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.
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