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HEPA Filter - What is it and how does it work?-Solenco South Africa

HEPA Filter - What is it and how does it work?

Take a deep breath. Have you ever wondered what it is that you are breathing in? We know that we breathe in a mixture of oxygen and nitrogen, and 1% of it is a mixture of other gases and particles. But what are the other particles?

Particle pollution is the mixture of solid and liquid particles that are so tiny they’re practically invisible, but that doesn’t mean they’re harmless. No matter what the size, particles can harm your health.

We use air filters to remove airborne particles, pollutants, and microorganisms that are hazardous to our health. One of the most effective and commonly used filters is a HEPA filter.

What is a HEPA filter?

HEPA stands for high efficiency particulate air (filter), or sometimes referred to as high-efficiency particulate absorbing (filter). It is a very efficient filter that is able to remove 99.97% of contaminated particles from the air. HEPA filters are frequently utilised in hospitals and by pharmaceutical companies in order to establish a safer workplace and avoid the spread of infectious diseases.

What is the mechanism behind it?

A HEPA filter works by arranging sheets of polypropylene (a plastic polymer) or fibreglass fibres, between 0.5 and 2.0 microns, in a tangle. The fibres are tightly packed, but still leave enough room for air to pass between them. Unwanted particles get trapped in the tangle of fibres through the processes of inertial impaction, diffusion, interception, or sieving depending on the size of the particle. This all basically means that 99.97% of particles that have a diameter greater or equal to 0.3 microns will be trapped by a HEPA filter.

  • Tiniest particles: through the process of diffusion small particles flow into the filter and diffuse within the air flow’s stream. Moving among the filter’s fibres and slowing down and getting lost. They end up bouncing around among the fibres. They get stuck in the filter, but they don’t settle (Brownian Motion).
  • Mid-sized particles: these particles are caught among the fibres through the process of direct interception. The particles aren’t large enough to have inertia and aren’t small enough to diffuse. They simply go with the air flow and are caught by the fibres when they’re touched.
  • Large particles: large particles are caught by the fibres due to inertial impaction. When entering the filter, the particles are unable to adjust or change direction and, due to its inertia, will continue along its original path and collide with the filter’s tangle of fibres.
  • Largest particles: these particles are caught by the fibres during the process of sieving, which is simply when they’re far too big to fit through the tangle of fibres and are stopped or caught.

So, as you may have guessed, HEPA filters work on a microscopic level, where a micron or micrometre is the unit of measurement for the particles they trap. A micron is so incredibly tiny that it is 1 millionth of a meter. The smallest object visible to the naked eye is 40 microns in diameter, which is about the size of a grain of salt.

A HEPA filter can capture pollen, dirt, mould, dust, moisture, bacteria, viruses, and submicron liquid aerosol. Even some viruses and bacteria which are ≤0.3 microns. On their own they do not filter out gases and odour molecules. To filter out odours, you would need to use an activated carbon (charcoal) or other type of filter instead of or in addition to a HEPA filter.

1 centimetre is 10 000 microns
1 millimetre is 1 000 microns
1 micron is 1 000 nanometres

Some common particle sizes in microns:

  • Pure oxygen: 0.0005 microns
  • Tobacco Smoke: 0.01–4 microns
  • Viruses: 0.02-0.3 microns
  • Covid 19 virus: 0.125 microns
  • Corn Starch: 0.1–0.8 microns
  • N95 face mask: 0.3 micron
  • Veld fire smoke: 0.4–0.7 microns
  • Car emissions: 1–150 microns
  • Bacterium: 1–3 microns
  • Spider web: 2–3 microns
  • Dust: 2.5 microns
  • Cement Dust: 3–100 microns
  • Mould: 3–12 microns
  • Spores: 3–40 microns
  • Ground Coffee: 5–400 microns
  • Red blood cell: 7–8 microns
  • Standard coffee filter: 10–15 microns
  • Pollen: 15 microns
  • Skin cell: 20–40 microns
  • White blood cell: 25 microns
  • Grain of salt: 40–60 microns
  • Human hair width: 50–180 microns
  • Grain of beach sand: 90–10 000 microns
  • Eye of a needle: 1 200 microns

     (click on the image to enlarge it)

    Infographic on Particle Sizes
    Image credit: Visual Capitalist.

     

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