I have already written at length about the mechanism of transmission of COVID-19. Based on basic science and the transmission of other viruses, such as influenza, it was easy to predict in early 2020 that COVID-19 would spread through the airborne route. As more super-spreader events occurred, and more data was collected, it became very clear by the end of 2020 that COVID-19 is spread through aerosols, and that the airborne route is probably the predominant mechanism of spread. And yet, for some unfathomable reason, hospitals and governments have generally clung to the idea that aerosols are of minor importance. (Although those same organizations will emphasize the importance of ventilation, which only matters is COVID-19 is airborne, and so their own recommendations are illogical and unscientific.)
I don’t feel the need to re-write my entire review of this literature, as the evidence was compelling a year ago, and has only become stronger since. However, there have been a number of important publications since my last post, so I thought I would provide a very brief update.
(Please feel free to send me any papers I have missed. More importantly, if you disagree with me, please send me any science that demonstrates that COVID-19 is not airborne, or that droplet-contact is truly a predominant mechanism of spread.)
Coleman KK, Tay DJW, Sen Tan K, et al. Viral Load of SARS-CoV-2 in Respiratory Aerosols Emitted by COVID-19 Patients while Breathing, Talking, and Singing. Clin Infect Dis. 2021 Aug 6:ciab691. doi: 10.1093/cid/ciab691 PMID: 34358292
This study collected samples from 13 patients with confirmed COVID-19. Fine aerosols (defined as less than or equal to 5 um) accounted for 85% of the viral load detected. In other words, there is far more virus emitted in aerosols than there is in droplets. (The major problem with the paper is that the 5um cut off is inaccurate. Particles much lager than 5 um remain airborne. Therefore, much more than 85% of emitted virus will be found in airborne particles. ) 94% of virus was emitted during talking and singing, and 7 participants emitted more virus when talking than singing. (In other words, the concept of aerosol generating procedures makes absolutely no sense.)
Hawks SA, Prussin AJ, Kuchinsky SC, et al. Infectious SARS-CoV-2 is emitted in aerosols. bioRxiv preprint. doi: https://doi.org/10.1101/2021.08.10.455702
This is an animal study, but also found that the majority of emitted virus is found in small aerosols (<8um), indicating that “SARS-CoV-2 is an airborne virus.”
Port JR, Yinda CK, Owusu IO, Holbrook M, et al. SARS-CoV-2 disease severity and transmission efficiency is increased for airborne but not fomite exposure in Syrian hamsters. bioRxiv [Preprint]. 2020 Dec 28:2020.12.28.424565. doi: 10.1101/2020.12.28.424565PMID: 33398267
In the initial post, I discussed a study in ferrets that pretty definitively demonstrates airborne spread of SARS-CoV-2. This paper again clearly demonstrated airborne spread, in hamsters, and demonstrates that the airborne route is more efficient than the fomite route of spread. (This paper does demonstrate transmission through fomites, which means that SARS-CoV-2 is unlikely to be 100% airborne, like tuberculosis.)
Santarpia JL, Herrera VL, Rivera DN, et al. The size and culturability of patient-generated SARS-CoV-2 aerosol. J Expo Sci Environ Epidemiol. 2021 Aug 18:1–6. doi: 10.1038/s41370-021-00376-8. PMID: 34408261
From the very beginning of the pandemic, we were able to find SARS-CoV-2 in air samples from patient rooms, in air ducts, and outside patient rooms. However, some argued that this might just represent dead viral RNA, and not actually present an infectious risk. (There was never any evidence for this claim, just strong bias against airborne transmission.) This paper (among others) demonstrates living, culturable SARS-CoV-2 from aerosol samples in patient rooms, providing “additional evidence that airborne transmission of COVID-19 is possible.”
Wang CC, Prather KA, Sznitman J, Jimenez JL, Lakdawala SS, Tufekci Z, Marr LC. Airborne transmission of respiratory viruses. Science. 2021 Aug 27;373(6558):eabd9149. doi: 10.1126/science.abd9149. PMID: 34446582
I obviously love this article, as it reproduces a great deal of what I have written in my 2 main blog posts. I love it when people share my work, but if you want an article with a little more prestige to share with hospital administrators or government officials with their heads in the sand, this publication in Science is probably the definitive review on the airborne transmission of viruses. Some key points:
- The traditional cut off of 5 um to define an aerosol is not evidence based. (100 um is probably a more reasonable cut-off.)
- There is very strong evidence that SARS-CoV-2 is transmitted through the airborne route, including “the strong effect of ventilation on transmission, the distinct difference between indoor and outdoor transmission, well-documented long-range transmission, the observed transmission of SARS-CoV-2 despite the use of masks and eye protection, the high frequency of indoor superspreading events of SARS-CoV-2, animal experiments, and airflow simulations”.
- Conversely, fomite transmission has been found to be far less efficient, and droplets only predominate when people are within 20cm of each other while talking.
Greenhalgh T, Jimenez JL, Prather KA, Tufekci Z, Fisman D, Schooley R. Ten scientific reasons in support of airborne transmission of SARS-CoV-2. Lancet. 2021 May 1;397(10285):1603-1605. doi: 10.1016/S0140-6736(21)00869-2. Epub 2021 Apr 15. Erratum in: Lancet. 2021 May 15;397(10287):1808. PMID: 33865497
I included this paper in the May Research Roundup, but if you missed it, here are 10 scientific reasons in support of airborne transmission of SARS-CoV-2:
- Superspreading events have been incredibly common, and are clearly best explained by airborne transmission
- There are multiple documented cases of long range transmission
- Asymptomatic spread – ie from people not producing significant droplets – is incredibly common
- Indoor transmission is higher than outdoor – that only makes sense with airborne spread
- Nosocomial infections are occurring even with strict adherence to droplet contact PPE
- Viable SARS-CoV-2 has been detected in the air
- SARS-CoV-2 has been found in air filters and air ducts
- Animal studies, where animals are separated in a way that only aerosols can spread between cages, show airborne spread
- There is really no evidence against airborne spread
- There is really no direct evidence that COVID is droplet spread
Morgenstern, J. COVID is airborne: A brief update, First10EM, September 20, 2021. Available at:
3 thoughts on “COVID is airborne: A brief update”
Thanks for this round-up/summary. Especially appreciate you pointing out the inconsistency between recommending ventilation and insisting the virus is in droplets. Now if we could just get the full range of protections consistent with airborne transmission! That’s where we need the voices of occupational hygienists, ventilation engineers, etc. Tme to rememer that ocupational health (and its practice) is part of public health.
Hi Justin, Thank-you for your article and evidence revue. You make a strong argument for the mode of transmission for SARS-CoV2 (Covid). Really enjoy reading your publications. Justin your doi link to your article is broken and therefore unable to retrieve your article: Justin Morgenstern. COVID is airborne: A brief update, First10EM, 2021. Available at:
Can you please look into this?
Thanks for noticing.
The DOIs take a little while to get assigned to the post – so the link often won’t work right away. Hopefully it will be up soon.