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hepa filters and how hepa filters work

How do Hepa filters work? Everything you need to know

While you may not always be aware of their presence, high-efficiency particulate air (HEPA) filters are all around us, including in vacuum cleaners, vehicles, medical and scientific facilities and in buildings’ heating, ventilation and air conditioning (HVAC) systems – and of course in Respiray Wear A+, our very own wearable air purifier. HEPA filters are fine filters designed to remove harmful or unwanted particles from a moving stream of air or fluid. This filtration technology is important in a multitude of industries thanks to its unique ability to trap a broad range of microscopic particles, including pollutants, pathogens like bacteria and viruses and airborne allergens such as pollen, dust mites and pet dander.

How do Hepa filters work?

HEPA filters actually trap different-sized particles from the air through four contrasting working principles – sieving, inertial impaction, interception and diffusion. Essentially, the largest particles don’t fit, the smallest float around and get trapped, and the rest can’t follow the airflow through the filter. Thanks to this combination of filtration methods, HEPA filters can trap particles of all sizes including pollen, pet dander, dust mites, bacteria, viruses and air pollution particles. This is especially important for elderly people and those with pre-existing respiratory or cardiovascular diseases, who’ve been shown to be more vulnerable to these dangers.

How do the different filtration methods work?

Without understanding how HEPA technology works, it would seem obvious that larger particles are prevented from passing through more efficiently than smaller ones, with a tighter mesh ensuring better filtration. However, the reality isn’t so simple because the different methods they use work in completely different ways. Because of this phenomenon, particles that are 0.3 microns in diameter actually have more chance of passing through a HEPA filter than both larger and smaller ones, for example. The variety of methods used by HEPA filters is the reason they’re able to efficiently trap such a broad range of particle sizes. Here’s a breakdown of how these filtration principles work:

Method 1: Very large particles (sieving, also known as straining)

HEPA filter filtration. Method 1: Very large particles (sieving, also known as straining)

The particles are simply too large to fit through the gaps between the fibers and get caught.

Particle size: >10 microns

Examples: Dust, pollen, mold, PM10 pollution

Method 2: Large particles (inertial impaction)

HEPA filter filtration. Method 2: Large particles (inertial impaction)

These large particles are small enough to fit through but too heavy to change direction in time with the air stream, colliding with the filter fibers instead.

Particle size: 1–10 microns

Examples: PM2.5 pollution; combustion particles, volatile organic compounds

Method 3: Small particles (interception)

HEPA filtration. Method 3: Small particles (interception)

The particles try to flow around the filter fibers in the air stream but move too slowly and come into contact with the fibers, becoming trapped.

Particle size: 0.3–1 micron

Example: Bacteria

Method 4: Very small particles (diffusion)

HEPA filtration. Method 4: Very small particles (diffusion)

Tiny particles are so light that rather than following the stream of air they change direction randomly (called Brownian motion), ultimately getting caught in the filter fibers before their randomized route can carry them through the gaps.

Particle size: <0.3 microns

Example: Coronavirus

Here’s a diagram illustrating all four methods that HEPA filters use to capture particles:

HEPA filtration methods.

Why are particles measuring 0.3 microns important for Hepa filters?

Particles with a diameter of 0.3 microns are considered the “most penetrating particles” due to being too heavy to diffuse but too small to be easily intercepted. This is why HEPA filters are focused on being able to trap these particles. For example, the filter in Wear A+ traps 99.7% of 0.3 micron particles, meaning it captures close to 100% of particles smaller or larger than that size. While particles much larger than 0.3 microns are captured by sieving, much smaller ones are trapped by diffusion. This is how HEPA filters can trap particles of all sizes, including viruses, and not just those that are too large to pass through the mesh.

Why did we choose H12 Hepa filters for Wear A+ rather than H13?

HEPA filters come in different grades, including the H12 filters used in Respiray Wear A+. A higher number denotes a finer filter – but are H13 filters always better for air purification? Not necessarily, because although finer filters can trap more particles, the air also passes through them slower, leading to a trade-off in terms of airflow. In turn, this means the air can be passed through an H12 HEPA filter more times than an H13 one while delivering the same airflow, ultimately leading to more efficient filtration.

Respiray Wear A+ – HEPA filters

H13 filters are effective – but H12 filters are more efficient

While this description doesn’t capture the true complexity of how these different grades of HEPA filter compare, it explains the principle behind our decision to use H12 HEPA filters in the Wear A+ wearable air purifier rather than H13 filters. Because the air purified by Wear A+ gets mixed in with the surrounding air and therefore requires constant filtration, rather than straining for that extra 0.27% of filtration effectiveness at the expense of airflow we’ve opted for a system that offers better overall filtering efficiency with lower energy consumption, while also providing a higher air speed to create an optimal buffer zone of clean air for users to breathe.

H12 filters save energy and provide better airflow

Our dedicated team of scientists, engineers and aerodynamics specialists at Respiray worked tirelessly, thoroughly testing the device to create the best possible airflow. With an H13 HEPA filter the airflow would have been weaker, meaning the clean air buffer zone created by Wear A+ would be less dense and more energy would be needed to achieve the filtration required, resulting in higher noise levels and increased power consumption. Thanks to the H12 filters used, the device has the ideal rate of air delivery to produce a laminar flow (where the air streams move in the desired direction with minimal sideways movement), minimizing turbulence and maintaining a more precise, consistent buffer zone of clean air around the user’s face while also making the device more energy efficient.

Want to find out more about how Wear A+ provides effective protection against airborne allergens, pollutants and viruses? Check it here.

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