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we review the application of Standards as relates to the Quality of Air. This relates to whether we are referring to protection from Bio/Chem agent infiltration or normal Indoor Air Quality indicators. Key areas discussed here are:

- Filter efficiency levels

- Airflow influence

- System Integrity

- Performance verification

- Recommendations

FILTER EFFICIENCIES

This discussion must begin with MERV. In 1999 the ASHRAE committee brought us Minimum Efficiency Reporting Value or ASHRAE 52.2, a testing procedure that finally gave us a measure of filter performance that made sense. Interestingly, nearly five years later and we still find most individuals writing specifications and purchasing filers using the “old ASHRAE 52.1 Standard”. We should briefly look at what the two Test Standards offer in the way of information. To synopsize the test procedures it is very obvious that they are worlds apart from each other.

ASHRAE 52.1 – Average Atmospheric Dust Spot efficiency. This test assumes that we should select filters based upon their average efficiency. The efficiency is a measure of opacity change of pieces of litmus paper under a white light meter comparing upstream to downstream against atmospheric air. This means that very little knowledge is generally imparted to the end user as to the initial efficiency of a filter. Now when looking at the need for proper filtration when would one need the 95% efficient filter? Most folks when faced with this choice would respond – “immediately”. However the five best selling minipleat filters rated at 95% Average Dust Spot Efficiency have initial efficiencies that range from 57% to a high of 81%. These will achieve their rated value when loaded with enough ASHRAE dirt, which is a mixture of very large particles as compared to atmospheric dirt.

ASHRAE 52.2 – Minimum Efficiency Reported Value means that the worst value is determined over the life of the product and that is where the filter is reported on performance. The same story applies on most filters, that is as the filter loads the efficiency improves. This test is done on specific size ranges measured in microns and is accomplished by the generation of particles to test 12 size ranges.

As the world of contaminants becomes more thoroughly understood their size, transport mechanism, and life characteristics allow us to make choices of filtration geared to removing specific target sizes. Recognition of the effects of inadequate filtration helps us to identify areas of needed improvement. It is time that we start writing specifications on the size of particle that we must remove for the intended use.

It is no secret that many filters, equal in ASHRAE 52.1 are very different in performance according to MERV. This variation is not just limited to one end of the spectrum it affects the whole scale from MERV 5-16. It is possible to have 30% filters that range from MERV 6-9 and 90% filters that range from MERV 12-15. Do we ignore this knowledge or do we change our Standards to specify the efficiency for the particle size of protection the application requires?

Prefiltration - Most specifiers are in agreement that the minimum prefiltration or single stage filtration for any areas should be 30% according to ASHRAE 52.1 test procedure. This level is predominantly intended to protect the HVAC equipment from fouling. By looking at the possible MERV values that have derived from the 30% efficient value we find a significant variation in the protection against the type of particulates that could cause fouling. The E-3 category is based on the 3 – 10 micron size challenge, recognizing that this is a composite curve of efficiency on the range let’s review:

· MERV 6 35 – 50%

· MERV 7 50 – 70%

· MERV 8 70 – +%

· MERV 9 85 % minimum

With this knowledge when we compare particulates that we expect these filters to remove several natural occurring microbial organisms fit into this size range. Pollens run 5-10 microns; spores from the fungi world are regularly above three microns including stachybotras spores that run 5-8 microns in size. If the best we can expect from our filters is around fifty percent performance on removal of this range of contaminants then we must also realize that we did not select the efficiency for the intended use. What develops is a known future need to clean and disinfect the system.

In systems that utilize HEPA filters downstream we must recognize the need to remove a higher percentage of contaminants in the E-2 range or 1 – 3 micron size, as the under three micron world of microbial organisms can find a home in the HEPA filter. HEPA’s, in a cooling system, have enough restriction to airflow to cause a thermal change that can create enough moisture for microorganisms to grow.(Kevin Corfmann Phd Physical Science “Mass Flow Characteristics”) In air handling equipment in the humid climates of the world the air supplied to the HEPA is 100% saturated allowing organisms to grow. Once growing these can send their hyphae through the media and regrow themselves on the downstream side thus shortening the life of HEPA filters. (Daniel L Price -Microbiologist) For this reason the use of higher MERV value prefilters will reduce the possibility of microbial contamination in the HEPA bank. Efficiency of prefiltration upgrade to higher level MERV

1-3 micron 3-10 micron

· MERV 10 50 – 65% 85% +

· MERV 11 65 – 80% 85% +

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