Wearing a mask: effective or idiotic?

I read a post on Facebook recently that suggested that wearing a mask was akin to trying to stop a mosquito with a chain link fence.  It is a pretty apt analogy, especially when you look at the relative sizes of viruses and mask pore sizes:

• Typical Coronavirus size  ~ 0.125 microns (reference 1)
• Size of pores in a surgical mask = 80-500 microns (reference 2)

So the virus is approximately 1000 times smaller than the holes in the mask!  How, then, can the mask possibly stop the transmission of a virus and keep us all safe?

Evidence suggests that masks are, indeed, effective.  Further, state governments are recommending them and businesses such as supermarkets are requiring them, so what gives?  Is this foolish or a good idea?

It turns out that the answer to this seemingly simple problem is a little bit more complex than meets the eye.  We need to examine two different aspects of the problem in detail:  First, we will look at the structure of the virus and its immediate environment and second, how masks actually work.  

Coronavirus Structure

In order to get some insight into the virus and the environment that it lives in, let's shrink ourselves down in the magic shrinking machine (sort of like the movie "Fantastic Voyage") and examine the virus particle.  We are going to reduce your size 1 million times so the virus appears to be about the same size as a softball.

The first thing you will notice is that the virus is covered with spikes that resemble the cloves you'd use in your recipe to cook a ham (see Figure 1 below).  These spikes are made up of a three part protein complex that the virus uses to enter and infect a cell (perhaps we will explore this more in a future blog post).  

Figure 1: The Coronavirus

 The next thing you will see is that, between the spikes, the virus is coated in a soft, kind of fatty and soapy looking membrane.  This is the viral envelope and it wraps the core.  The Coronavirus is an enveloped virus, and it needs this membrane (and the spike proteins) in order to survive and propagate (reference 2a).  

(Note that non-enveloped or "naked" viruses have a protein-based shell, and can survive in more types of environments than enveloped viruses (reference 3).)

The Viral Membrane

It turns out that most enveloped viruses need water to survive.  Let's take a closer look at one of the reasons - this has to do with the viral membrane.  It consists of thousands of long molecules called phospholipids (refer to Figure 2 below).  These molecules have a phosphate at one end (the little oblong disc in the picture) followed by two long chains of oily carbons (the twin tails in the picture below).  

Figure 2: A phospholipid molecule

The phosphate carries an electrical charge which makes this end of the molecule like to be in water.  The oily tails hate to be in water (oil and water don't mix).  So, (referring to figure 3 below), when you put a bunch of these molecules into water, they self-assemble into a double-layered structure, with these charged phosphates facing outward and the oily carbon chains facing inwards. This allows all the oily tails to be together without the water and the water-loving heads to all stick out into the aqueous environment.  

Figure 3: A Phospholipid bilayer

This structure is called a phospholipid bilayer (and is actually the key to all cellular and multicellular life on the planet).  Further, this is the membrane that wraps the virus, holds the spike proteins, and is absolutely required for the virus to infect someone.

What would happen to the virus if the water were removed?  All of those phosphate "heads" would be unhappy.  The membrane would become unstable and the virus would not be able to remain viable for very long.  So, in order for the virus to remain happy, it needs to be surrounded by water.  

Viral Transmission

As we learned in the discussion above, when a person is infected and shedding viruses, the virus particles that are most likely to survive and infect other people will be surrounded by water - water from the droplets that are produced when people talk, sneeze, cough, or even just breathe.  It is highly unlikely that individual virus particles will survive outside of these droplets since their membranes will quickly fall apart.  Droplets from an infected person typically contain many (thousands...) of viruses and will either be suspended in the air or precipitate out and collect on surfaces (infected surfaces are called "fomites" by epidemiologists).  

(Note that there are several paths of transmission that epidemiologists specify that are important for the coronavirus (reference 4). Perhaps we will explore these in a future blog post.) 

The sizes of these droplets are considerably larger than the virus itself.  As an example, recent measurements of the size of droplets that occur while people are speaking have been found to be between 4 and 21 microns in size (reference 5).  These are the sizes of particles that we must filter out!

So we have made some progress.  We started with a particle size of 1/8 of a micron and we now know that they come in much larger droplets of at least 4 microns in size.  But the pore sizes of masks are far larger than 4 microns.  The question remains of how mask can be effective with a pore size of 80 (or more) microns and protect us agains these far smaller particles?  

To find the answer, we must delve a bit into the theory of operation of masks.  

Masks:  Theory of Operation

Filtration by masks do not operate with the same principles as filters for liquid media.  A mask must allow the wearer to continue to breathe as particles are trapped - so the design is quite different.   If a mask were designed as a sieve (i.e. directly blocking particles as they come through the mask), then the wearer would have an increasingly difficult time breathing as the mask filled up with more and more particles.

Medical masks (surgical and N95 for example) are not constructed as a simple sieve, but feature non-woven material that creates a torturous path for the particle to take through the mask.  As it is passing through, there are three ways that a particle can be stopped (refer to Figure 4):

Figure 4:  Filtration mechanisms for masks

Inertial impaction, for the larger particles, means that the particle has too much momentum to twist and turn thru the torturous path of the fiber and it gets trapped.  Diffusion is the mechanism where the smaller particles are vibrating and themselves will tangle into the fiber by moving back and forth.  Electrostatic attraction is for negatively charged particles which will cling to the positively charged fibers.  

You can now see that it is not the blocking of pores that causes the filtration of particles in the mask, but these multiple mechanisms that lead to effective filtration.  And experimentally, this has been verified quite a few times.  Here is one set of results showing the efficiency of filtration as a function of particle size (reference 6).

You can easily see that these masks are very efficient for filtering out particles of 4 microns or larger.  But they are not perfect.  Real world measurements (which also must account for misfitting and air leaking around the mask) suggest that efficiency lies between 50 and 80% (reference 7).  So if we can't be 100% safe, why wear a mask?

Viral Load

It turns out that getting a virus is not an all-or-nothing proposition.  There is a lot of evidence that the number of viral particles that you inhale will determine not only the likelihood of you getting sick, but can determine how sick you get. If you lower the overall number of viral particles (the "load"), then you lower the probability of getting sick, or if you do get sick, you'll have a less severe illness (reference 8).  

The viral load of a particular environment (say, a grocery store) is a strong function of the number of people wearing masks.  Even though the masks aren't perfect, they will significantly reduce the number of viruses floating around in droplets, especially if EVERYONE wears a mask. 

 

Asymptomatic Infections

One more objection that is often heard is that a person shouldn't have to wear a mask if he or she is not sick.  It turns out that the Coronavirus is unique in that it has an exceptionally high number of cases where people are infected and don't show any symptoms.  As many as 50% of the people who have the disease may be asymptomatic (reference 10).   

In Conclusion

It turns out that masks are more efficient at filtering outbound air (protecting others) than they are in filtering inbound air (protecting the wearer).  Therefore, you are really showing respect for other people and their loved ones by protecting them from a virus that you might not know you have.  

You are protecting your neighbor more than you are protecting yourself by wearing a mask.  Not doing so for selfish reasons can become a classic example of something called the Tragedy of the Commons (reference 9).  Read all about it in the reference.  We can avoid this if and only if each of us wears a mask when in a public setting.