must be added to the other precautions taken, toreduce the severity of the pandemic and save lives.

Disclaimer: The views and opinions expressed in this article are those of the authors and do notnecessarily reflect the official policy or position of any agency/institution.

Acknowledgment

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Together with the authors, 239 scientists support this Commentary, and their affiliations and contact
details are listed in the Supplementary.

usThe following scientists contributed to formulating this commentary: Linsey C. Marr, William
Bahnfleth, Jose-Luis Jimenez, Yuguo Li, William W. Nazaroff, Catherine Noakes, Chandra Sekhar,an

Julian Wei-Tze Tang, Raymond Tellier, Philomena M. Bluyssen, Atze Boerstra, Giorgio Buonanno,Junji Cao, Stephanie J. Dancer, Francesco Franchimon, Charles Haworth, Jaap Hogeling, ChristinaMIsaxon, Jarek Kurnitski, Marcel Loomans, Guy B. Marks, Livio Mazzarella, Arsen Krikor Melikov, ShellyMiller, Peter V. Nielsen, Jordan Peccia, Xavier Querol, Olli Seppänen, Shin-ichi Tanabe, Kwok Wai

edTham, Pawel Wargocki, Aneta Wierzbicka, Maosheng Yao.

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The following scientists reviewed the document: Jonathan Abbatt, John Adgate, Alireza Afshari, KangHo Ahn, Francis Allard, Joseph Allen, Celia Alves, Meinrat O. Andreae, Isabella Annesi-Maesano,Ahmet Arısoy, Andrew P. Ault, Gwi-Nam Bae, Gabriel Bekö, Scott C. Bell, Allan Bertram, MahmoodAcBhutta, Seweryn Bialasiewicz, Merete Bilde, Tami Bond, Joseph Brain, Marianna Brodach, David M.
Broday, Guangyu Cao, Christopher D. Cappa, Annmarie Carlton, Paul K. S. Chan, Christopher Chao,Kuan-Fu Chen, Qi Chen, Qingyan Chen, David Cheong, Per Axcel Clausen, Ross Crawford, Derek
Clements-Croome, Geo Clausen, Ian Clifton, Richard L. Corsi, Benjamin J. Cowling, Francesca Romana
d'Ambrosio, Ghassan Dbaibo, Richard de Dear, Gianluigi de Gennaro, Peter DeCarlo, PhilipDemokritou, Hugo Destaillats, Joanna Domagala-Kulawik, Neil M. Donahue, Caroline Duchaine,Marzenna R. Dudzinska, Dominic E. Dwyer, Greg Evans, Delphine K. Farmer, Kevin P. Fennelly,Richard Flagan, Janine Fröhlich-Nowoisky, Manuel Gameiro da Silva, Christian George, Marianne

Glasius, Allen H. Goldstein, João Gomes, Michael Gormley, Rafal Górny, David Grimsrud, KeithGrimwood, Charles N. Haas, Fariborz Haghighat, Michael Hannigan, Roy Harrison, Ulla HaverinenShaughnessy, Philippa Howden-Chapman, Per Heiselberg, Daven K. Henze, Jean-Michel Heraud,Hartmut Herrmann, Philip K. Hopke, Ray Horstman, Wei Huang, Alex Huffman, David S. Hui, Tareq
Hussein, Gabriel Isaacman-VanWertz, Jouni J.K. Jaakkola, Matti Jantunen, Lance Jennings, Dennis

criptJohansson, Jan Kaczmarczyk, George Kallos, David Katoshevski, Frank Kelly, Søren Kjærgaard, Luke D.
Knibbs, Henrik N. Knudsen, GwangPyo Ko, Evelyn S.C. Koay, Jen Kok, Nino Kuenzli, Markku Kulmala,Kazukiyo Kumagai, Prashant Kumar, Kazumichi Kuroda, Kiyoung Lee, Nelson Lee, Barry Lefer, Vincent

us

Lemort, Xianting Li, Dusan Licina, Chao-Hsin Lin, Junjie Liu, Kam Lun E. Hon, John C. Little, Li Liu, Janet
M. Macher, Ebba Malmqvist, Corinne Mandin, Ivo Martinac, Dainius Martuzevičius, Mark J. Mendell,anDavid Miller, Claudia Mohr, Luisa T. Molina, Glenn Morrison, Roya Mortazavi, Edward Nardell,M

Athanasios Nenes, Mark Nicas, Zhi Ning, Jianlei Niu, Hidekazu Nishimura, Colin O'Dowd, Bjarne W.
Olesen, Paula J. Olsiewski, Spyros Pandis, Daniel Peckham, Tuukka Petäjä, Zbigniew Popiolek, UlrichedPöschl, Wayne R. Ott, Kimberly Prather, Andre S. H. Prevot, Hua Qian, Shanna Ratnesar-Shumate,James L. Repace, Tiina Reponen, Ilona Riipinen, Susan Roaf, Allen L. Robinson, Yinon Rudich, ManuelceptRuiz de Adana, Masayuki Saijo, Reiko Saito, Paulo Saldiva, Tunga Salthammer, Joshua L. Santarpia,
John H. Seinfeld, Gary S. Settles, Siegfried Schobesberger, Paul T. J. Scheepers, Max H. Sherman, AlanShihadeh, Manabu Shiraiwa, Jeffrey Siegel, Torben Sigsgaard, Brett C. Singer, James N. Smith, Armin

AcSorooshian, Jerzy Sowa, Brent Stephens, Huey-Jen Jenny Su, Jordi Sunyer, Jason D. Surratt, KazuoTakahashi, Nobuyuki Takegawa, Jørn Toftum, Margaret A. Tolbert, Euan Tovey, Barbara J. Turpin,
Annele Virtanen, John Volckens, Claire Wainwright, Lance A. Wallace, Boguang Wang, Chia C. Wang,
Michael Waring, John Wenger, Charles J. Weschler, Brent Williams, Mary E. Wilson, Armin Wisthaler,Kazimierz Wojtas, Douglas R. Worsnop, Ying Xu, Naomichi Yamamoto, Xudong Yang, Hui-Ling Yen,
Hiroshi Yoshino, Hassan Zaraket, Zhiqiang (John) Zhai, Junfeng (Jim) Zhang, Qi Zhang, Jensen Zhang,Yinping Zhang, Bin Zhao, Tong Zhu.

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