Biological contaminants in the air conveyance systems of buildings are not a “new awareness” rather only more closely scrutinized in the face of current events.  The world has not changed since September 11, only our perception has changed.

 

With the issues at hand education of the most effective methods of protecting your facility starts with knowledge of the organisms.  Once understood in particle size and potential for distribution, plans for preparedness can be developed.  Areas and levels of risk must be established by each facility in relationship to what they can budget.

 

ASHRAE has developed building design and HVAC system guidelines for human comfort.  Interestingly, the new Test Standard 52.2 on Air Filtration Efficiency helps to identify filter performance against the very organisms we fear.  This is done in particle size efficiency values versus particle challenge, labeled MERV Values.  This test method far surpasses the old standard by outlined performance on particle size in minimum performance (or worst) efficiency rather than average lifetime removal.  ASHRAE 52.2 also uses “neutralized” particulate, which means that all “normal surface adhesion charges” are eliminated in an attempt to get true mechanical particle removal efficiency.  What does this mean to Bio-Terrorism?

 

First it is important to recognize the viable particle size of spores, bacteria, and virus and how they could enter our air conveyance systems.  Second of all, how is disease transmitted and developed in people.

 

SPORES

Many organisms grow from spores, from bacterial colonies to redwood trees.  Most spores begin in the 0.5 to 2.0 micron size range.  These are not normally round balls of smooth surface, but rather fuzzy little seeds with design to travel airborne when convenient.  Length is usually greater than diameter, with the smallest mean particle size around 1 micron.  The structure of spores makes them very likely to agglomerate together into larger particles.

 

BACTERIA

These living organisms vary in size from sub-micron diameter to as much as 10 microns in length.  Specifically the Anthrax bacterium is approximately 1 micron in diameter up to 8 microns in length.  As living organisms bacteria require water and food to sustain life.

 

VIRUS

Particle size of virus ranges from .05 to less than .005 microns.  This size particle travels literally by bouncing off other molecules in the air stream, acting more like a gas than solid particle.

 

DISEASE TRANSMISSION

Anthrax is a unique organism in that disease can develop from inhalation, skin surface, or ingestion.  Each of these involves contact with spores, which, once imbedded in an area of nutrient, grow into the bacteria.  Disease is a result of the body defense mechanism reaction to toxins produced by the bacteria.

 

Other known spores from molds such as stachybotrys or aspergillus also produce toxins in ideal conditions.  Again disease is usually a result of body reaction to the mycotoxins produced by the organism.

 

Thus direct contact with spores is the biggest danger in disease transmission, while direct contact with growing mold or bacteria are less likely.

 

Virus is transmitted from host to host, or in other words, from person to person.  Remaining even a few feet away from infected individuals dramatically reduces the chances of infection.

 

FILTRATION

Historically most buildings have utilized filter systems that provide protection of only the HVAC mechanical equipment.  Some proactive organizations have taken steps to improve Indoor Air Quality through upgrades of filter systems.  Protection of the equipment involves filtration that does not even address the 1-micron size range of spores.

 

Using ASHRAE 52.2 Test Standard as a guide to selecting filters for removal of 1 micron and larger at greater than 50%, MERV 10 filters are required.  For removal of 0.5 micron and larger and greater than 80%, MERV 15 is required. (Insert Chart from Page 7.2.4 Third Edition NAFA Guide).

 

While Hepa filtration based upon 99.97% at 0.3 microns would be wonderful to have in all areas, it is not feasible.  Most HVAC systems do not have the fan capacity to handle the pressure drop necessary to utilize Hepa Filters.

 

 

 

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