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Does smaller particle always penetrate filters easier?

Common sense would suggest smaller particle always penetrate filter easier. But this is not always, as state in the FAQ of CDC site:

1. How effective are the Part 84 filter respirators against particles smaller than 0.3 micrometer in diameter?

The 0.3-micrometer diameter used in the certification testing is approximately the most penetrating particle size for particulate filters. Although it seems contrary to expectation, smaller particles do not penetrate as readily as 0.3-micrometer particles. Therefore, these respirators will filter all other particle sizes at least as well as the certified efficiency level.

2. How effective are the Part 84 filter respirators against asbestos fibers or other rod-shaped particles?

Although fibers or rod-shaped particles may have very small cross-sectional diameters relative to their lengths, the Part 84 particulate filter respirators will be at least as efficient against this particle shape as the certified efficiency level.

CDC is right, this is contrary to expectation. The reasons is explained in another document in more details. Here is a diagram from pg. 9 of the document showing there are actually four different collection mechanisms govern particulate air filter performance: inertial impaction, interception, diffusion, and electrostatic attraction:

4 different collection mechanisms of particulate filters

The first three of these mechanisms apply mainly to mechanical filters and are influenced by particle size.

  • Impaction occurs when a particle traveling in the air stream and passing around a fiber, deviates from the air stream (due to particle inertia) and collides with a fiber.
  • Interception occurs when a large particle, because of its size, collides with a fiber in the filter that the air stream is passing through.
  • Diffusion occurs when the random (Brownian) motion of a particle causes that particle to contact a fiber.
  • Electrostatic attraction, the fourth mechanism, plays a very minor role in mechanical filtration. After fiber contact is made, smaller particles are retained on the fibers by a weak electrostatic force.

Impaction and interception are the dominant collection mechanisms for particles greater than 0.2 ?m, and diffusion is dominant for particles less than 0.2 ?m. The combined effect of these three collection mechanisms results in the classic collection efficiency curve, shown in the following Figure:

Fractional collection efficiency versus particle

The TOTOBOBO filters is a type of electrostatic filters which contain electrostatically enhanced fibers. Such fibers actually attract the particles to the fibers, in addition to retaining them. Electrostatic filters rely on charged fibers to dramatically increase collection efficiency for a given pressure drop across the filter.

Following table gives a rough idea of how small a micron is:

 Substance  Micro-meters (microns)  Inch
 90% of Wood smoke particles are smaller than   1 micron  0.00004
 Bacteria (average)   2  0.00008
 Red Blood Cell   8  0.0003
 Talcum Powder   10  0.0004
 White Blood Cell  25  0.001
 Human Hair   70  0.003
 Grain of Table Salt  100  0.004

Read more:

Used filters from India

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