How Do Filters Work?
There are many types of filters that vary in their properties depending on the media that is to be filtered and what is being captured. This discussion is focused on the filtration of particles in air. Filters in this form are typically made of non-woven textiles. For N95 respirators, melt blown polypropylene is commonly used and a charge is applied to the material to increase its efficacy. So how does it work? Well, the answer is not as easy as one might think. There are four main parameters involved in capturing particles: inertial impact, interception, diffusion, and electrostatic attraction (Figure 1).
Inertial impact is the mechanism that prevents larger particle from moving around the fibrous matrix due to their mass, which traps them in the material. Interception is the mechanism that prevent large particles from passing through the fibrous matrix because they are intercepted by fibers that capture them. Diffusion is the mechanism, which is a little more complicated to understand, that cause particles to leave the flow paths of air and intercept with fibers. This method helps to capture smaller particles. Finally, electrostatic attraction, like used in N95 respirators, causes particles that are the opposite charge of the fibers to attract and be captured. This mechanism is not dependent on the particle size but on the charge of the particle.
Figure 1. Filtration Mechanisms
How are Filters Rated?
Filters are tested to determine how effective they are at capturing particles. They types of test used and the standards used to evaluate their efficacy depends on the intended use of the filter. For HVAC systems, filters are rated using the minimum efficency rating value (MERV). This system describes how effective a material is at capturing different particle sizes. The higher the MERV rating the better it is at filtering smaller particles (Figure 2). This information is important because it can tell you if your filter is able to capture larger particles like dust and pollen or if it can capture much smaller particles like bactria and viruses as well.
Figure 2. MERV rating chart
Masks are rated using a different method. Many people don't realize that the N95 is actually a classification for two key parameters: resistance to oil aerosols and percent of aerosol collection. The letter in the classification describes its resistance to oil aerosols and the number describes its percent of aerosol collection (Figure 3).
Oil Aerosol Resistance
Not Oil Resistant
Somewhat Oil Resistant
Strongly Oil Resistant
Percent Aerosol Collection
Figure 3. Respirator classification
Key Design considerations
In the absence of mask that have been tested using government approved standardized test, one must consider the filtration preformance of other material options that may be avalible. The size partical that a filter can capture is an important factor with viruses like coronavirus ranging in the submicron dimension.
Breathability is important for a few reasons. First, if you can't breath when wearing the mask, then you won't wear it. Second, when a mask is less breathable it can reduce the filters efficacy at capturing particles. The more you have to work to breath, the greater the force of air flow the mask will experience. This will make the filter parameters discussed with figure 1 less effective at traping particles by diverting them from the airstream.
The fit of the mask is an important part of a filters efficacy. Gaps around the edges of the mask provides the opportunities for particles (viruses) to bypass the filter and contaminate the air you breath. While this is a major consideration for the actual design of the mask, it should also be considered when selecting a filter material and how it will be incorporated into the mask design. Filter pockets in DIY mask provide spaces where particles can enter without being filtered. Making mask with stiffer materials like vaccuem bags may not allow for a tight fit around the face.
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MERV Filter Rating Charts – Filter Application Chart. LakeAir. [accessed 2020 Apr 13].
RobertsonPaddy P. What Are The Best Materials for Making DIY Masks? Smart Air Filters. 2020 Apr 10 [accessed 2020 Apr 13].