The challenges in developing a new air filtration technology are somewhat different than those for liquids. Because of the inherent limitations in air handling systems and human performance, air filtration must be carried out at a considerably lower pressure drop than liquid filtration. The most significant technical concern, then, is being able to achieve sufficient biological and chemical removal levels at the required flow rates without undue pressure drop. Fortunately, the low density/viscosity of air leads to higher levels of turbulence and easier deflection of small particles allowing for thinner material and lower pressure drops as compared to liquid filtration media. Further, because of the absence of surface tension issues it is easier for contaminants to interact with the fibers within the filter.


Early air filtration tests showed that several non-optimized designs operated in the performance range of HEPA filters for bio- contaminants (see Figure 1). Samples of Seldon's candidate air filters achieved a log 2 reduction (~98%) for a weaponized form of Bacillus subtilus (orange squares at right), the common surrogate for the bio-warfare agent, Bacillus anthracis (anthrax). This level of removal is comparable to the early results achieved in the development of a water filtration product that currently has log 7 bacteria and log 6 virus reductions in water (99.99999% and 99.9999%, respectively).


    Figure 1: Neutral particle (Blue) and surrogate bio-warfare agent (orange) removal efficiency for several Seldon filters. Red curve is typical filtration relationship.

Comparing the performance of Seldon's media to that of a flat sheet of HEPA media under the test conditions used in Seldon's laboratory shows that several of Seldon's media designs achieve higher capture efficiency than HEPA media at nearly three times the permeability. Recently, a third party laboratory evaluated two types of Seldon air filtration media and confirmed internal test results showing that the medias could capture between 98.8% and 99.7% of particles 0.35 m in diameter. (Particles sized around 0.3 m are considered to be the most difficult to capture.)


A third party has also evaluated the performance of Seldon's air filtration media in a cross flow design. Figure 2 shows how the test filter was challenged with particles 10 to 120 nm in diameter. Even without design optimization, the media was capable of removing well over 90% of the challenge particles in all size categories. Most critically, the media is able to remove particles 50 nm in diameter the approximate size of particulates found in heavily polluted urban areas.


    Figure 2: Performance of Seldon's air filtration media when tested in a cross-flow configuration.