Studies stoke concern about coronavirus contagion through air via speech

17 April 2020. Related: COVID-19: transmission & prevention, COVID-19.

Mark Mascolini, natap.org

Accumulating evidence indicates that simply speaking can emit coronavirus-containing particles that waft through air for tens of meters, hover there, and possibly transmit SARS-CoV-2 to a person who inhales these virus-tainted aerosols.

“Based on the trend in the increase of [SARS-CoV-2] infections, and understanding the basic science of viral infection spread,” Australian and Chinese researchers write, “we strongly believe that the virus is likely to be spreading through the air”. [1]

They “plead that the international and national authorities acknowledge the reality that the virus spreads through air, and recommend that adequate control measures . . . be implemented to prevent further spread of the SARS-CoV-2 virus.” 

Since the days after World War II, researchers recognised that merely speaking produces oral fluid droplets [2] that can carry infectious virus particles. Big droplets quickly fall to the ground, but small droplets can dehydrate and linger in the air as “droplet nuclei” [3] that “behave like an aerosol and thereby expand the spatial extent of emitted infectious particles” [4]

Researchers from the National Institutes of Health (NIH) and the University of Pennsylvania conducted a laser light-scattering experiment that visualised speech-generated droplets and determined how they spread and linger [4]. A researcher spoke through an opening in one side of a carboard box painted black inside, repeating the words “stay healthy” at different volumes, without or with a damp washcloth over his mouth. An iPhone 11 Pro video camera positioned at the other end of the box aimed at a laser light sheet through which droplets passed. Ultrahigh-resolution recordings estimated the size of these droplets, represented by flashes of light. 

The brightness of flashes reflected particle size and time present in a 16.7-msec video frame. The number of flashes in a single video frame ranged from 227 at low-volume speech to 347 at the highest volume. Flashes in a single frame reached their maximum with the tongue-on-teeth “th” sound in “healthy.” When the researcher spoke through a slightly damp washcloth, the flash count fell to the background level averaging 0.1 flashes.

The investigators note that droplets emitted during speaking in one study were smaller than those ejected during coughing or sneezing [4]. But some research found the same number of droplets during speaking and coughing [5]. The NIH-UPenn team did not assess the potential roles of speech-propelled droplets and aerosols in viral transmission. 

Writing about this experiment, Harvard researcher Matthew Meselson explains that larger droplets and smaller aerosols take different routes if inhaled [6]. The bigger droplets settle in the upper respiratory tract, from which they can be removed in nasal secretions or ascend the “mucociliary escalator” and then be expelled or swallowed. But smaller aerosolized particles can descend deeply into the lung, nest in alveoli, and start infecting lung cells.

Merelson sites another recent study showing that aerosols containing SARS-CoV-2 remain infectious in tissue culture assays for three hours [7]. That finding suggests to Merelson that aerosols from infected people may “pose an inhalation threat even at considerable distances and in enclosed spaces, particularly if there is poor ventilation.” He suggests “wearing a suitable mask” when infected people may be nearby or providing adequate ventilation in enclosed spaces currently or recently inhabited by SARS-CoV-2-infected people.

Hand washing and 6-foot social distancing remain the primary measures for avoiding SARS-CoV-2 infection. But Australian and Chinese researchers Lidia Morawska and Junji Cao argue those strategies “do not prevent infection by inhalation of small droplets exhaled by an infected person that can travel . . . meters or tens of meters in the air and carry their viral content”. [1, 8]

Morawska and Cao remind readers that SARS-CoV-1 did spread in air, a route that explained transmission of this coronavirus in Hong Kong’s Prince of Wales Hospital and in healthcare facilities in Toronto [1]. They site studies demonstrating airborne transmission of Norwalk-like virus in school children and influenza A/H5N1 in ferrets. At a single choir practice near Seattle, Washington, speaking and singing apparently contributed to SARS-CoV-2 spreading to 45 of 60 choir members. [9]

Findings like these, Morawska and Cao say, mean “it is highly likely that the SARS-CoV-2 virus also spreads by air” [1].

Measures that can lower chances of indoor transmission, Morawska and Cao propose [1], include:

• Increased ventilation rate.
• Natural ventilation.
• Avoiding air recirculation.
• Avoiding staying in another person’s direct air flow.
• Minimising the number of people sharing the same space.

Implementing measures like these depends on countries recognizing the risk of airborne transmission, but “currently, this is not the case anywhere in the world”. [1]

References

1. Morawska L, Cao J. Airborne transmission of SARS-CoV-2: The world should face the reality. Environ Int. 2020;139:105730. doi: 10.1016/j.envint.2020.105730.
   https://www.sciencedirect.com/science/article/pii/S016041202031254X
2. Duguid JP. The size and the duration of air-carriage of respiratory droplets and droplet-nuclei. J Hyg (Lond). 1946;44:471-479.
3. Marr LC et al. Mechanistic insights into the effect of humidity on airborne influenza virus survival, transmission and incidence. J R Soc Interface 2019;16(150).
4. Anfinrud P et  al. Visualizing speech-generated oral fluid droplets with laser light scattering. N Engl J Med. April 15, 2020. doi: 10.1056/NEJMc2007800. https://www.nejm.org/doi/full/10.1056/NEJMc2007800
5. Chao CYH et al. Characterization of expiration air jets and droplet size distributions immediately at the mouth opening. J Aerosol Sci. 2009;40:122-133.
6. Merelson M. Droplets and aerosols in the transmission of SARS-CoV-2. N Engl J Med. April 15, 2020. doi: 10.1056/NEJMc2009324. https://www.nejm.org/doi/full/10.1056/NEJMc2009324
7. van Doremalen N et al. Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1. N Engl J Med. doi: 10.1056/NEJMc2004973. April 16, 2020.
   https://www.nejm.org/doi/full/10.1056/nejmc2004973
8. Morawska L, et al. Size distribution and sites of origin of droplets expelled from the human respiratory tract during expiratory activities. J Aerosol Sci. 2009;40;256-269. https://www.sciencedirect.com/science/article/pii/S0021850208002036
9. Read R. A choir decided to go ahead with rehearsal. Now dozens of members have COVID-19 and two are dead. Los Angeles Times. March 29, 2020. https://www.latimes.com/world-nation/story/2020-03-29/coronavirus-choir-outbreak