The Concept of HEPA 

How To Design An Effective Anti-epidemic Mask

What’s the concept of HEPA?

And why ACTife’s mask is the only mask design on the market that uses both HEPA principles and physical antibacterial materials to achieve the effective epidemic prevention?


What is HEPA?

HEPA stands for High-Efficiency Particulate Air Filter, which can be made of filter paper, non-woven fabric, or glass fiber. According to the definition of the U.S. Department of Energy, a filter that can block more than 99.7% of suspended particles with a size of 0.3 microns is a HEPA filter. It is interwoven by very fine fibers, which can capture particles very effectively.

The way how HEPA filters air is not like sieving particles, in fact, HEPA filters rely on the adsorption of fiber to hold the fine dust below PM2.5. The point is, how to use a larger grid to block particles smaller than the grid? The answer is down below.

We can consider the following two benefits:

  1. Filtering efficiency: Basically, particles smaller than the mesh can pass through, and those larger ones will be blocked. It is suitable for the filtration of large particles (if the diameter of the particle in the air dielectric is more than 3μm, that is consider as bacterial aerosol, sometimes it can be as large as 20μm).
  2. Brownian motion efficiency: If the particle is small enough (the diameter of the particle in the air dielectric is smaller than 1μm, which is the size of the virus aerosol), we cannot use the Newtonian mechanics (macro inertia). Under this situation, we must look at the problem with the microscopic model of molecular collision gas dynamics, which is the concept of Brownian motion. To put it simply, in the case of general air environment and small enough particles, it must be based on the aerodynamics, which is the Brownian motion caused by collisions between gas molecules in the air and virus aerosol particles whose filtering effect must be calculated.

When the particles are collided by molecules in the air, and the virus aerosol particles that produce Brownian motion move toward the mask, they will move in the space in a non rectilinear random motion. This motion is divided into a combination of linear motion (when not impacted) and non-differentiable transient reversible motion (when impacted), causing these virus aerosol particles to be effectively blocked by the fiber of the mask, even if the density of the mask fabric or fiber does not reach the mesh size that is smaller than the virus aerosol, it will still stick to the mask fiber due to these random movements.

When we further analyze the Brownian motion of these virus aerosol particles in the air, we can get the following four conclusions:

  • The smaller the mass of the particle is, the more obvious the Brownian motion is, that is to say, the easier it is to be hit by the molecules in the air to produce a random disturbance.
  • The smaller the size of the particle, the less likely it is for the surrounding molecules to produce balanced collisions (two particles with the same direction and force collides, and the two force will be offset, which means nothing happens), which makes the Brownian motion more obvious.
  • The higher the temperature is, the faster the movement of molecules are, and the more obvious the Brownian motion is.
  • The lower the density of the air, the smaller the number of molecules hitting the particles, and it is easier to caused unbalanced force and form more obvious Brownian motion.

The simple conclusion is, the smaller the mass and the volume of the virus aerosol, and the hotter the weather, the thinner the air, the more intense the Brownian motion of the virus aerosol. So we find that the virus is more likely to be grabbed by the mask fabric.

In the design of mask, in order to block these particles that’s doing Brownian motion, we can usually see the following strategies:

Strategy 1: Use electrostatic adsorption. When the virus aerosol particles approach the mask in a Brownian motion, they will be very easily attracted by the electrostatic adsorption layer and stick to the mask.

Strategy 2: Use various design methods of HEPA filter. For example, let the surface of the filter show a 3D texture, which increases the proportion of virus aerosol particles adsorbed by the mask material.

The above two strategies seem to have a high barrier effect, but we have to pay attention to several important risks:

  1. The risk of virus particles remaining on the outer layer of the mask is that we will touch, adjust, and take off to drink. So if you get the virus on your hand, you can touch the eyes or other parts of your face, causing various risks of cross contamination.
  2. Using the principle of Brownian motion, with larger pores (more breathable, but can block virus particles on the fabric as well) to improve the air permeability and comfort, the particles will eventually enter the respiratory tract due to repeated inhalation and exhalation, gradually pushing the particles into the inner layer, it’s just that each aerosol particles stays on the mask for a longer time.

An extremely important key concept here is to allow virus aerosol particles to stay on the mask longer due to Brownian motion in the air. If the mask has no antibacterial function, all we can do is passively wait for the virus to slowly lose its activity over time. The time that the virus can survive in the aerosol is often longer than the time the particles stay on the mask that has HEPA or electrostatic design. The conclusion is that the epidemic prevention effects of these methods are actually very limited.

Using the ACTife’s ACT sputtered antibacterial material in the mask design can achieve breathability and extend the time that the particles stay on the mask. When the virus can stays on the mask for more than tens of seconds, the antibacterial ACTife material on the mask will achieve effective sterilization (please refer to the test report), allowing the mask to not only breathable, but also achieve a very safe epidemic prevention effect. This is one of the important concepts of ACTife antibacterial protective masks and 3D beatify masks. This is a technology that makes the mask have both breathable and anti-epidemic ability.


The term “aerosol” refers to solid or liquid particles suspended in the gaseous medium, including all common substances in life such as dust, fog, smoke, PM2.5, fungal spores, droplets, and inhaled drugs for clinical treatment of respiratory diseases, etc., all belong to aerosols, and the particle size are different from the types and sources. Everyone will produce many visible or invisible particles in the process of breathing (quiet breathing and deep breathing), speaking, sneezing, and coughing, with a diameter ranging from 0.01 to 500 microns (μm,10-6 m). When the particle diameter is greater than 10 microns, it can sink to the ground due to gravity, which is called “dust fall”, and when the particle diameter is less than 10 microns, it can be suspended in the air, which is called “suspended particles”. Simply use the thickness of the hair to illustrate the concept of particle diameter. For example, the diameter of a 10-micron particle is about 1/7 of the diameter of a human hair, and a 2.5-micron particle is about 1/28 of the diameter of a human hair.

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