Brief Discussion
CDC Guidelines for TB

  1. For ventilation, the guidelines refer to "engineering controls" in lieu of 100% outdoor air at the recommended rate of 18 air changes per hour (ACH). See Dilution Ventilation.

  2. The first recommendation is for HEPA filters in any air conveyance system where TB microbes are thought to exist as it is theorized that a HEPA filter of 99.97% at 0.3 micron or better would remove 100% of the TB organisms. Therefore, a number of air changes called equivalent air changes may be passed through a HEPA filter before being sent back to the conditioned space. This would be less the amount used as exhaust to maintain a negative pressure relationship to adjacent spaces. As an example, 15 air changes could go through the HEPA (recirculated) as equivalent air changes and when combined with 3 air changes of 100% OSA used for makeup, the total would be 18 ACH.

  3. The second is for UVC light energy (UVGI) an acceptable control. The guidelines do not refer to UVGI in lieu of HEPA filters but in addition too so as to include the upper air UVC fixtures. However, due to the recognition by the CDC of UVGI's efficacy against TB, it is commonly done in HVAC equipment where 100% outdoor air and/or HEPA filters are not an option. The most notable UVGI user is the National Jewish Center in Denver and it is also the country's leading treatment center for drug resistant TB. UVGI has been used there exclusively for over 12 years! They also have the direct distinction of having the fewest number of conversions to active TB.

When HEPA's are used alone then the design engineer must consider and the end user must perform the proper maintenance and disposal of them. Certified Bag-in / Bag-out side access housings should be utilized to change-out the HEPA filters so as not to expose the airstream and service personnel to viable TB. The spent (used) filters must be properly disposed of, usually as hazardous waste. It is not uncommon to average about $600.00 each for this service. In this application, Steril-Aire has a more logical approach using UVGI upstream of the HEPA (which must be equipped with aluminum separators) which kills the organisms throughout the media (patent pending). There are many advantages:

  1. Eliminating the first, operational and installation cost of a bag-in / bag-out unit.

  2. Eliminating the cost of hazardous waste disposal for a spent filter.

  3. Making the plenum a safer place to enter and the filter much safer to handle.

TB exhaust:

Many installs exist wherein UVC lights are used in lieu of a HEPA filter to treat the exhaust from a TB application. Here is the rationale:

  1. Only requires a pre-filter allowing for common exhaust fan-sets and reduced horsepower (energy savings) to get the job done.

  2. Eliminates big pressure drop swings thus enabling the assurance of maintaining proper airflow (ACH) as well as the design negative pressure at all times.

  3. Eliminates the hazardous waste issue as mentioned above.

It is not uncommon for engineers to select a lower single-pass efficiency (95-99% UVC kill ratio) for TB exhausts because they are not reusing this air but sending it to the outdoors. Note too that the resulting concentration of TB per unit volume in this exhaust air could be lower than that of the steady state level as would be found inside a TB space. TB exhausts should always aim away from air intakes, maintenance personnel and passers-by.

There is currently a lot of discussion on the TB microbe relating to the drug resistant varieties and to the fact that TB can easily become dormant and not just die-away as most might believe. So burying and digging it up later could be a catastrophe, similar to opening up the pyramids only to have some researchers die as a result of their exposure to long forgot microbes. The question becomes "should we catch them or kill them" and of course once you have some history and a working knowledge you choose the latter!

When the preference is to kill them we must look at the "how" and this is where the interest in scientifically testing UVC on TB is renewed, as a way to kill without secondary contamination. Steril-Aire fixtures are currently the only products that work in cold and moving airstreams. So they were selected by NIOSH for the CDC to test UVC's efficacy against drug resistant TB in moving airstreams - and at G.W. Long, Hansens Disease Center's lab in Louisiana. The reason is that these products are the only ones that work predictably under these conditions. When the CDC and Hansens lab finish this job, science would have the evidence it wants to validate what users have demonstrated for many, many years. UVC may then be a direct recommendation to kill TB rather than to catch it.

When lots of air handlers in the US are equipped with UVC, big numbers of every type of microorganism would be killed as a result. The amounts are staggering and would significantly reduce the incidences of colds and flu as well allergies and allergy asthma and of course the many other maladies that give our immune systems a real workout. The gross national output would surely to go up.

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