Masking for Covid-19
How masks affect the transmission of respiratory viruses
A legacy article originally published August 16, 2021 under the title “Big Covid Post #2”
A family member recently asked me the following question:
There has been a lot of mixed messaging from public health officials about masking. I have heard arguments about why masks don’t (and cannot) work and have even read some studies concluding that masks do not work. I am also aware of data on the benefits of masking. And beyond the question of whether or not masks can reduce the spread of Covid there is the issue of the negative externalities of masking. That is, even if masks are beneficial, are we sure they don’t do more harm than good? Can you untangle this web?
I promised to try. But this is going to require a bit of a journey, particularly into the amazing science of N95 masks. I will start by dividing this question into parts. First, what exactly do masks do physically to impact the spread of any respiratory virus, Covid or otherwise? On this topic I will take both a micro and macro approach. In the micro approach,I will examine the physical interaction between air and mask fibers as air moves antegrade and retrograde over the mask. Since as Tim Minchin said, “most of society’s arguments are kept alive by a failure to acknowledge nuance”, I will not gloss over that nuance. I will explore with you the impact of proper mask fitting and handling as well as the effect of a “false sense of security”. In the macro approach to follow in a separate post, I will examine the effect of population-wide masking on the parameters in the differential equations of pandemic spread and what this means for how the pandemic can unfold over time.
First, what do masks do to reduce the spread of respiratory viruses? Respiratory viruses reproduce in the lining of the respiratory tract. This tissue is called the mucosa. Here, the concentration of viral particles (the viral load) is very high. Particulates are constantly leaving our mouths and noses and may do so in large, heavy droplets or small, light aerosols. Particles 60 micrometers (1/400th of an inch) in diameter or larger are called droplets. They are heavy and usually fall to the ground within 6 feet which is the reason 6 feet is used for social distancing. But in a sneeze they can travel 3 or 4 times farther. Loud talking or singing or coughing can project them some middle distance. Particles 5 micrometers in diameter (1/5000th of an inch) are called aerosols. They are so light that they can remain suspended in the air indefinitely. If you have trouble believing that aerosols are constantly leaving on your breath you needn’t look any further than a cold winter day for proof. Your foggy breath is that very aerosol. Another example of an aerosol is the perfume or cologne that hangs in the air after being sprayed from an atomizer. Surgical or dense cloth masks act like a particle sieve and block the larger droplets. But if you wore one of those masks last winter or have ever wrapped a scarf over your nose and mouth you know from watching that ethereal fog appear on the other side that these face coverings do not block aerosols. To do that requires an N95.
N95s are really interesting. They are designed so that any small particle that touches a fiber of an N95 masks will stick to that fiber by a force called the Vanderwaal force in a process called capture. Any particle larger than 1/25,000th of an inch is guaranteed that its straight-line path will bring it into contact with a fiber due to the multilayered, dense weave of the N95 mask. This is called capture by inertial impaction.
Particles smaller than 1/250,000th of an inch bounce around randomly in a process called Brownian motion. (Aside: it was Einstein who first described the physics of Brownian motion in 1905, the same year he published the special theory of relativity and mass-energy equivalence). This random bouncing around guarantees that these super small particles will get bounced into a fiber. This is called capture by diffusion.
Intermediate size particles in the neighborhood of 1/100,000th of an inch are the most likely to be able pass through an N95 because they follow the airstreams through the mask. However, N95s have one more trick. The fibers produce an electric field inside the mask. This electric field exerts a force on any charged particle pushing it into a fiber. But even neutral particles will experience an electric force since the electric field in the mask will induce a charge separation within the neutral particle creating a dipole. This is called capture by electrostatic attraction.
By these methods, N95 masks will filter 99% of particles larger than 1/50,000th or smaller than 1/150,000th of an inch. The most likely particle to pass is a particle 1/75,000th of an inch. The name N95 refers to the mask’s ability to filter 95% of particles of this size.
But here, details are important. All of this only works if the only way for air to get in or out of an N95 is through the fibers. If even one slipstream of air can go around the mask, its effectiveness is diminished. So an N95 must be properly fitted. Even before Covid, those of us who work in certain positions in healthcare would periodically go through the process of N95 fit testing. The most common healthcare associated use of N95’s in the “Before Times” was in the care of a patient with Tuberculosis. Tuberculosis is a bacterium that spreads efficiently via aerosols. Because of TB, healthcare workers would from time to time need to be fitted for and use N95s. If an N95 fits properly, you won’t smell anyone’s perfume nor will you smell the stuff your coworkers left in the fridge for a month.
Because of the fact that the only way in or out of an N95 mask is through the fibers, it protects both the wearer and everyone else. Dense cloth masks and surgical masks stop droplets expelled by the wearer but allow particles of viral size (aerosols) to pass through or around them in either direction. They are therefore most useful for protecting from droplet transmission.
For all types of masks. Handling is important. Masks of course are destined to be intimately associated with your face, especially that part of your face around your nose and mouth. Suppose you take a “mask break” removing your mask and setting on a nearby table. If you place that mask on the table face-side-down and the table happens to have 1-hour-old droplets from a Covid-infected person’s sneeze on it, now the inside of your mask is covered in Covid. When you put that mask back on, you have just smeared Covid directly on your nose and mouth. The same can happen if you drop your mask on the floor or if you touch your mask with contaminated hands.
Last year, healthcare workers were trained in proper mask handling. Once I put my mask on in the morning, I often never took it off again until arriving home 8–10 hours later. This usually meant not eating or drinking all day. Those times I had to take it off, I first cleaned my hands with an alcohol solution and never set my mask down on anything. I either held it by the straps or hung it on a mask stand designed to hold masks by the straps and be easily sterilizable. Before putting it back on, I again cleaned my hands first with alcohol. At no time did I ever touch the inside of my mask. And I never stacked my masks front-to-back so that the inside of one came into contact with the outside of another. Usually, I wore a surgical mask over my N95 to protect the outside from splashes which can damage the mask and shorten its lifespan. In my car at the end of the day, I placed my N95 in a cardboard tray alone to dry out so it could be used again 2 days later. On back-to-back days, I alternated between two N95s
The other handling error that non-experts make is face-touching. Masks can be itchy. This might lead you to touch your face more than when not wearing one. I frequently see people reach under their mask to scratch their nose. This behavior punches a huge hole in the effectiveness of a mask. For 14 years I have spent about half my work week doing invasive procedures including pacemaker operations. These procedures are done under sterile conditions so I am used to wearing a mask. Doctors and nurses in procedural and surgical specialties are used to ignoring those face itches for hours on end. Do our foreheads and noses itch in the middle of surgery? Of course they do. But our gloved hands are sterile and must stay that way. Also, they are covered in nasty stuff. If I scratch my face, my hands are no longer sterile and my face will be covered in blood and guts. So we learn to ignore all those sensations. In fact, for a while in the spring of 2020, I wore gloves not because I thought they would directly protect me from Covid, but because wearing the combination of gloves-and-a-mask put me mentally into “surgery mode” where I was tuned-in to what my hands were doing and where they had been. In that minset, it was natural for me to ignore itches and not touch my face.
Often, people opposed to masks will quote one of the (very few) studies that purport to show that masks don’t work. All of these studies suffer from some kind of flaw. The flaw may be a logical fallacy (usually correlation-causation fallacy) or a methodological flaw (such as comparing a population trained in mask handling with a population lacking such training). On the other hand, proponents of masks will declare masks to be the most effective way to halt this pandemic. This, too, is false. Make no mistake, in the absence of a vaccine, the best way to end a pandemic is to sequester the infectious (or possibly-infectious) from the susceptible. This is the role of isolation, quarantine, and contact tracing (collectively called “containment”). To deal with the reality that contact-tracing, isolation and quarantine are often leaky, masking effectively plugs those leaks. Containment will be covered further in the post on epidemiology. But as discussed above, when it comes to masks, details matter. When properly selected, fitted, and handled, and in combination with isolation, quarantine, and contact tracing, masks are a powerful tool for containing a respiratory pathogen.
Tomorrow, we will dive into epidemiology to see how masks and vaccines affect the mathematics of epidemic spread.
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Credit to https://preonsaw.blogspot.com/2020/07/how-smart-is-n95-mask.html for the Gif’s and the science of N95 masks.
Credit to cdc.gov also for the science of N95 masks.