AEROSOL TRANSMISSION OF SARS-COV-2: PHYSICAL PRINCIPLES AND IMPLICATIONS. UPDATE ON MASK DESIGN AND PERFORMANCE (JANUARY 18, 2021)
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Aerosol transmission of SARS-CoV-2: Physical principles and
implications.
Update on mask design and performance (January 18, 2021)
Michael C. Jarvis
School of Chemistry, Glasgow University, Glasgow G12 8QQ, Scotland, UK.
Email: michael.jarvis@glasgow.ac.uk
________________________________________________________
Summary
Low expectations of the reduction in SARS-CoV-2 transmission that can be achieved by
wearing masks in public are currently justified by their poor effectiveness in trapping
infective aerosols. This can be changed. The key improvement needed is in quality of fit,
which can be achieved with ‘mask fitter’ devices. Simple cloth masks can be effective but
are less likely to be useful where infective aerosols build up over time.
________________________________________________________
Introduction
In August 2020 I completed a review on aerosol transmission of SARS-CoV-2, which was published in
November1. Because this paper spent so long in the review process while a deluge of new publications
were appearing, I forwarded to the Committee an update on implications for ventilation of public
buildings2. It was my intention to prepare a further update on implications of aerosol transmission for
the design and efficiency of masks, but at the time a consensus on the effectiveness of masks used by
the public against aerosol infection was hard to establish. Since the New Year that has changed, and I
think an update on mask design might now be helpful, with relevance for example to the prospect of
re-opening schools. The number of relevant new publications is quite small so Instead of providing a
detailed review, I have merely prepared some notes alerting the Committee to the key papers.
At the beginning of the pandemic, when the importance of aerosol transmission was not yet
recognised, it was assumed that almost any form of face covering would catch large droplets emitted
when a symptomatic infected person coughs. Face coverings would then give some protection to
others, but not to the wearer. All that remains true for transmission through droplets above aerosol
size. For a face covering to stop transmission by aerosols, including transmission from asymptomatic
infected people, the requirements are different and more challenging.
Do masks currently reduce overall transmission?
By the Summer of 2020 it was being suggested that wearing of face coverings in the community
seemed to reduce transmission of SARS-CoV-21, even though the wide variety of face coverings in
public use were known to have low average efficiency in blocking aerosol transmission, compared with
masks used by health professionals.Since December 2020 two systematic reviews 3,4 and several regional studies5-9 have confirmed that despite their inefficiency the wearing of face coverings in the community leads to significantly reduced overall transmission, although quantitative variability is wide and the low quality of the evidence has been highlighted3. These studies do not distinguish between aerosol-transmitted infection and infection by other routes. The relative importance of aerosol transmission is still not well quantified overall and varies according to the setting, with the relative risk being greatest in inadequately ventilated spaces, e.g. in public transport2. The previous review included evidence that, although many of the cloth materials used for improvised or fashion-based face coverings were probably inefficient in filtering aerosols, a more universal problem was poor fit to the face of the wearer2. Poorly fitting face coverings might catch large infective droplets when the wearer coughed, but aerosols move freely in either direction with the airflow round the edges of the mask, the ‘steamed glasses’ effect being diagnostic10. Because the gaps expand when the wearer is breathing out, the advice that a mask gives less protection to the wearer than to others may not apply to infection via aerosols. These problems were evident when the previous review1 was published, but solutions were not. That has changed. There is now enough information to make substantial improvements to the average effectiveness of masks worn by the public, focusing on the least effective designs. Reducing leakage Medical respirator masks are fitted to the individual and a leak-proof fit is tested with specialised equipment11. That is not practicable for masks worn by everyone12. A recent preprint on masks commonly used in the US13 showed that where the filter material was effective, as much as 50-80% of particles in the aerosol size range could escape through gaps around the edges. Also, where cloth masks are fitted with a pocket to take a high-performance filter, much of the airflow is likely to pass through where the resistance is least, through the cloth outside the filter pocket. A recent preprint by Rothamer et al.13 showed that adding a ‘mask fitter’ device to hold the outer part of the mask in contact with the face reduced escape to the ̴10% expected for the better filter materials, although it remained >70% where the filtration efficiency of the material itself was low13. These observations are not peer reviewed, but if accepted they would have two implications: first, that masks can contribute much more to preventing transmission in both directions if the average quality of fit can be improved; and second, that ‘mask fitter’ devices are a potentially practicable way to achieve improved fit. A ‘mask fitter’ is a flexible frame fitted over the top of a conventional mask and manufactured or adjusted to fit closely against the wearer’s face. Their design is suitable for improvisation https://making.engr.wisc.edu/mask-fitter/, inexpensive mass production https,://www.kickstarter. com/projects/essentialbrace/essential-brace-designed-to-seal-your-loose-fitting-mask or tailored manufacture by 3D printing based on facial recognition software https://bellus3d.com/ solutions/facemask.html. The principle could probably be combined into the design of improved masks.
Filtration materials Medical masks with relatively high-efficiency filters are now more readily procurable than at the start of the pandemic and are well suited to the addition of a mask fitter device to stop leaks and bring their effectiveness closer to the design specification. The alternative is cloth masks6, requiring that the fabric is chosen to give reasonable filtration efficiency when leaks are avoided14. It was known in mid- 2020 that there were some fabrics and nonwoven (felted) materials that performed quite well in filtration tests15; not as well as the nonwoven polypropylene fibres used in medical masks, but enough to be useful if leakage could somehow be avoided14. However the lack of systematic comparisons and of accessible descriptions of the theoretical basis of aerosol filtration made it difficult to draw useful conclusions. One recent paper in particular goes some way towards filling this gap. Drewnick et al16. compared twenty readily available fabrics and nonwovens, showing that several common fabrics such as terry cotton or silk, especially in multiple layers, filtered >80% of particles in the >5 m size range. Their efficiency dropped off steeply for particles further down into the aerosol size range16, prompting the question, which droplet size range matters most in SARCoV-2 transmission? A clear answer to this question is not yet available, partly due to confusion between number-weighted and volume-weight size descriptors1 and partly due to variation in the relative importance of aerosol transmission between environmental settings2. Drewnick et al. also give a convenient description16 of the physical basis of aerosol filtration, much of which is non-intuitive. It might be imagined that materials with smaller pores than the particles being filtered would be needed, but it is not as simple as that. Diffusion and electrostatic attraction can contribute to capture by filters with larger pores, although electrostatic effects require the aerosol to be charged16 and for respiratory droplets the charge is uncertain. Single-layer fabrics have relatively low airflow resistance, but airflow obviously limits the scope for multiple layers 16. The underlying physics of airflow resistance, like filtration efficiency, is non-intuitive for finely fibrous materials17,18. Despite these uncertainties it seems worthwhile to persist with cloth masks based on multiple layers of cotton or other fibres, either using them with a mask fitter or incorporating a similar frame into the mask design. From the range of particle sizes for which cloth materials offer adequate filtration, it is likely that they will be most effective in reducing direct, non-aerosol, disease transmission between people who are not adequately social distanced19. Their reduced efficiency for particles under 5 m16 i.e. approximately within the aerosol range, means that they will be relatively less effective in scenarios where infective aerosols build up through the working day in a confined space with inadequate fresh-air ventilation2,20. In these scenarios, for example in public transport, medical-type masks with provision for improved fit are more promising. It is not clear if earlier advice that face coverings are worn to protect others, not the wearer, might lead to their wearing being regarded as ‘just’ an official requirement and inhibit the public’s motivation to find more efficient masks. This advice is probably not correct in view of the importance of aerosol transmission. In Scotland compliance with guidance on wearing masks is quite good, but this should not be taken for granted. Where masks are to be worn for extended periods, for example by shop assistants or in schools, comfort is a significant consideration and deleterious effects of mask wearing need to be kept under observation.
Conclusion
The poor performance of many current masks leaves considerable room for reductions in transmission
to be achieved by improvements in mask design.
References
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