The main air pollutant causing dark soiling indoors is the general category “soot” from outdoors.  Soot causes “ghosting”, the discoloration deposited on walls just above the floor, and around other cracks where air leaks in.


  Soot is a byproduct of the incomplete combustion of hydrocarbon fuels.  Sooty exhaust comes largely from diesel trucks, plus also from aircraft engines.


  Soot is a mass of impure powdery particulate matter coated with black carbon.

  Tiny aerosol droplets are also formed in exhaust.  They keep getting smaller as their volatiles evaporate, and they cause an odor both outdoors and indoors.  But they do not darken as much as soot particles.

  Other particles are also black, especially common tire wear, mostly larger (so they fall out on window sills).


  The visible darkening is caused by medium size soot particles, about 2 to 100 micrometers (µm, or µ) diameter.  Soot is formed very small, but can grow very large by agglomeration.

  Particle sizes found outdoors are “bimodal”, grouped into the fine mode 0.1-1.1 µ formed by combustion, and the coarse mode 1.1-30 µ formed by grinding (like road dust), not black like soot.

  Larger (>100 μ) particles settle out of the air quickly (in seconds), medium-size (1-100 μ) slowly (hours), and small particles (<1 μ) maybe never (days).

  Also, any particle smaller than the wavelength of light (<1 µ) appears black.


  A MERV-13 “submicron” (stops >1 µ) filter is considered effective on fine soot.  A MERV-8 “coarse” (stops >3 µ) does not capture the smaller soot, but might be adequate to avoid soiling.


  To demonstrate and compare the effectiveness of filters (MERV-8 vs. -13) to avoid soiling, we will test airborne concentration levels in the supply air vs. outdoors.  To test a filter’s particle capture efficiency, we can use these methods:

  (1) Delos #DC-1100 Laser Particle Counter: a continuous direct reading instrument that displays 2 size ranges: “small” >1 µ, and “large” >5 µ.  This will be the main monitoring tool.

  (2) Zefon Air-O-Cells: sampled 7 minutes, later analyzed using light microscopy >3 µ, to check for major particle types.

  (3) White filters to compare darkness after days  of pumping, through pore sizes of 17, 6, and 1 µ.

  (4) Tape lift samples of settled dust particles for detailed microscopic analysis.


  Testing will include days with wind from both our prevailing directions, NW and SW.  In both cases, these apartments on the E center are downwind from the exhaust plumes from the dirty take-off ends of the runway whether N or S.

  Testing capture effectiveness can be done after installation, with the system ON vs. OFF.