Masks and Airflow: Covid 19 Special

Returning to normalcy in Bratislava, with “terraces” allowing people to enjoy their lives.

When it comes to masks, advice and practice has been quite different across Europe, Evan Burgess, based in Bratislava Slovakia, interviews Danish air and sensor specialist Thor-Bjørn Ottosen in order to shed some light on the science informing varying approaches…
European countries such as Austria, Czech Republic, Slovakia and Ukraine were quick to introduce blanket policies on wearing masks. There is some correlation between countries that don’t wear masks and a rise in Covid 19 cases, however there are many other issues involved which could skew the data. These include population density, and countries such as Denmark, Norway and Finland having no mandatory mask wearing yet very few cases. But what are some pertinent issues to wearing masks? Will it harm or will it help?

Take your pick of folk art inspired masks!

In this article we will look at Thor-Bjørn Ottosen’s views on airflow and whether a mask could help. Thor-Bjørn is a specialist in air and sensor technology at the Danish Technological Institute.

The Knowns
Many people are wearing masks across the world right now. They can be makeshift, or with expensive filters. These covers seem to have a dampening effect and though may not be able to stop direct infection to the wearer, could well reduce the amplitude of an out breath, sneeze or cough. Governments are now encouraging people to get out more, on the basis most people’s infections seem to have occurred in a confined space. What is not clear is the long-lasting impact of wearing masks again and again. Living in Slovakia, I see people using medical masks, scarves and homemade cloths. None of which were intended for long term use, but numbers of cases seem to be low. Though it is not clear if this is because of masks, a lack of testing, a lack of susceptible demographics compared to Italy, Spain, France and the UK, or any combination of factors. What is definite though is many Slovaks travelled to hotspots before the borders closed in the middle of March.

The Unknowns
Diffusion of air is no doubt quite important in reducing contact. To what extent and how is not quite known, but guesses seem to inform policy. In Slovakia many councils have asked public transport users to only use the window seats whilst wearing masks, and where possible all windows are open on buses and trams. It also isn’t exactly clear which methods of transmission respiratory viruses such as Covid-19 are mostly spread by. Recent headlines from New York seemed shocked as those who had tended to stay in were often found to be coming into hospital with symptoms (ref 1). Due to the very generic symptoms, it may be that in some cases other agents were the cause. It might be that touch spread is a key issue. One town in Germany reported a “super spreader event” (where multiple people seemed to contract Sars Cov 2) during a carnival where kissing each other on the cheek was a key feature, along with an organiser shouting instructions to the audience and a lot of close proximity in a confined space (ref 2)

With that, it is not known how much of a dose (viral load) is needed for someone to display symptoms. It could be different from one person to the other, just like a quantity of a toxin affects people differently. Despite all these unknowns, there is  evidence that proper use of face coverings could help.

It is known the weave and filters of masks are larger than the viral particle. What is not certain is whether or not this matters. This is because particles often travel in a droplet or aerosol, which may be blocked by PPE. 

Mobile Masks

Thor-Bjørn studied the flow of air relevant to exhaust fumes from cars. Previously a postdoctoral researcher at the University of Worcester, he explains that simply standing to the furthest side of a pavement to the road could reduce exhaust fume exposure, due to dilution, and carrying a baby instead of pushing it in a pram could reduce the baby’s exposure to fumes. It really seems inches could make an impact when dealing with small particles. This is because the air moved in vortexes propelled by oncoming traffic that sucked particles into their own trajectory. Though it isn’t clear exactly how other particles spread, it indicates that a respiratory virus is not a ghost chasing people into the forests and hills. Sensible measures can be taken based on this kind of research, such as avoiding confined spaces (increasing public transport instead of reducing it may help) and when forced to do so, being aware of how air flows.

Thor-Bjørn Ottosen

1. First off, Denmark seems to be riding out this pandemic quite well. What is going on in Denmark day to day?
Denmark has followed a strategy similar to many other countries, where the entire society was shut down and is now gradually being opened again. This has apparently worked remarkably well in Denmark, where people have been very good at keeping distance and following the municipalities advice. This is most likely the reason we do not have more infected people than we have.

2. When the head of the WHO said that the virus was airborne, people were confused. Can a particle the size of a virus travel through the air, and are there any examples of distances involved from other sciences? (i.e. Tree disease).
Viruses themselves are as small as 20-30nm (10 to the power of minus 9 meters). They’re contained in droplets that range from less than 3 µm (10 to the power of minus 6 meters) up to over 6 µm. This compares similarly to other atmospheric particles such as exhaust fumes or biological particles such as pollen and fungal spores. For other atmospheric particles of different sizes, travelling 10 km, 100 km or even 1000 km would not be unusual. Thus atmospheric transport of virus particles for both short and long distance is entirely likely. Now, the main thing to worry about in this case is probably not long-distance transportation, since the longer the distance the more dilution happens along the way, so the most likely transmission through air is probably only measurable in a few meters.

3. Can you tell us about the people who normally wear masks before this crisis, and what were they used for?
Masks are a way to filter gas and particles from the air before entering your nose and mouth. They come in various materials and price ranges and target various purposes. In Asia, it is quite common to use cheap fabric-woven facemasks, which filter out coarse particles, like dust, and thus makes the air more comfortable to breathe. In Europe, some bikers have chosen to buy more expensive tight-fitting facemasks, which will filter out more particulates and gaseous material and thus give a greater reduction in inhalation of undesirable elements. Since this is a tight-fitting mask and you breathe the air through a filter, this is probably not comfortable to wear for the full day.

Take a doorway pizza! For the moment the staff must wear masks, and you must order with a mask.

4. What kind of sizes are we talking about here when it comes to filters and particles?
In general, the largest health effects of air pollution results from the smallest particles, since they penetrate deeper into the lungs. (See question 2 for more details of size)

5. Do you think it is likely a single virion could actually travel on its own, as opposed to in a droplet or vapour? Therefore perhaps the filter sizes are somewhat a red herring to effectiveness?
This is the process that will happen after a virus droplet is released into the atmosphere:

FIG. 2. Evaporation of a liquid droplet (left) to a droplet nucleus (right). As the liquid evaporates, the nonevaporative content concentrates until a droplet nucleus is obtained.

(Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2546863/pdf/0002-08.pdf)

It thus depends on the size of the droplet and the mesh size of the mask.

6. If you were sitting in a room, does the room have its own airflow dynamics? Would you sit in a library next to a stranger or away from them?
Naturally, the room will have its own airflow dynamics. Usually the indoor wind speed is too small to feel on your skin. But it doesn’t change the fact that the air is moving, as we know from when a window is open. In general, the closer you sit to another person, the more exchange of matter and energy will happen between you and the other person. For instance, you can feel the other person’s body heat and smell their breath. If this person is carrying a sickness, the chances that it spreads to you increases as you get closer. That’s why the municipalities recommend keeping your distance. Also, if the person coughs or sneezes droplets will be released in a cloud around this person. The larger droplets will quickly deposit on the floor, but the smaller droplets will diffuse through the air and get diluted by the turbulence. The larger the distance the smaller the concentration reaching you and the less chance of being infected.

7. The British government has recently encouraged cycling and walking over other forms of transport. Do you think this makes sense on an airflow level?
This probably doesn’t have too much to do with airflow, but rather with how close you are to other people during these activities. When walking or biking you will usually have a longer distance to your fellow commuters compared to a busy London tube train with reduced air exchange. The more air exchange in the microenvironment, the more dilution of particles, and the smaller the risk of getting infected.

Scenes like this may return to the UK soon, perhaps masks will be involved.

8. Are you personally wearing a mask, what reasons informed your judgement?
I am not wearing a mask, but I will not extend that to a general recommendation. Each person must make their own assessment. If you are wearing a hospital-grade mask (and use it properly), you will reduce your risk of exposure, but it will not be very convenient in the long run.

9. Why do you think the message on masks has been so erratic internationally?
As with many other aspects of this disease, there is a lack of evidence on the effectiveness of using masks in the general population. If the difference between masks and no masks is only a few infected cases, the measure is not proportional to the effect.

10. Are there any downsides to the long term uses of masks you could think of?
The downside is that they are inconvenient, and they cost money. I don’t know if there are any health effects of long-term mask use.

11. There has been a lot of talk about UV light, have you heard any of these theories and do they make sense to you? (i.e. putting lights in a ventilation system or key locations to kill bacteria/virus)
UV-light is a well-known air cleaning technology, which has a good effect on things such as bacteria and fungal spores as well as other corona viruses such as SARS. Research needs to be done to confirm that this is also the case for COVID-19.

Conclusion: Is it worth wearing a mask?
Though masks are not intended to be worn non-stop, there is consistent evidence they can dampen the effect of a cough or sneeze. Wearing masks and PPE in a professional setting (Doctor, Soldier, lab technician) requires training and even a sequence to what comes off when, and sometimes teamwork to facilitate this. There is also a lot of discipline to be aware of. Wearing masks can mean a lot of touching your face and readjustment, but if you clean your hands regularly (and the mask) then you will probably avoid or mitigate a lot of problems. Cleaning or changing masks should be part of any routine.

Ref 1 https://www.dailymail.co.uk/news/article-8293417/66-New-York-coronavirus-hospitalizations-people-staying-HOME.html
Ref 2 https://www.youtube.com/watch?v=vrL9QKGQrWk

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