How N95 Masks Stop Viruses

Preventing a pathogen from entering our respiratory system, at first glance, may seem obvious. The first thought might be to trap them by preventing particles from moving through a filter. But looking deeper at the problem reveals the true scope of the challenge.

With every normal breath we take, we inhale around a half-liter of air. The pressure difference between the atmosphere and our lungs during inhalation, peaks at around 8 cm of water. For comparison, a typical shop vac can pull a vacuum of around 200 cm of water or about 25 times that of our lungs.

Pathogens vary widely in size with bacteria generally ranging in size from 1-20 um to viruses which can range from 17nm up to 750nm. The rhinovirus that causes the common cold, for example, is around 30nm in diameter, while HIV, SARS-COV-2, and some strains of influenza hover around 120nm.

TYPES OF RESPIRATORS

N95 respirators are part of a class of respiratory protection devices known as mechanical filter respirator. These mechanically stop particles from reaching the wearer's nose and mouth. Another form of respiratory protection is the chemical cartridge respirator. These are specifically designed to chemically remove harmful volatile organic compounds and other vapors from the breathing air.
Both classes of respirators are available in powered configurations, known as powered air-purifying respirators.

N95

The N95 designation is a mechanical filter respirator standard set and certified by the National Institute for Occupational Safety and Health in the United States. The number designates the percentage of airborne particles removed, not their size. While ratings up to N100, that can filter 99.97% of airborne particles exist, N95 respirators were determined to be suitable for short-term health care use, in the 1990s.

Other designations include oil-resistant R and oil proof P respirators, which are designed to be more durable and maintain filter effectiveness against oily particles in industrial use. Surgical grade N95 respirators possessing fluid resistance were specifically cleared by the United States Food And Drug Administration for medical use.

HOW THEY WORK

Modern mechanical filter respirators work, not by ‘netting’ particles but rather by forcing them to navigate through a high surface area maze of multiple layers of filter media. This concept allows for large unobstructed paths for air to flow through while causing particles to attach to fibers due to a number of different mechanisms.

In order to achieve the high surface area required, a non-woven fabric manufacturing process known as "melt-blow" is used for the filter media. In this technique high temperature, high-pressure air is used to melt a polymer, typically polypropylene, while it’s spinning. This produces a tough yet flexible layer of material composed of small fibers. Depending on the specifications of the layer being produced, these fibers can range from 100um all the way down to about 0.8um in diameter.

How these fibers capture particles are determined by the movement of air through the filter media. The path of air traveling around a fiber moves in streams. The likelihood of a particle to stay within this stream is primarily determined by its size.

The largest particles in the air tend to be slow-moving and predominantly settle out due to gravity.
Particles that are too small for the effects of gravity, down to around 600 nm, are primarily captured by inertial impact and interception.

Inertial impaction occurs on larger particles in this size range.

In contrast, particles below 100nm are mainly captured through a mechanism known as diffusion. Random movements of air molecules cause these very small particles to wander across the air stream due to Brownian motion. Because the path taken through the filter is drawn out, the probability of capture through inertial impact or interception increases dramatically, particularly at lower airflow velocities.

EFFICIENCY

Because of the complex, overlapping methods by which particle filtration occurs, the smallest particles are not the most difficult to filter. In fact, the point of lowest filter efficiency tends to occur where the complementing methods begin to transition into each other, around 50-500 nm. Particles in this range are too large to be effectively pushed around by diffusion and too small to be effectively captured by the interception or inertial impaction. This also happens to be the range of some of the more harmful viral pathogens.
Interestingly, the more a respirator is worn, the more efficient it becomes.

FLAWS

The weakest point on any respirator is how well it seals against the face. Air will always pass through facial leaks because they offer much lower resistance than the respirator, carrying particles with it.

SUPPORT NEW MIND ON PATREON

SOCIAL MEDIA LINKS

[bangyahead]

You make some of the best and most interesting videos on Youtube, and not one beg for likes and subs. That crap gets so old.

[klauserji]

So.. same process as making cotton candy.

[WarpedYT]

Awesome Awesome! I have alot of filter experience from my Past Hospital HVAC contracts, Static filter and UV are very powerful mechanisms of filtration.

[ReevansElectro]

The fellow at 7:17 doesn't realize that he is wearing it wrong. The bottom elastic goes below the ears and the top elastic goes above the ears.

[kashmirha]

This should be broadcasted on every television about covid air filtering and general air filtering.

[Muonium]

It's very important to note that the effect mentioned around 6:00 that "the more a respirator is worn the more efficient it becomes" is really only relevant when the mask is being used to filter HUGE amounts large suspended particles in air such as wood dust in a shop, and it is definitely not the main mechanism of mask degradation at play for masks worn for eg. aerosolized viral particle protection. The main mechanism for mask degradation worn in the clean air of for instance an office or grocery supermarket is going to be the neutralization of the electrostatic potential of the mask's electret layer. The high humidity of exhaled breath will allow electric charges to be conducted away from the electret charged fibers and reduce the mask's efficiency for particle filtration over the time noted in the video. Immersing an N95 in any liquid, eg. sterilizing alcohol solution, will immediately destroy the electret layer and reduce the efficiency to likely below 80% filtration of 300nm dia. particles.

[robertschlesinger]

Excellent overview on protective masks, particularly N95 masks. A must see video for all to see.

[mogh]

One of the most informative videos on the YouTube 👍✌️

[trekkienzl]

If you told me back in May 2019 that by May 2020 I'll be incredibly into and fascinated with quality face masks and 60% alcohol hand sanitizer, I would have thought your crazy.

Today, I carry hand sanitizer with me at all times and I'm really into my N95 and KN95 masks.

[leemylks]

amazing vid mate. One note: you should end with someone showing how to wear the N95 mask properly.

[kirianf]

what's up with the dude kitesurfing?

[BradsWorkbench]

Wow this channel really exploded in size. Congrats man, keep up the great videos

[Scrogan]

The importance of fit is why I see no point in those rectangular masks with the elastic on either end, they never seal against the face at all.

[tootalldan]

Nice video. Can you do one on the Hepa house filters?
Btw, the mucus membrane of our nose is a good defense as long as we don't get too dry such as winter air.

[XiwithHighPing]

India produced tiny amount of N95 masks and no PPE kits in until mid march. After received faulty goods from china and returned it, they started to produce it by themselves and now India produces 5 lacks PPE kits per day and thousands of n95 masks.

[ryanburbridge]

Your doing pretty good on those subs bud keep up the good work

[Stonehawk]

it's literally the cotton candy process applied to molten polypro instead of sugar.

[anonymous.youtuber]

In the context of sars-cov2 is it not fso that masks fitted with valves only protect the wearer of the mask, not the people in the vicinity. When used in a department where all patients are already infected, this might be less important though I would like to ask the question if contamination with other viruses the wearer could be spreading could be an additional burden for the patient.

[saygr]

I don't know about the type of flow of air through the mask but if we were to make it a turbulent flow, since micro vortice creation could actually help the diffusion process considerably. I believe that it will greatly further increase the efficiency of the filtration. In case if it already turbulent or unresearched yet, Here's a good topic for someone's thesis.

[johnj]

That graph at the beginning seems to be inverted?