By Enrico Di Pietro
, P.Eng., Member
The term HEPA around filter has never been so exceptionally
used in every present time ventilation discussion, but grievously mishandled
in its true nature.
HEPA is an abbreviation of High-Efficiency Particulate Air, and
is classified as a MERV 17, following the Minimum Efficiency Reporting Value, a
scale created d in 1987 by the American Society of Heating, Refrigerating and
Air-Conditioning Engineers (ASHRAE) to document the effectiveness of air
A myth that requires rectifying is that HEPA filters won’t stop
viruses, such as COVID-19. In fact, closer review of the HEPA filter
will demonstrate the opposite, withal efficiency closer to 100%. 1
Let us move back in time and recall the first laboratory need of
submicron particulate removal. During radioactive research in the 1940’s,
conducted under what was known as the Manhattan Project, the HEPA filter was
developed to prevent the spread of radioactive particles. It was determined
that the most penetrating size particle would be of 0.3 micron, in such a way
determining the standard around HEPA filter being capable of stopping 0.3
micron particles with an efficiency of 99.97% following the latest
DOE-STD-3020-2005. Despite the Standard, one should be aware of HEPA filter
behavior with other than 0.3 micron particle size.
HEPA filters are made from multiple layers of specific micro
fibers with diameters ranging from 0.002 to 0.500 micron. Straining is the
natural interpretation of any filter, or simply comparing it to a mesh, without
any regard to other occurring mechanism in the filter. Howbeit, once a particle
enters the tortious pathway established by the micro fiber structure, residence
time is increased allowing supplementary capturing methods such as natural
electrostatic attraction, inertial impaction, interception and diffusion.
Natural electrostatic attraction can mildly favorably influence
any other mechanism, or act alone to force the particle to contact a fiber.
Inertial impaction occurs when air rapidly changes direction and the principles
of inertia separates the particulate from the air stream. Interception involves
having the particulate make physical contact and becoming attached to the media
fiber. Diffusion occurs when the random, also called Brownian motion of a
particle causes the particle to collide and stick to a fiber. The figure below
illustrates the different mechanism.
This brief column offers a different view of the HEPA filter in
an attempt to add and expand common knowledge of this technology. Although one
cannot warrant that the usage of HEPA filtration alone can reduce the risk of
Covid-19 contagion, it is critical to understand the limit of the technology,
follow adequate installation methods and understand the final effectiveness
once applied to a global ventilation system.
1. Perry, J. L., Agui, J. H., Vijayakimar,
R (2017) Submicron and Nanoparticulate Matter Removal by HEPA-Rated Media
Filters and Packed Beds of Granular Materials. 20170005166, NASA, Huntsville,
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