What type of filter should be used to remove tuberculosis
(TB) bacteria from the air?
A: The Center
for Disease Control (CDC) recommends that HEPA filters
be utilized to help reduce the amount of TB bacteria
in the air.
What is a HEPA filter?
A: A true
HEPA filter is one which when tested manitains a minimum
of 99.97% efficiency on particles 0.3 microns in size
when challenged with thermally generated DOP according
to Federal Standard 209D or latest issue.
How do I know if my filter is a HEPA filter or not?
A: All true
HEPA filters are tested and then certified that they
are a minimum of 99.99% efficient on particles 0.3 microns
in size when challenged with thermally generated DOP.
The certification label should be on the filter itself.
If the filter does not meet federal standard 209D (or
latest issue) or IES-CS-1T it cannot be termed a HEPA
Is a filter that claims to be 99.99% efficient on
particles 1 micron in size actually a HEPA filter?
A: No! These
filters look like a HEPA filter and are constructed in
much the same way but do not have the same efficiency
characteristics as a true HEPA filter. Federal specifications
call for 0.3 microns as the challenge agent size, not
1 micron. Quite a few companies try to sell less efficient
filters under the guise that they are true HEPA filters.
Are HEPA filters effective in removing airborne TB
A: The average
TB bacteria is a rod shaped particle that is 0.3 - 0.6
microns wide by a length of 1-4 microns. This is well
within the capture range of the filter. Most airborne
TB bacteria are enveloped in a particle of moisture called
a droplet nuclei. The droplet nuclei is larger than the
bacteria itself which increases the efficiency of its
removal by the filter.
Will an air purifier using a HEPA filter eliminate
any chance of contracting TB in the space that it
An air purifier
using a HEPA filter greatly reduces the quantity of all
airborne particulate matter including TB bacteria by
the removal and dilution process therefore reducing the
risk of contracting TB. However, no single process can
totally eliminate all risk.
If the rod shaped bacteria enters the filter "width
wise", isn't it possible for the bacteria to
pass right through the filter?
A: It would
be highly unlikely for this to happen. HEPA filters depend
on a capture mechanism known as diffusion in order to
capture sub-micron size particles. Particles which, are
less than one micron in size, collide with air molecules
which cause them to move in an erratic manner. This random
or zig-zag motion greatly increases the probalility of
the contaminant colliding with filter fibers and ultimately
Why can't the filters currently used in our HVAC
system provide the contaminant removal that we are
A: Most health
care facility HVAC systems do not contain filters with
a high enough efficiency level to remove the TB bacteria
from the air. These filters will only capture a very
small percentage of the submicron to 5 micron size particles
which constitute the size range of TB bacillus.
Why can't I replace the existing low efficiency filters
in my HVAC system with a true HEPA filter?
A: It is
usually very impractical to replace existing filters
with HEPA filters. Most HVAC systems cannot accommodate
the increased resistance to the air from that a HEPA
filter would create. This would mean that new filter
housings capable of holding at least twice as many HEPA
filters as the previous amount of low efficiency filters
would have to be installed in order to overcome the resistance
factor. The filter housing would also have special locking
mechanisms in order to provide an adequate seal around
the filters so that no air could bypass the filter.
If it is impractical to upgrade my HVAC system to
HEPA's, then how and where would I use them?
A: It would
be desirable to have a portable air purifier which utilizes
a HEPA filter while having it's own fan and motor. The
unit should be of sufficient air volume to supply the
required number of air changes to effectively cleanse
the air in the space in which it is being utilized.
-End of Section