This comment was initially a response to a youtube video from Tech Ingredients – a channel I have in the past thoroughly enjoyed for their in-depth dive into scientific and engineering aspects of various heavy on engineering DIY projects. Unfortunately, I am afraid that panic around COVID19 has prevented a lot of people from thinking straight and I could but disagree with the section on masks.

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Hey there – Engineer turned biomedical scientist here. I absolutely love your videos and have been enjoying them a lot, but I believe that in this specific domain I should have enough experience to point out what appears to me as overlooked and is likely to chase drastically your recommendation on masks.

First of all, the operation room masks and the standard medical masks are extremely different beasts – if anything their capacity to filter out small particles, close in size to droplets transporting COVID19 at the longest distance is much closer to N95s than those of standard medical masks:

The standard medical masks let through about 70% of droplets on the smaller end of those that can carry SARS-CoV-2. A decrease in exposure of such magnitude has not been associated with a statistically significant reduction in contagion rates in any respiratory transmitted disease.

So why are standard medical masks recommended for sick people? The main reason for that is that in order to get into the air, the viral particles need to be aerosolized by coughing/sneezing/speaking by a contaminated person. The mask does not do well at preventing small particles from getting in and out, but it will prevent, at least partially the aerosolization, especially for larger droplets – that will contain more viruses and hence be more dangerous.

Now, that means that if you really want to protect yourself, rather than using a mask, even surgical, it’s much better to use a full face shield – while useless against aerosolized particles suspended in the air, it will protect you from the largest and most dangerous droplets.

Why do medical people need them?
The reality is that without the N95 masks and in immediate contact with the patients, the risk of them getting infected is pretty high even in what is considered as “safe” areas – as well as passing the virus to their colleagues and patients in those “safe” areas. If let spreading, due to the over-representation of serious cases in the hospital environment, it is not impossible that the virus will evolve to forms that lead to more serious symptoms. Even if we can’t protect the medical personnel, preventing those of them who are asymptomatic from spreading the virus is critical for everyone (besides – masks are also for patients – if you look at pictures in China, all patients wear them).

Second, why did WHO not recommend the use of N95 masks to the general public at the beginning of this outbreak, whereas they did that for SARS-CoV in 2002-2004 outbreak almost as soon as it became known to the West?

Unlike the first SARS-CoV, SARS-CoV-2 does not remain suspended in aerosols for prolonged periods of time it does not form clouds of aerosolized particles that remain in suspension and can infect someone who is passing through the cloud hours after the patient who spread it left. For SARS-CoV-2, the droplets fall to the ground fairly rapidly – within a couple of meters and a couple of minutes (where they can be picked up – hence hand washing and gloves). Due to that, unlike SARS-CoV, SARS-CoV-2 transmission is mostly driven by direct face-to-face contact with virus-containing droplets landing on the faces of people in direct contact.

Situation changes in hospitals and ICU wards – with a number of patients constantly aerosolizing, small particles do not have the time to fall and the medical personnel is at less than a couple of meters from patients due to the place constraints. However, even in the current conditions, the N95 masks are only used in the aerosol-generating procedures, such as patient intubation.

Once again, for most people, face shield, keeping several meters of distance and keeping your hands clean and away from your face are the absolute best bang-for-buck there is with everything else having significantly decreasing returns.

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PS: since I wrote this paper, a number of science journalists have done an excellent job at doing in-depth research on the subject and write up their findings in an accessible manner:

• Should Everyone Wear A Mask In Public? Maybe—But It’s Complicated
• The Atlantic: Everyone Thinks They’re Right About Masks

In addition to that, a Nature study has been recently published, indicating that while masks are really good at preventing large droplets formation (yay), when it comes to small droplets formation (the type that can float for a little bit), it’s not that great for Influenza. The great news is that for Coronavirus, since there are few droplets of that size formed, it works great and containing any type of viral particles emission: Nature Medicine Study.