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Why Masks Don’t Work Like Sieves!

Why Masks Don’t Work Like Sieves!

Recently, more than ever before, there have been a lot of misconceptions floating around about masks and filtration. From how effective masks are against small particles to how they actually operate, we've seen many questions arise over the past two years.


A few weeks ago, we put together an article discussing ten common mask misconceptions. While that article covers many of the most common misconceptions that we see, today we wanted to take a deeper dive into a particular misunderstanding that regularly arises.


This topic revolves around what particle sizes masks and filter media are effective against. Specifically, we want to tackle the false belief that masks are less effective against smaller particles. In actuality, smaller particles are often filtered more effectively than their larger counterparts!


When it comes to air pollution, it is usually PM1.0 and PM2.5 particles that users are seeking protection from. These particles average 1.0μm (1000nm) and 2.5μm (2500nm) in diameter, and masks are effective protection methods. 


However, while these particles average 1000nm and 2500nm in diameter, there are far smaller, potentially harmful particles out there. Recently, all attention has turned to COVID-19 particles. While seldom naked, these particles can, in theory, be as small as 70-90nm (1). 


With COVID-19's naked particles being less than 1/10th the size of PM1.0, it's drawn into question whether or not masks are adequate respiratory protection. Unfortunately, this has led to a common belief that masks are ineffective against such small particles. After all, these tiny particles can pass through the sieve that is a mask filter. Right?


Not at all. The microfibre and nanofibre filters used in masks and respirators are often very effective against even nanoparticle. In fact, these protective devices have higher filtration efficacy against these ultra-fine particles than some larger particles! In this article, we want to explain exactly how this phenomenon occurs. 


In order to best explain mask filtration performance across particle sizes, there is one more misconception that first needs to be addressed. That is, how masks filter particles. While commonly thought to filter through a sieve-like function, masks are far more complex and rely on various methods to filter. With that in mind, this article will discuss the following points in detail:

 

  • How masks filter particles (not like a sieve!)
  • How smaller particles are filtered with higher efficiency 
  • Most penetrating particle size phenomenon

Masks are Not Sieves

There is a belief that the fibrous filters used within masks operate similarly to a sieve. That is to say; they have holes (pores) that are extremely small. As particles try to pass through these pores, they are captured as the pore diameter is less than the diameter of the particle. 


This leads to the belief that these filters are ineffective against particles that are only nanometres in diameter. From this point of view, this logic makes total sense. If the particles are too small to be caught by the filter, they can flow through and reach the wearer -  much like how water or fine particles can pass through a sieve. 


However, this is merely a misconception. In actuality, masks are not at all comparable to a sieve. A range of mechanisms are simultaneously in action to catch particles attempting to navigate through a mask filter. 


Some particles, such as pollen particles that average 30μm in diameter, can indeed be sieved due to their large size. These fine particles cannot enter the filter as they quickly encounter pores too small for them to pass through. The issue is that most particles are too small to be sieved - especially PM1.0, PM2.5 and viral particles. 


In fact, melt-blown filters commonly used in N95 and equivalent respirators have an average pore size of 29.285μm (2). At this relatively large size, sieving isn’t effective at all. Nearly all the harmful particles that masks are designed to filter are smaller than 30μm! For example, PM1.0 and PM2.5 are only 1μm and 2.5μm, respectively.


What actually happens is that particles enter the filter and are then subjected to a range of mechanical filtration mechanisms. These mechanisms are all physics-based and can capture the vast majority of particles navigating through the filter - even those as small as virions.


Together, these mechanisms can remove almost all particles from the airstream before they reach the wearer. In this way, masks can provide high filtration efficacy despite not sieving particles that attempt to pass through. 


Filtration Mechanisms

There are four key mechanical mechanisms that allow for particle filtration within masks and respirators. These mechanisms each impact different particle sizes, with some being more effective against larger particles and some being more effective against smaller particles. Let’s begin by introducing the four physics concepts that lead to filtration.

  • Gravitational Sedimentation: Gravitational sedimentation occurs on larger particles and is sometimes called gravitational settling or simply settling. Usually impacting particles between 1μm - 10μm, gravitational 
    sedimentation occurs when the forces of gravity act on particles. Gravitational and ballistic forces will cause particles to fall, where they will come in contact with a fibre and adhere to it.

Gravitational settling affects only larger particles as the forces of gravity have a greater impact on them. Gravitational and ballistic forces have less impact on smaller particles, and therefore, this mechanism can’t be relied upon to filter particles <1μm in diameter.

  • Inertial impaction: inertia is the resistance of a physical object, in this case a particle, to any change in speed or direction of motion. In the case of particles, inertia is the resistance to a change in directional motion. 

Filters are made up of thousands of microfibres, each randomly placed. Since air needs to pass through the filter in order to allow the wearer to breathe, air streams will navigate through the fibrous media. 


However, as particles attempt to follow the air streams, some will have too much mass, and inertia will impact them. This means that the particle will not be agile enough to follow the tight turns that are encountered along the air streams. Particles will then stray from the airflow and get caught by a fibre, settling on it.


Since only bigger particles and those with higher densities or velocities are significantly impacted by inertial impaction, this mechanism only affects particles > 1.0μm. Therefore, for larger particles > 1.0μm, inertial impact and gravitational settling play an essential role.

  • Interception: Interception is more straightforward. This occurs when a particle with a large enough diameter follows the streamline yet passes too close to a fibre and adheres to it. 

For impaction to occur, particles must stray from a streamline. However, interception occurs when particles follow a streamline, yet that streamline passes too close to a fibre. In other words, there is no divergence from the streamline when interception occurs. 


Interception is most effective against particles that are 0.6μm or smaller. These particles are less affected by impaction and instead fall prey to interception. Along with Brownian diffusion, interception has a key role in removing ultra-fine particles from the airstream.

  • Brownian Diffusion: the final mechanical filtration mechanism, diffusion, catches particles as they bounce around the filter. This phenomenon is labeled Brownian Diffusion and occurs when particles > 0.2μm in size move seemingly randomly. In reality, these tiny particles are actually being bombarded by even smaller gas molecules which influence their movement.

This causes these tiny particles to move choatically through the filter, often straying from streamlines. During this random movement, most particles will come in contact with a fibre and adhere to it. 


Brownian Diffusion as a filtration mechanism becomes more prevalent as particle sizes become smaller. Both smaller particles and those will less velocity will be more greatly impacted by this mechanism.

  • Electrostatic attraction: This mechanism is not mechanical, but it is worth mentioning as it is commonplace in respirators and masks. On top of this, it has some further implications that are discussed in more detail here (link to article). 

Electrostatic charging is a process through which filter media in a device is given a static charge. These electrically charged fibres can then filter both neutral and oppositely charged particles in the airstream. This filtration method is especially effective against nanometer-sized particles that are otherwise able to navigate through the network of fibres that make up a filter.


In conjunction, these four mechanical filtration mechanisms, sometimes in collaboration with electrostatic attraction, can capture the vast majority of particles attempting to navigate through the filter of a mask. Only a single mechanism will capture particles at some particle sizes, while at other sizes, mechanisms may work simultaneously. 


In order to give these mechanisms the best chance of capturing passing particles, manufacturers tend to use multiple layers of non-woven materials. These materials are the opposite of woven - sporadically laid out in a web-like fashion. These individual web-like networks are then layered to a point where they can capture particles yet are still breathable.


While the mechanisms above are highly effective at removing most fine and ultrafine particles from air streams, there are a few weaknesses. Not all particle sizes are filtered equally, and that creates an occurrence called the MPPS.


What Is MPPS?

MPPS stands for ‘most penetrating particle size’, and it represents the size at which particles are most likely to penetrate a filter media. When I first heard of this concept, I felt sure that it must be the smallest particle - after all, surely the smallest particles are the ones most likely to navigate through a filter successfully?


Surprisingly, this is not the case. All of the filtration mechanisms described above actively remove particles from airstreams in the mask. However, while some mechanisms are more effective against larger particles and some against smaller particles, there are weaknesses along the way.


The most penetrating particle size (MPPS) is typically around 0.3μm (4). Therefore, filtration efficacy is at its lowest against particles in this particular size range. That is to say, both particles larger AND smaller than 0.3μm are filtered with more efficiency than particles that are 0.3μm.


While some filter media will have a different MPPS, the concept exists on all masks and filters. There is always a particle size that is most likely to penetrate a filtration device, which can sometimes differ. On typical masks and respirators, however, the MPPS is close to 0.3μm. 


If you have ever dug into the certifications behind a mask or respirator, you will likely have encountered the filtration efficacy test results. These results are carried out in a lab to judge whether or not a mask is capable of a given filtration standard (such as N95, KN95, KF94, or otherwise). 


These tests are often carried out using test particles with a mean diameter of 0.3μm. While some testing procedures will vary, this is the industry standard test size. This is because these tests represent a ‘worst case' scenario for the mask - this includes testing the device against the most penetrating particle size.


Why Does MPPS Occur?

Now that we know what the MPPS is, it’s time to discuss what causes this phenomenon. After all, it seems very counterintuitive that a mask is actually more effective against smaller particles! So let’s jump straight into it.


The most straightforward way to describe the MPPS is to say that it is a weakness between the mechanical filtration mechanisms present in masks. This weakness occurs as the core filtration mechanisms switch. To explain this in more detail, we can identify two critical, opposite regimes regarding filtration. 


The first regime is the interception-dominant regime. Interception most greatly impacts particles in the mid-size range. For particles larger than 0.3μm, interception will be the most prevalent filtration mechanism. As such, filtration above 0.3μm (to around 1.0μm) is named the interception-dominant regime. However, as particles become smaller, diffusion takes over to become the most important filtration mechanism (5).


Below 0.3μm, the diffusion-dominant regime will come into play. Diffusion is most effective against even smaller particles and operates less efficiently against particles 0.3μm or greater in diameter. 


The issue is that while the interception-dominant regime is highly effective against particles larger than 0.3μm and the diffusion regime is the opposite, being most effective against particles smaller than 0.3μm, this leaves a gap in filtration. 


At 0.3μm, neither interception nor diffusion is dominant. Instead, they both operate at less effectiveness. This leads to the MPPS, where diffusion and interception occur simultaneously but at reduced efficiency.


Some factors can lower the MPPS of a filter media. One of the most common ways this is achieved is by using electret filters. Electrostatic filtration works very differently from the aforementioned mechanical filtration mechanisms and is present in masks that have undergone static charging.


Where the MPPS for filters that rely on mechanical filtration occurs at 0.3μm, filters with an electrostatic mechanism tend to experience a far lower MPPS. For example, it was found that electret filters have an MPPS of around 30-40nm (0.03-0.04μm) (3).


It is worth noting that while the MPPS is the worst-case scenario for personal protective devices, filtration is still highly effective at this size. For an N95 device, the filtration at the MPPS must be equal to or greater than 95%. For N99 devices, the filtration must be consistently > 99%, even at the MPPS.


While not always the case, it’s common for an N95 or KN95 device to have significantly better filtration for particles above and below 0.3μm. The device will then fall in filtration efficacy near the MPPS but will never dip below 95% - if it does, a certification can not be received.


What Particles Exist in the MPPS?

Assuming that a mask or respirator has a typical MPPS of around 0.3μm, these are some of the particles that may be considered most penetrating due to their size (6).


Smoke (tabacco, resin, and coal): 0.01μm - 1μm

Bacteria: 0.1μm - 10μm

Paint pigments: 0.1μm - 5μm

Allergens (cat, dust mite, etc): 0.1μm - 10μm


Of course, there are many more particles out there that exist within the MPPS. However, these are a few common examples of particle types that could potentially be most penetrating for a mask.


Interestingly, other harmful particles such as virions (usually 20-100nm) are significantly smaller, while common allergens such as pollen are significantly larger at 10μm+. In theory, respiratory protection devices are more effective against these particle sizes.


However, it is worth mentioning that while the particles mentioned above range through the MPPS, they are still filtered at a high rate. While the MPPS is the worst-case scenario of a device, filtration even at this point is > 95% on an N95 or KN95 respirator.


Conclusion

In today's myth-buster article, we wanted to dispel the common thought that masks are less effective at filtering smaller ultrafine particles. However, the myth that masks act like sieves first had to be addressed to address that misconception directly. 


After reading this article, we hope you can take away some information about how masks operate on a scientific level. While seemingly very basic devices, a lot of research and development goes into creating these necessary pieces of protective equipment. 


If you're interested in reading about more common mask myths, please feel free to check out this article discussing ten common mask misconceptions. Some of them are very surprising! 


Sources:

  1. https://www.nature.com/articles/s41598-021-81935-9
  2. https://www.mdpi.com/2073-4360/10/9/959/htm
  3. https://www.sciencedirect.com/science/article/pii/S2452199X20301481
  4. https://iopscience.iop.org/article/10.1088/1757-899X/609/3/032044
  5. https://www.eeer.org/journal/view.php?number=1044
  6. https://www.coloradoci.com/bin-pdf/5270/ParticleSize.pdf


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