May 14, 2020

expert reaction to study looking at speech-generated droplets and COVID-19 transmission

Research, published in PNAS, reports on speech-generated droplets and COVID-19 transmission.

Dr Michael Cotterell, NERC Research Fellow, University of Bristol, said:

“The laboratory research techniques used are novel and certainly provide compelling evidence that micron-sized speech droplets have significant lifetimes (several minutes). One detail that is less clear is whether the aerosol introduced to the researchers’ enclosure could be contaminated by simply opening the speaker shutter, and therefore whether all the particles sampled correspond to speech droplets.

“It must be stressed that the authors do not measure the viability of SARS-CoV-2 in aerosol droplets at the small (micron sized) droplet sizes associated with the long airborne lifetimes. Moreover, regarding the authors’ assumptions of viral load, this value is not well known/constrained for SARS-CoV-2. Therefore, making recommendations on social distancing, face masks/coverings and hygiene on the back of this work alone is not possible. More work is needed to understand the viability (including aerosol size dependence to rates of death) of SARS-CoV-2 in aerosol droplets, viral load, and contexts such as filtration efficiencies for various mask/face coverings, before any recommendations on the aforementioned health and safety precautions can be made in respect of mitigating airborne transmission.

“The research methods are novel. The findings are neither novel nor surprising. The lifetimes and number concentrations of speech aerosol have been characterised in previous work by other researchers. It does not transform our understanding, but adds further evidence to building consensus on the range of lifetimes for speech-generated aerosols.”

Prof Lawrence Young, Professor of Molecular Oncology, University of Warwick, said:

“This study measured the size and spread of oral fluid droplets using a laser. It convincingly shows that normal speech generates airborne droplets that can remain suspended in the air for tens of minutes or longer. This suggests that virus from an infected individual could be transmitted this way in confined spaces, however there is no direct analysis of the presence of viruses in the droplets or their ability to pass on infection.

“The work is a physical study using a laser scattering method. One of the main assumptions in this paper is that each virus particle in a droplet is equally capable of causing an infection. We don’t know that this is the case for SARS-CoV-2. 

“The study is novel and supports the view that respiratory and aerosol transmission are significant mechanisms of virus spread. It does not really change our understanding of how this virus is transmitted,  just confirms and extends previous data.

“It adds weight to the need for social distancing and raises important issues about the potential for the virus to spread in confined spaces such as offices and factories. It also highlights the problem of virus transmission from infected individuals who do not have symptoms.”

‘The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission’ by Stadnytskyi et al was published in PNAS on Wednesday 13 May. 

All our previous output on this subject can be seen at this weblink: www.sciencemediacentre.org/tag/covid-19

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