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Which particles can get captured by a filter or by my lungs?

Which particles can get captured by a filter or by my lungs?

This is an incredibly important and somewhat complicated question.  I want to present the following graphic, which shows relative particle size.  I’d say it’s pretty surprising how large our hair is and how small those bacteria, smoke particles and viruses are, right?  The “µm” stands for micrometers, which is one-millionth of a meter.  Micrometers are also called “microns”, so the human hair is 50-180 microns.

In our FAQ on “What is HEPA?”, we explored how filters actually trap contaminants like these.  In order to start answering our original question, let’s review that!

“Sieving” is the most simple way a filter “strains” out the particles that are larger than the openings between the fibers of the filter.  

                  (image from classicfilters.com)

Then, there are three types of motion that particles experience in a stream of air, such as the airstream being drawn through a filter.  Filter design actually takes advantage of the size of particles and the way they move through the air, in order to capture them.  

  • Obviously, some particles are larger than others.  Inertia acts on larger particles by causing them to continue in their original path of motion when entering the filter, thus colliding directly into fibers instead of sweeping around them with the accompanying airflow.  Particles 0.3 micron and larger (coronavirus size and larger) are affected by inertia. 

                (image from classicfilters.com)

  • Air is made up of all kinds of molecules and particles, which are getting thrown about in erratic ways by collision with one another.  This is called Brownian motion.  Taking Brownian motion into consideration is filtering by diffusion, which occurs when smaller particles travel in an erratic fashion through the airflow and are impeded by fibers.  The particles trapped by diffusion are typically smaller than 0.1 micron.

                         (image from classicfilters.com)

  • In direct interception, a particle follows the airstream but travels closer than its radius to the fiber, “brushing” it and becoming stuck.  This can happen for many different sizes of particles and just depends on where they are in the airstream relative to the fibers.  

                    (image from classicfilters.com)

So there you have it– four ways a filter uses to capture particles.  Interestingly, because of the gap between inertia and diffusion, particles around 0.3 microns in diameter are the most difficult to filter out of the air, because they are least affected by these methods.   This is called the most penetrating particle size (MPPS)--the size that is most likely to slip through the filter.  

HEPA filters are rated on their ability to remove this size particle—0.3 microns.   They do so with greater than 99.95% efficiency.   There are 2 ratings of HEPA: H13, which removes at least 99.95% of 0.3 micron particles, and H14, which removes at least 99.995% of 0.3 micron particles.  

Without a HEPA filter in your home, you are probably relying on furnace filters of various lower filtration ratings.  That means that more particles are getting through the filter, and remaining in the air for you to breathe.  Yikes!  Particles of 10 microns or less can be inhaled, and particles of 2.5 microns or less are more likely to deposit deep in the lungs.  (California Air Resource Board).  This size of particle is extremely lightweight, and once it’s in your lungs, it can move all the into the deepest part of your lungs and lodge there.  Without filters to capture particles from the air, unfortunately your airway and lungs will act as filters.  Here is how (Canadian Center for Health and Occupational Safety):

  • The respiratory system can be divided into upper and lower regions.  The upper region starts at the nose and mouth, and goes down to the vocal cords in the larynx. The lower region starts at the larynx and extends through the trachea all the way to the bronchioles and aveoli (smallest air sacs).

  • Particles 10 microns and larger are most likely to lodge in the upper respiratory region by inertia (also called impaction), because in this region, air is flowing at a higher velocities and during the twists and turns particles tend to continue in their original direction and “impact” a wall of the airway and stick there.

  • Interception is likely to occur with fibers, like asbestos fibers.  In the range of 1 micron in diameter and 200 microns in length, if one end brushes an airway wall, it will lodge there.  These can make it into the bronchial tree.

  • Sedimentation occurs when particles larger than 0.5 microns loose their momentum and gravity takes over, causing the particles to settle in the bronchi and bronchioles.  

  • For particles smaller than 0.5 microns, diffusion is in effect (Brownian motion).  This can happen in the small airways and alveoli, when air has virtually slowed to stillness.  

  • Although the mucous lining of our respiratory tracts was designed to clear airways by moving it up and out of the respiratory system, it does not always function well in older and diseased people, and the sheer number of ultrafine particles can clog airways. 

Fine particles (2.5 microns down to 0.1 micron) and ultrafine particles (0.1 micron and smaller) are shown to cause inflammation and inflammatory diseases.  Ultrafine particles can migrate from the alveoli to the bloodstream and to other organs, resulting in delayed sicknesses such as reactions to vehicular pollution, welding fumes and burning trash.  The danger of ultrafine particles is not just their size and number (much greater numbers PM2.5), it is also their charge, which can adsorb toxic chemicals onto their surface.    Fine particles have been shown to cause cardiovascular disease; they travel up the olfactory nerves to the brain; they are inhaled by electronic cigarettes, and have been shown to increase risk for hypertension and diabetes and cause cancer (animal studies) . (study)

For all these reasons, a HEPA filter is a great addition to your home, vacuum, and even a mask when you are performing high-risk activities like welding, spraying paint, or caring for a sick relative, so that the filter will do the work instead of your lungs and body!

Photo by Joshua Newton on Unsplash