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A recent article by the Lancet reflected that where previous research on the use of masks in non-health-care settings had predominantly focused on the protection of the wearers, there is an alternative rationale to also protect the people around users from respiratory droplets, which in the context of a virus pandemic, places weight on different operating criteria and benefits. This is also supported by recent studies in South Korea, and Hong Kong .
My colleague Richard Owen recently wrote a blog on personal ‘bio-security’, the importance of hygiene and the limitations of some types of face masks as PPE for the general public. The benefits of masks derive from a complex interplay of technical, anatomical, and even emotional and social factors contributing to their performance, use and adoption.
While a complete understanding of how Coronavirus is transmitted is still being researched by scientists across the world, basic engineering can help us understand some of the issues and choices facing the general public. The ideal solution would be a mask that forms a two-way barrier to viruses while allowing the user to breathe normally. That way you are protected from others, and others are protected from you.
Unfortunately, there are some technical challenges. Firstly the Coronavirus is very small, in the order of 100nm in size, so they can exist in droplets that hang in the air and filters have to be quite dense, or utilise complementary technologies to form a barrier to them. Whilst it is relatively easy to make filter media that meets this technical specification, avoiding unwanted leakage between the mask and the face, that short circuits the filter, becomes the main design challenge. Because everyone has face contours that are different, and masks must fit tightly to seal, those that seal well are less comfortable to wear for long periods of time. This discomfort is exacerbated by the resistance experienced by inhaling and exhaling through the barrier membrane, leading to fatigue.
Then there is the problem of condensation inside the mask caused when the wearer’s warm humid breath condenses on the cooler mask components. This adds to discomfort caused by pressure against the areas of the face where skin is close to maxillofacial sub-structure, and by skin sensitivity to the material choice in some users.
It also stands to reason that if the mask successfully catches viruses from the environment, then over a day it will effectively concentrate them in one place. So removal and disposal of the mask requires care to avoid it becoming a direct source of infection itself.
Finally, there is a good argument that increased demand for masks from the general public will mean that supplies may be diverted away from those most at risk and in most need, healthcare workers and carers treating sick patients.
As with any medical device, design is a compromise based on the likelihood and risk of different outcomes. To completely minimise risk requires a military style NBC (nuclear-biological-chemical) suit, which is obviously not a practical solution. So the mask performance is determined by the possibility of meeting enough virus to cause disease, the likelihood of the virus getting through, or more likely around, your mask, together with the usability, availability and cost of the mask that determines if you wear it in the first place. This means different designs of mask are the best compromise in different situations, but the wearer must understand the protection they have and not stray outside the environment their set up is designed for.
There is also the question of who the mask is protecting, the wearer or the people they meet? Is the virus inside or outside the mask? Many existing mask designs have small valves to let exhaled air bypass the filter to reduce condensation and breathing effort, so these don’t provide any protection from viruses escaping.
In a population where you don’t know who is infected, then public coughs and sneezes pose a high infection risk. As Richard pointed out in his blog, particles can easily travel several meters from the person who sneezes, well beyond the social distancing 2m radius. Logic suggests that even a leaky, home-made mouth cover would absorb a percentage of the sneeze or cough, and so reduce the dose acquired by others close by. In this situation the risk to the user of concentrating their virus in the mask is irrelevant as they have the virus anyway. However, sharing masks or asking others to handle or wash them poses new opportunities for infection.
So in an environment where governments and scientists are weighing up different lock-down exit strategies, even a simple, imperfect mask appears a positive measure for those who don’t know if they have the virus but want to reduce the potential of onward infection.
In addition, there is also a psychological effect to consider. In a world where many people have been indoors for weeks, emerging for the first time back into public spaces where there may be an unseen ‘enemy’, is likely to increase levels of social anxiety.
We recently spent a fortnight undertaking immersive field research in China to develop a specification for a new technology-enabled personal pollution mask. The aim of this project was to avoid the health risk associated with inhalation of pollution in busy cities, where particles, designated as PM2.5 (atmospheric particulate matter less than 2.5 microns in size) can reach into your lungs and even be absorbed into the bloodstream.
We found in addition to the practical concerns about pollution filtration, inhale/exhale experience, fit, carriage, and maintenance, even in a pre-pandemic world some wearers expressed a strong feeling of responsibility to wear a mask as a social courtesy to protect those around them from anything they may be carrying, especially in enclosed spaces such as packed rush-hour buses.
We were also struck by the tension between the reassurance of efficacy for many adult users provided by an industrial aesthetic, against masks with a more informal or fashion-oriented design language, which were perceived to reduce confidence in their functional credentials. Children reversed this insight, with use compliance being directly influenced by designs with comic or novelty value!
In the UK, like in many European countries, pre-pandemic it was incongruous to don masks in public places. However, the low cost and low burden of incorporating them into city life seems a small price to pay to reduce infection rates. Indeed, it is a habit that may well survive the current pandemic and become a new social norm.
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