WHICH FABRIC MASKS PROTECT US BEST AGAINST CORONAVIRUS?
If we want to know which masks are the best to protect ourselves, we have to consider a key point: much of the transmission of this new virus, SARS-CoV-2, occurs through aerosols. If we do not take into account aerosols, nothing I write below would matter; any fabric or material that acts as a physical barrier would suffice.
But if you also want to protect yourself from aerosols or, if still not convinced, prefer to apply the precautionary principle, read on.
You must be wondering: «If the N95 respirators (or its equivalent FFP, N95, etc.) are the best in laboratory tests, why don’t we use them all, and stop complicating our lives looking for alternatives?». The answer is simple: because there are no such masks for everyone. There were not enough respirators in March, nor are there now, nor will there be for a long time.
Almost all countries need many more masks than they have. And why not make them? Because there are not enough raw materials or machinery to manufacture all the PPE masks that any country needs, even if production increased dramatically.
The material of the N95/FFP filters is challenging to manufacture. The plastic polypropylene is first melted until it has approximately the consistency of liquid honey. Molten plastic emerges from hundreds of nozzles as threads.
But it is still far from being as thin as it will be later. In the airstream, the plastic forms a very fine fabric. The fibers have a diameter of less than 1 microns (µm) (one micron is one-thousandth of a millimeter). With a single 7-gram thread of this diameter, you could span the entire earth. Such a thread would, in turn, be enough for about two to four face masks, depending on the quality of the mask.
An electrostatic charge is added during the manufacturing process. This extra feature attracts and captures particles of all sizes.
Fibers are not the only factor in filtration. Masks of all kinds vary in filtration efficiency based on their shape and fit.
The effectiveness of any mask depends on three factors:
- Its filtering capacity (which, in our case, in turn depends on the role each particle size plays in virus transmission).
- It’s fit.
- Its breathability (the ease with which air passes through the material).
When choosing a material to make masks, the most difficult thing is finding a material that is effective enough to capture particles but allows you to breathe well enough. But it’s not just a matter of comfort: if a mask has a pressure drop that is too high (that is, breathability is low), the air will not leak but will pass around the edge of the mask (that is, low breathability), the air will not leak but will pass around the edge of the mask.
The effectiveness of a mask and the breathing resistance are inversely related. The more difficult it is for the air to pass through the mask, the more it will pass over the edges, which will decrease the mask’s effectiveness (air will follow the path of least resistance).
Loose-fitting masks allow aerosols to leak. Leaks increase the faster we breathe. For example, when we exercise (more breaths/minute). If the mask leaks because of the design or because we wear it incorrectly, it won’t be good, even if the material is good.
It has recently been shown that even very small leaks, in the order of one percent of the total sample area, can substantially reduce the overall filtration efficiency of a mask down to half or even less compared with the value of the material itself. Therefore, the leak area must be kept at a minimum.
And what does that imply in practice? That if we buy a mask with a valid certificate that only evaluates the material (filter, fabric) but that does not fit the face well, that mask will not protect us SO MUCH, even if the certificate says that it filters 99% of the particles.
Masks must fit properly to work well. Never use a mask without an excellent nasal fit (well-molded and pressed flexible strip). Give it back if they don’t fit well!
Beards and facial hair interfere with sealing and reduce the effectiveness of the mask.
Do you see any holes? Does your finger easily fit under the chin, along the sides, or above the nose? Can you feel the warm air from the sides when you exhale? Are your glasses fogging up? If you answer yes to any of these questions, the mask will not protect you well.
If a cut nylon stocking is placed over any mask with side leaks (including surgical masks), the mask’s effectiveness improves by 15 to 50%. Even if it’s a bit strange, it works great because it improves the fit and adds a layer of electrostatic charge (the nylon), making the particles stick together.
The material’s ability to trap partidles is called filtración efficiency. The filtration efficiency of any mask depends on the size of the particles. But the masks are not like strainers, but rather like spider webs.
Filtration is a combination of many capture mechanisms:
— Sedimentation, by gravity
— Diffusion, by Brownian motion
— Impaction, by inertia
— Interception, by size
— Electrostatic, per charge
These mechanisms are shown in this excellent animation.
There are friction forces and intermolecular forces. Viruses often have oily surfaces. Polypropylene is lipophilic, which means that it attracts fats. Any particle that has grease on its surface will adhere very easily to these substances.
The smallest and largest particles are the easiest to trap. The smallest particles because they are more easily removed by diffusion and electrostatic forces, and the largest because they are more easily removed by impact.
Medium-size particles are the hardest to filter. They evade capture because they follow the air flow, twisting and turning around the fibers. This is why the N95 tests are done at ~ 0.3 µm (microns), the “difficult” test, which is in the area of the most challenging particle size to retain.
But what sizes do we have to filter? It is not the same to block a ping-pong ball than a basketball.
We know that this SARS-CoV-2 virus is spread mainly through aerosols. And, thanks to experts like José-Luis Jiménez or Kimberly A. Prather, we also know that the drop-aerosol cut-off point is ~ 100 µm. But it is not the same to have to filter a 0.1 µm particle as a 100 µm one. They are all aerosols. But if a virus particle were a ball, a 100 µm particle would be the entire stadium.
If we listen to the WHO, we only have to worry about particles larger than 5–10 µm (‘droplets’). In that case, ANY mask is good.
ALL masks are good at blocking ballistic drops released by the user or those that can hit the user’s face when they are close to other people. According to the WHO, these drops are responsible for the contagion, along with the fomites. All cloth masks reduce the jet of expired air’s speed and radius, partially retaining the virus and thus limiting its spread. The puff of air doesn’t go that far. But we do know that most (more than 90%) of infectious aerosols range in size from ~ 0.5 µm to 10 µm, with a maximum between 1 and 2 µm.
Measuring cloth masks’ effectiveness (and paper masks) is not as easy as that of medical masks. The detachment of small cellulose fibers from the mask (from fabric or paper) increases confusion and “noise.”
ALL common fabrics have low filtration efficiencies for 0.3 µm particles; SOME blocks more than 50% of the 1–2 µm particles and MOST of them more than 50% of the particles larger than 5 µm.
In Europe, the hygienic mask (the cloth ones) test and the surgical mask test are done at ~ 3 µm. It is the so-called BFE (Bacterial Filtration Efficiency) test. As seen in the following figure, a mask can have good filtration efficiency at 3 µm and a much lower efficiency below 1 µm. Therefore, it would be convenient to design a test that considers the characteristics of infectious aerosols (size, etc.) and establishes parameters that better reflect the real situations in which masks are used.
The filtration efficiency of cloth masks depends on many factors, including the number of threads, thickness, type of cloth, water resistance, number and type of layers, design, and fit. With the right combination of all these factors, we can obtain homemade masks as good as medical masks. It has been estimated that a cloth mask, used correctly, can reduce the risk of exposure to SARS-CoV-2 by 2 to 10 times. But there are also terrible masks (silk scarves, wool bandanas …), which only filter 10 %.
There are several ways to improve a fabric mask’s effectiveness: with a denser weave and with more layers. A well-designed fabric mask should have a waterproof fabric, multiple layers (at least two or three), and a tight fit around the edges. It is essential to include a flexible material to adjust it to the bridge of the nose. It is also important that the stitches and seams are tight and tight. Furthermore, a good mask will have a large surface area that leaves space around your nostrils and mouth.
And what materials to use? In general, woven fabric is not as effective as the nonwoven material. The fabric is very resistant to airflow. The threads’ fibers are too tight for air to flow well, and the pores between the yarns allow particles to pass through.
The mask’s effectiveness improves as the filtration area increases and the size of the pores decreases. That is why the fabric structure is essential. The best fabrics have small fibers, small threads, many threads, and tight threads. Tightly woven and small-fiber fabrics hold better but have the disadvantage that they are more difficult to breathe with.
For each type of material, the filtration efficiency is higher for the material with the highest number of threads (threads per inch, TPI). However, this is not a general characteristic when different fabrics are compared.
Recently, 44 masks made from homemade materials and various medical masks were compared. The best materials were those designed to filter aerosols (medical masks and vacuum bags) and soft textiles, such as plush, fleece, felt, the cotton from which bandages are made, and velvet. Cotton muslin and microfiber were also acceptable. Very fine or very porous fabrics such as silk, polyester, linen, or cotton in T-shirts had extremely low filtration efficiencies.
Face masks made of cloth materials can reach decent filtration efficiency over a broad particle size range when stacked to an adequate number of layers — especially if we use materials designed to filter aerosol particles, or fluffy textiles like, e.g., French terry, fleece, felt, or velour.
On this website, you can compare some materials: http://jv.colostate.edu/masktesting/.
The performance of a mask can be increased by layering materials, but only up to a point. When increasing layers of materials, the capture of particles is not additive. But the pressure drop is additive.
Therefore, we can add layers and not increase the mask’s effectiveness because resistance to air passage is detrimental. Wearing two masks is not the same as doubling the protection. However, an outer layer that ensures a better seal (such as a cut over nylon stocking) can be beneficial.
In general, if we want to achieve filtration efficiencies greater than 50% for particles smaller than 2 µm, we need to add a FILTER to the fabrics. Filters can greatly improve the performance of a mask without making it difficult to breathe. The following figure shows an example of how you increase filtration efficiency by adding a filter. After adding the filter, an efficiency similar to that of the N95/FFP2 masks is achieved.
But we must pay close attention that the air does not leak around the filter’s outer edge. Because then the air will not pass through the filter, but through the edges, as shown in the following figure.
That is, designs like the one in the photograph are not worth it. Air follows the path of least resistance.
Vacuum bags and household HEPA and MERV13 air filters from furnaces and air conditioners retain a very high percentage of particles of different sizes (between 80 and 100%, depending on the particle size). Some of these filters are made from the same materials as those used to manufacture N95 and surgical masks..
This material consists of nonwoven fibers that carry permanent electrostatic charges to improve very small particles’ retention. The most widely used material is polypropylene.
The problem with air filters is that some might release tiny fibers that would be dangerous if inhaled. For this reason, it is sometimes recommended to insert the filter (one or several layers) between two layers of cotton fabric.
But there is a BETTER solution: get the filter raw material directly from the manufacturers, instead of reusing boiler filters, anti-allergens, etc. This polypropylene is the material of the filters used in the experiments shown in the figures, which can be consulted on this website: Melt Blown (MBP) — MBBFE95–42100 (airfilterusa.com) and MERV13 — M13FMR-20125 (airfilterusa. com).
In addition to polypropylene, a very promising material is made from finely cross-linked nanofibers. This material achieves better particle blocking efficiency with less thickness than conventional filters.
In summary, any mask reduces the likelihood of getting COVID, and probably any mask makes the infection milder. When everyone wears a mask, the combined filtration efficiency increases. Still, we must try to find the optimal materials and design, bearing in mind that what we have to filter are infectious aerosols of sizes majority between 0.5 and 10 µm.
Much of the information on this post has been extracted from this excellent webinar. I am grateful to Professor John Volckens (Colorado State University) for permission to disseminate the figures and results shown in this article. Likewise, I thank Professor José Luis Jiménez (Univ. Colorado) for reviewing and insightful comments.