How is it possible to filter out sub-micron particles?

This question is repeatedly asked:

How is it possible for Totobobo filters to filter out sub-micron particles?

This is an important question because sub-micron particles ( eg.PM2.5 ) contained in polluted air is the major health concern. The secret of Totobobo filters is the high intensity static charges on each fiber which made up the filter matrix. When you breathe in, airborne particles follow the air and pass through the filter matrix. Fine particles are very susceptive to the static charges and therefore most of them eventually are attracted onto the filter fibers. In fact, Nelson Lab test shows Totobobo F96 filter is able to cut down 99.86% of 0.1 microns particles. Under a microscope it shows how small particles are attached to the filter fibers.

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Fig. 1  Fine particles, size range from tenths of microns down to 0.1 microns are attracted to the fibers as they pass through the complex matrix of the Totobobo filter.
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Fig. 2 100X magnification view of a needle (0.6mm thickness) and filter fiber.
Totobobo filter with a needle of 0.6mm thickness
Fig. 3    Totobobo filter with a needle of 0.6mm thickness

 

Quoted from the NIOSH Science Blog below:

N95 Respirators and Surgical Masks

How do filters collect particles?

These capture, or filtration mechanisms is described as follows:

Diagram illustrating the filtration mechanisms of inertial impaction, interception, diffusion, and electrostatic attraction. In each case, fibers are shown filtering particles.

  • Inertial impaction: With this mechanism, particles having too much inertia due to size or mass cannot follow the airstream as it is diverted around a filter fiber. This mechanism is responsible for collecting larger particles.
  • Interception: As particles pass close to a filter fiber, they may be intercepted by the fiber. Again, this mechanism is responsible for collecting larger particles.
  • Diffusion: Small particles are constantly bombarded by air molecules, which cause them to deviate from the airstream and come into contact with a filter fiber. This mechanism is responsible for collecting smaller particles.
  • Electrostatic attraction: Oppositely charged particles are attracted to a charged fiber. This collection mechanism does not favor a certain particle size.

In all cases, once a particle comes into contact with a filter fiber, it is removed from the airstream and strongly held by molecular attractive forces. It is very difficult for such particles to be removed once they are collected. As seen in Figure 2, there is a particle size at which none of the “mechanical” collection mechanisms (interception, impaction, or diffusion) is particularly effective. This “most penetrating particle size” (MPPS) marks the best point at which to measure filter performance. If the filter demonstrates a high level of performance at the MPPS, then particles both smaller AND larger will be collected with even higher performance.

This is perhaps the most misunderstood aspect of filter performance and bears repeating. Filters do NOT act as sieves. One of the best tests of a filter’s performance involves measuring particle collection at its most penetrating particle size, which ensures better performance for larger and smaller particles. Further, the filter’s collection efficiency is a function of the size of the particles, and is not dependent on whether they are bio aerosols or inert particles.

Graph showing a filter's efficiency on the Y-axis and particle diameter in microns along the X-axis. Efficiency falls in the 'Diffusion and Interception Regime'.