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TB Questions and Answers - Section 3b

Containment Dilution By Removal With HEPA Filters

 

Q: 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.

Q: 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.

Q: 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 filter.

Q: 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.

Q: Are HEPA filters effective in removing airborne TB bacteria?

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.

Q: Will an air purifier using a HEPA filter eliminate any chance of contracting TB in the space that it is cleaning?

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.

Q: 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 being removed.

Q: Why can't the filters currently used in our HVAC system provide the contaminant removal that we are looking for?

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.

Q: 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.

Q: 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 3b-

Next- Section 3c

 

 

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