The Engineering & Science Behind N95 and FFP2 Respirators???

In today's health environment, the safety of high-quality personal protective equipment (PPE) remains paramount and whilst the immediacy of COVID-19 might have faded for many, healthcare professionals continually depend on N95 respirators to shield against pathogens and diseases.

Particle Filtration

Among the most critical pieces of PPE are N95 respirators, however you may have also seen other respirators; KN95, P2 and FFP2. All of these masks types are known for their ability to filter airborne particles, particularly when used as a medical device, they are used to protect the user from airborne viruses, bacteria and other pathogens.

Masks Filter Particles

A prevalent analogy likens the filtration mechanism of masks to an ultra-fine sieve where particles enter and become trapped between the mask's material threads, accumulating over time. One might deduce from this analogy that a mask can only filter particles down to a specific size, allowing smaller particles to pass through unobstructed. However, whilst this analogy is widespread, it is incorrect.

The mechanics of filtration

A typical respirator, whilst seemingly simple in design, is a marvel of physics and engineering. When examined under a microscope, the respirator appears as a dense matrix of non-woven fibres. Each of these fibres, with a thickness ranging between 1-4.5um?(O'Dowd, et al., 2020), creates a complex network that pathogens must navigate to reach the wearer.

It is essential to note that these fibres are not packed solidly; significant air pockets exist between them. Consequently, the proportion of fibre to the total volume remains surprisingly low, ranging from 5 – 15%?(Du, et al., 2021), which allows masks to still be relatively breathable.

Everytime pathogens/particles comes into contact with these strands of fibre, they tend to adhere to them. This adhesion can be attributed to VanDerWaals forces, in which two neutral particles are attracted to each other, or through electrostatic forces. What is particularly intriguing is the mechanism by which these particles come into contact with the individual fibres of the mask, which is called single fibre filtration theory.

Let's look into the specifics of this process.

Interception

The first method of particle capture is termed "interception". This occurs when a particle, which is lightweight, small and flows with the airstream. As the airflow circumvents the mask fibres, the particle follows the fluid stream. However, during interception, it gets trapped on the top and sides of the filament layer.

Impaction

The second method is "impaction". This form of filtration, perhaps the easiest to visualise, typically functions for denser particles that move at faster speeds. Instead of the particle adhering to the airstream, its translational inertia causes it to continue straight, colliding with the filament and adhering to it. The particle's Stokes Number often significantly influences the number of particles captured by this method.

Diffusion

The third form is called "diffusion", caused by Brownian motion. Brownian motion is a curious and fascinating phenomenon in physics, where small particles move in a somewhat random fashion due to the many impacts from gas atoms and other particles. This results in a particle that wukk typically move with the air streamline but do so in an erratic manner, increasing its path travelled. This effect is most pronounced with very small particles, leading to the conclusion that smaller particles are more likely to be filtered by the filter media than larger ones.

Gravitational Settling

The 4th form is gravitational settling. This is where particles simply settle due to gravity. As you can imagine, for small particles, this effect is somewhat minimal and is dependent on the orientation of the mask.

Electrostatic Attraction

The 5th and final form is electrostatic attraction. Skilled mask manufacturers enhance their respirator's ability to capture particles by imparting an electrostatic charge to the particles. This means particles will either be attracted to the respirator through electrostatic induction if they are neutral, or if charged, they'll be attracted under Coulomb's law.

Testing Parameters

Particle Diameter

A frequently posed question regarding respirators, "What is the size of virus or bacteria particles and whether respirators can effectively filter them out."

Tests for particle penetration typically measure a particle with what's termed a 'Count Median Diameter' of 0.075 um, and this is polydisperse. In simpler terms, this means the aerosol doesn't consist of just one particle diameter but a range of diameters across a lognormal distribution.

There's a prevalent misconception that this particle size is chosen because it's akin to the size of a virus, bacteria, or a virus bound to a saliva droplet. However, the actual rationale is more straightforward.

The specific particle range is selected as, particles of this size have the highest likelihood of penetrating the majority of respirators.

Upon analysing the equations previously discussed, the following graph illustrates a peak penetration occurring around the 0.1 to 0.2 um particle diameter range.

For every given particle size, there is a specific mechanism to filter that particle. Diffusion works particularly well on smaller particles, as Brownian motion effects smaller particles more than larger particles. However, as the particle size grows, interception and impaction take over as main factors. It is worth noting the presence of the most penetrating particle in Figure 7 correlating to the low point of the black line, being typically sized between 0.1-0.3 μm. This size range aligns with the benchmarks set by our current Particle Filtration Efficiency (PFE) standards.

Fibre Diameter

N95s consist of thousands of tiny non-woven fibres. The manufacturing of these fibres is generally undertaken by dry formed non-woven methods such as melt-blown extrusion. In this process, polymer material is melted, then extruded through tiny nozzles, similar to that found in film extrusion. High velocity hot air attenuates the fibres down to diameters of 2 to 10 microns, where they are deposited as a sheet, in a random orientation?(Gahan & Zguris, 2000).

It is evident from the below graph that there is a general trend: the smaller the fibre size, the more efficient the mask becomes at filtering particles, this is due to the increased surface area available with smaller fibres, however fibres cannot become too small, otherwise they will become mechanically brittle and have the potential to dislodge, and cause airway impairment.

Particle Velocity

The efficiency of an N95 mask is influenced by the speed or intensity of your breathing.

Interestingly, for particles that are most likely to penetrate the mask, the faster they travel, the less effective certain mechanisms like diffusion and gravitational capture become. This reduced effectiveness is attributed to the shorter duration these particles spend within the mask. In contrast, the impaction mechanism, which does not depend on the time a particle spends inside the mask but on its contact with the mask's fibres, becomes more effective at higher speeds. The accompanying graph illustrates the varying impacts of these mechanisms on the filtration efficiency of individual fibres.

It is noteworthy that the average breathing rate during moderate exercise lies between 0.06 and 0.01 m/s. However, this rate can vary based on the mask's surface area. Generally, masks with a design that don't cling tightly to the face, providing more surface area, tend to offer better filtration performance.

Particle Density

The density of particles plays a small role in filtration efficiency. Lighter particles, such as fibrous materials, are generally less effectively filtered compared to denser, moisture-laden particles. However, the disparity between the filtration of these two types is relatively marginal.

The accompanying plot illustrates the influence of various capture mechanisms on particles of different densities.

Types of Masks

You might have observed various mask shapes available on the market. The primary designs include the

• Dome;
• Tri-fold; and
• Duck-bill;
• Bi-fold.

Each of these masks offer unique benefits, particularly concerning facial fit for diverse face structures. Particle velocity is a function of both how hard someone is breathing, whether there is going to be any direct blast of air onto the mask, such as speaking or coughing, and the surface area of the mask.

The total surface area of the mask contributes to comfort by ensuring it doesn't cling too closely to the face, however there is another metric and that is the effective surface area of the mask.

The Effective Surface Area: the portion not obstructed by facial features such as the cheeks or nose is equally vital for filtration efficiency.

For instance, the tri-fold, duck-bill, and dome masks snugly fit around the edges of the face but protrude beyond these edges. This design increases not only the total surface area but also the effective surface area, ensuring a uniform airflow through the mask.

Conversely, the bi-fold design has a reduced total surface area and tends to adhere to the wearer's cheeks. This reduces its effective surface area, potentially diminishing its filtration capability, regardless of the filter media used. This fact has been acknowledge by some health departments such as the Victorian government in Australia which is now reccomending the use of P2 and N95 masks over the use of KN95 masks.

Conclusion

I wrote this article to shed light on the remarkable physics underpinning the FFP2 or N95 respirator. The next time you are in the market for a reliable face mask, I hope this knowledge prompts you to consider if you can trust this company to deliver a reliable product, or if that emerging startup genuinely invested in the intricate engineering required for these protective items?

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P.S.

All views expressed in this article are my own and not of my employer.

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Bibliography

Du, W., Iacoviello, F., Fernandez, T., Loureiro, R., Brett, D. J., & Shearing, P. R. (2021). Microstructure analysis and image-based modelling of face masks for COVID-19 virus protection. Communications materials, 1-10.

Gahan, R., & Zguris, G. (2000). A review of the melt blown process. Fifteenth Annual Battery Conference on Applications and Advances.

Hinds, W. C. (1999). Aerosol technology : properties, behavior, and measurement of airborne particles. New York: Wiley.

Kwon, S.-B., Park, J., Jang, J., Cho, Y., Park, D.-S., Kim, C., . . . Jang, A. (2012). Study on the initial velocity distribution of exhaled air from coughing and speaking. Chemosphere (Oxford), 1260-1264.

O'Dowd, K., Nair, K. M., Forouzandeh, P., Mathew, S., Grant, J., Moran, R., . . . Pillai, S. C. (2020). Review of Current Materials, Advances and Future Perspectives. Materials, 3363.

Ogbuoji, E. A., Myers, A., Haycraft, A., & Escobar, I. C. (2023). Impact of common face mask regeneration processes on the structure, morphology and aerosol filtration efficiency of porous flat sheet polysulfone membranes fabricated via nonsolvent-induced phase separation (NIPS). Separation and Purification Technology.

Roberge, R. J., Bayer, E., Powell, J. B., Coca, A., Roberge, M. R., & Benson, S. M. (2010). Effect of Exhaled Moisture on Breathing Resistance of N95 Filtering Facepiece Respirators. The Annals of occupational hygiene, 671-677.

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