Allergenic Particles

TRUTH: The Clearing the Air report by Institute of Medicine(2000) states that “theoretical and empirical data suggest that air cleaners are most likely to be effective in reducing indoor concentrations of particles smaller than approximately 2 microns. However, much of the airborne allergen appears to be within larger particles” and “air cleaning was not found to be consistently and highly effective in reducing respiratory symptoms since much of the airborne allergens appear to be carried on larger particles.” The EPA clarifies this issue by stating “portable air cleaners are not effective at removing large particles because large particles settle out of indoor air at a substantial rate.” 

Why would this happen? 

Gravity is a major factor. Small particles suspended in the air can take five days to fall even three feet, but large particles like cat dander (at PM2.5) bend to the physics of their mass and fall quickly. This makes them difficult to capture into a filter because more of them are on the floor. Plus, the ones that are suspended have greater inertia (greater mass, so greater inertia) that keeps them from moving with the air toward a filter. Other forces come into play as well. When particles collide with a surface, they usually adhere as a consequence of van der Waals forces, electrostatic forces, and surface tension of liquid aerosols. Adhesive forces increase in proportion to the first power of particle diameter (Hinds, 1982). So bigger particles are harder to dislodge from surfaces.

That said, the exhaust vent, with its higher velocity, occurs resuspension predominantly for particles larger than approximately 2 μm like cat dander, from the floor and surfaces in the room. (Hinds' Theory and Thatcher and Layton, 1995). However, large particles resuspended from surfaces may not mix through-out the indoor air because their weight causes them to fall and redeposit in less time.

Therefore, few of these resuspended particles reach the inlet of the air cleaner 

(Richard B. Johnston, Jr. et al Clearing the Air: Asthma and Indoor Air Exposures, National Academy of Sciences pp. 413, 2000).

Theoretically Predicted Rates of Settling of Unit Density Spheres in Still Air

Aerodynamic Diameter
Terminal Settling VelocityTime to Fall 1 m
0.2 μm2.2 x 10-6 m/s5.3 day
0.5 μm1.0 x 10-5 m/s28 h
1.0 μm3.5 x 10-5 m/s7.9 h
5.0 μm7.8 x 10-4 m/s21 min
10 μm0.003 m/s330 s
20 μm0.012 m/s83 s
30 μm0.027 m/s37 s
50 μm0.075 m/s13 s