May 21, 2020

expert reaction to preprint (not a published paper) looking at face coverings and aerosol dispersion

A preprint (not a published paper) from the University of Edinburgh has investigated aerosol dispersion from a variety of different types of face covering.

Prof Trish Greenhalgh FMedSci, Professor of Primary Health Care Services, University of Oxford, said:

“This new, very detailed and thorough laboratory study seems to have two key findings. First, that whilst medical-grade specialist respirator masks protect the wearer from inhaling germs, these devices may not protect others from exhaled germs (because the valve is one-way only).  Second, that standard surgical masks, home-made cloth masks and face shields are all extremely effective at blocking exhaled germs escaping through the front of the device but they may generate jets downwards and to the side.”

“This was a laboratory experiment so we don’t yet know what the results imply for the real world, but I would suggest a) don’t wear respirator masks (because you could spread your germs to other people through it); b) don’t get too close to someone who’s wearing a respirator mask unless you’re wearing a mask yourself (because their mask won’t protect you from their germs); c) if you’re wearing a standard surgical mask or a home-made cloth mask to protect others (e.g. when on public transport), make sure it fits closely all around; d) if you want to wear a face shield, choose a design that includes a protective ‘skirt’.”

Dr Ben Killingley, Consultant in Acute Medicine (Clinical Lead) and Infectious Diseases, University College London Hospital, said:

“This is a detailed paper describing the ability of face coverings and masks to prevent or reduce respiratory secretion dispersal by the wearer.  This is not about wearing masks to prevent becoming infected.  It is a laboratory based experimental set up that tested masks on both real people (though not infected) people and on manikins.  As would be expected all masks do modify the dispersal of respiratory secretions but only tight fitting respirator masks (without valves) were shown to appreciably contain the dispersal.  Most other types allow significant leakage around the mask edges.  As the authors point out, the amount of respiratory dispersal reduction achieved, say 50%, would not translate into a 50% infection transmission reduction, many more factors would be at play.

“Previous studies have shown that masks do contain, to some degree, respiratory secretions.  This study uses methodology that visualises the respiratory secretions as apposed to detecting the physical secretions or measuring virus levels.  Importantly, the study is not able to say which type of particles escape or leak.  One would hypothesise that larger droplets would be more contained than smaller aerosol sized particles which are more likely to escape.  This is important because reducing large droplet dispersal would be expected to help reduce droplet and contact routes of transmission.  This study therefore suggests face coverings and most masks may not provide significant protection against aerosols dispersion.  The relative contribution that aerosol transmission plays in COVID is a key unknown.

“So, this study alone can’t tell us much but taking into account the evidence base as a whole, face coverings may be better than nothing in some settings where social distancing isn’t possible.  But this study does enforce what we already know; face masks/coverings are not the complete answer to controlling infection transmission.  They need to be worn properly and fit as tightly as possible.  They are no substitute for self isolating if you are unwell and frequent hand hygiene.”

Dr Antonio Lazzarino, Department of Epidemiology and Public Health, UCL, said:

“The authors did not measure viral transmission and this is not a clinical study involving people from the public, which is what we need to make some evidence-based decisions.

“However, the engineers show that face covers can reduce the front flow of air to some extent, although for many widely-used masks the exhaled air can still outreach the distance of 1 meter.  In their paper the authors say “surgical and hand-made masks, and face shields, generate several leakage jets, including intense backward and downwards jets that may present major hazards”, implying that the infectiousness of people wearing masks is not relevantly reduced.

“But based on this study alone we cannot conclude that face coverings present a major hazard, because this study is a laboratory study in which one person only was subject to the tests.  This study is meant to describe the physics of airflows with masks; it is not meant to experiment on the uses of masks to stop epidemics.

“However, this study confirms that the mechanisms of air-flow transmission are very complex and therefore we cannot use common sense when prescribing equipment for billions of people.  We must make evidence-based decisions.”

Preprint (not a paper): ‘Face Coverings, Aerosol Dispersion and Mitigation of Virus Transmission Risk’ by Ignazio Maria Viola et al is under embargo until 01:00 UK time on Thursday 21 May 2020.  This work is not peer-reviewed.

All our previous output on this subject can be seen at this weblink:

www.sciencemediacentre.org/tag/covid-19

Declared interests

None received.