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· MERV 12 80% + 85% +

One area that is not addressed at all, in any of the current Standards is the use of Molecular (gas phase) filtration. While this is an arena where there is more to question that what is known, use of molecular filters will be increasing significantly in the next few years. VOC’s, emissions, MVOC’s, toxins all these are being studied for their effects on indoor air quality. New system installations, as well as retrofit installations should be preparing for the installation of

some form of gas phase filtration. Sarin gas and other weapons gases are effectively controlled with proper

molecular filtration systems.

Final Filtration – Many specifiers have set minimum level of filter efficiency for habitant areas of 90% per ASHRAE 52.1 The targeted particle size for the protection from respirable particulates is generally considered to be one micron. This brings us into the E-1 or 0.3 – 1.0 micron size range in the MERV system. While it is possible for some spores and viruses to fit into this categorical size it is unlikely that they would travel as singlets, meaning that these smaller organisms require a vehicle for transport. This could be droplet nuclei or other microscopic particulate.

It is important to recognize that weapons of mass destruction even though man made still fit into the same physical size as naturally occurring organisms or particles. The differences can be surface adhesion treatment as the makers of these agents attempt to distribute organisms in smaller particle size in an effort to improve the effective distribution.

Again reviewing the 90-95% range according to ASHRAE 52.1found filters in the range of MERV 12 to MERV 15. Simply beginning with MERV 13 the discrepancies are significant;

0.3-1.0 microns 1.0-3.0 microns

· MERV 13 less than 75% 90% +

· MERV 14 75 – 85 % 90% +

· MERV 15 85 – 95 % 90% +

While penetration is not a frequently looked at value it becomes important to look at the efficiency at the E-1 category measured in percentage of penetration. 25 – 50% penetration versus 5-15% penetration can be significant when factoring dilution rates etc. This kind of information and the variation of performance levels on the final filters is what led to the use of HEPA filters in surgical areas and other critical treatment rooms within healthcare.

Yet even in the more exact science of HEPA filtration the difference should be measured in penetration through the system rather than the small numbers of difference between .97 and .99 fraction behind the first value. Looking at these values in reverse reveals a three-fold improvement in the penetration protection against small particle intrusion.

Example: 1,000,000 particle challenge @ 0.3 microns

99.97 allows 300 particle penetration

99.99 allows 100 particle penetration

(Efficiency Numbers can be Deceiving – Air Designers Inc #0001-0588-500)

When we consider that there are greater than 5 million particles per cubic foot of air in a typical outdoor sample the penetration values become significant. While the values are slightly different on particles smaller and larger than the tested 0.3 microns the proportional difference in efficiency value remains. Current technology allows for this increase in efficiency without sacrifice of airflow or significant pressure drop increase to the HVAC system.

AIRFLOW INFLUENCE

Airflow plays an important role in filtration performance in a couple of ways that should be reviewed, the flow in or out of a facility by pressure differential and the airflow speed (face velocity) approaching the filter bank.

Face Velocity - All to often we see in filter data sheets that a filter is “rated” at a maximum feet per minute, or some other value. While this may be true, ambient elements such as humidity, temperature, or dirt load can effectively destroy a filter’s integrity at the maximum flow rates. Additionally, filter life doubles with a reduction of only 20% below the maximum rated airflow of the same filter. Particle collection and retention efficiency improves significantly as well. It is possible a filter could move into the next higher MERV category at 75 – 80 % of the maximum rated airflow. Turbulence is reduced in the system allowing a more uniform loading and performance of the filter bank; in addition the effects of ambient conditions are less significant.

The use of filtration devices selected at “Minimum efficiency Standards” while operated at “Maximum rated velocities” leaves no room for error in HVAC system design or building performance. Commonly this leads to premature filter failure or higher maintenance costs to the facility. When using constant volume fans, an increase in the filter pressure drop will start to affect the volume of air supplied, which could start to have negative effects on the building pressurization balance. At lower filter face velocities these changes are minimized allowing for more effective management of the indoor air environment.

Pressure differential – Positively and negatively pressured isolation rooms are terms discussed in most

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