The results of a new independent study on the benefits of incorporating antimicrobial chemicals into HVAC air filters give the IAQ industry more fodder for debate in this often controversial subject. IE Connections has obtained an advance copy of the final report of ASHRAE research project 909-RP, entitled, “Determine the Efficacy of Antimicrobial Treatments of Fibrous Air Filters.” Karin K. Foarde and James T. Hanley of North Carolina’s Research Triangle Institute (RTI) performed the study under contract to ASHRAE.

According to ASHRAE the research project objective was to, “determine the efficacy of antimicrobial agents on fibrous air filters and the potential ability of these treatments to inhibit filter media surface growth and subsequently reduce indoor air microbiological particulate contamination.” To fulfill that objective the RTI scientists performed both laboratory and field research on pleated panel filters and ring panel filters.

Filters from three different manufacturers were obtained, each of which incorporates a different antimicrobial agent in their product. ASHRAE specified that the manufacturers and the antimicrobial agents remain anonymous in the project report. However, IE Connections has obtained exclusive information regarding their identity.

Project 909-RP identifies the antimicrobials by the numbers 1, 2, and 3. Antimicrobial 1 is Sporax, a chemical used in the filtration industry exclusively by Fiberbond Corp. Antimicrobial 2 is INTERSEPT^®, licensed for use on air filters to American Air Filter International and sold under the PolyKlean Gold brand name. Antimicrobial 3 is Aegis Microbe Shield, used by a diverse group of filter manufacturers including Air Kontrol, Environmental Products Group, J. Barba, Koch Filter Corp., Martin Color Fi, Mountville Mills, Purolator and Tri-Dim Filter Corp.

Study Results Mixed

ASHRAE sought answers to specific questions through the study, the most important of which is whether the incorporation of an antimicrobial inhibits biological growth on both clean and dust-loaded fibrous filters. In growth studies the answer depending the antimicrobial, the ERH, and dust loading – varied significantly.

Antimicrobial 1 was the most efficacious. When incorporated into a pleat filter, it prevented microbial growth both with and without dust loads at all ERH levels tested. In the ring panel filter, antimicrobial 1 prevented microbial growth on the clean filter but failed on the dust-loaded filter, where it allowed growth at ERH levels over 85 percent.

Antimicrobial 2 thwarted biological growth on both the clean pleat filter and clean ring filter at all ERH levels tested. It failed to prevent growth on the dust-loaded filters, performing more poorly on the ring filter where microbial growth was measured at ERH 80 percent and above.

Antimicrobial 3 performed worst of all. On the clean pleat filter and the clean ring filter growth was detected starting at ERH 85 percent and 94 percent respectively. When dust-loaded, growth was detected at ERH 80 percent and above on both types of filters.

The report notes that while the types of filters tested for each antimicrobial were identical, since filters were produced by different manufacturers for each antimicrobial, “the filter media composition may vary with each.” Some untreated filter media may be inherently hostile to growth.

Interpreting the Results

Al Veeck was chairman of the ASHRAE Project Monitoring Committee responsible for the study’s oversight. When asked whether there is an IAQ value to using a filter incorporating an antimicrobial agent, he replied, “two of the filters performed fair to poor, so the answer for those filters appears to be ‘no.’ One performed well and there may be some value to using it in certain instances.” Veeck went onto emphasize that antimicrobial treated filters are only a small part of the overall solution to preventing biological growth indoors.

“If you cut an artery, a band aid isn’t going to solve the problem. You need to sew the artery closed. It’s the same with mold inside HVAC systems. When you find filters with biological growth, it’s always due to a problem upstream in the system. If that problem were fixed, there would be no need for an antimicrobial filter,” argues Veeck.

Chuck Rose is the Director of Research and Development with AAF International and also serves as Chairman of ASHRAE TC 2.4 “Particulate Air Contaminants and Particulate Contaminant Removal Equipment.” He agrees that, “if you have a really grimy, dirty system the [antimicrobial treated] filter isn’t going to make much of a difference to IAQ, if any.”

Rose says the real value to using an antimicrobial filter is to prevent mold from growing on the filter. “Filters catch dirt, and microbial contaminants are often attached to the dirt. There are circumstances when biological contaminants would grow on filters, and an antimicrobial treatment may prevent that,” Rose told IE Connections.

According to Bob Williams of Fiberbond Corp., preventing growth on filters is why his company pioneered antimicrobial filters twelve years ago. “The independent testing shows that two things our distributors have been telling their customers for years hold true. First, including an antimicrobial in a filter does not change the effectiveness of the filter. Second, selecting the right antimicrobial can protect the filter for its entire life,” Williams said.

Veeck cautions that it is not wise to purchase an antimicrobial treated filter with the misconception that it can be safely left in the system longer than an untreated filter. “The ASHRAE study proves that dust-loading greatly inhibited the ability of all the filters to prevent microbial growth. Once that layer of dust coats the media the antimicrobial can’t do its job,” he said. However, Veeck does concede that, “in hot, humid climates the use of an antimicrobial filter may prove beneficial as part of a holistic approach to IAQ.”

Other Results of Interest

While the efficacy results are what will capture most people’s attention, Rose says other findings from 909-RP are equally important. “This study produced information that’s never been available before, such as the conclusion that antimicrobial treated filters show no signs of offgassing and present no unique disposal concerns. Equally important was the finding that antimicrobial treatments had no effect on filtration efficiency. These are good research results,” Rose remarked.

Project 909-RP also fostered the development of a new ASTM method to evaluate the susceptibility of a material to bacterial and fungal growth. ASTM Method 6329-98, Standard Guide for Developing Methodologies for Evaluating the Ability of Indoor Materials to Support Microbial Growth Using Static Environmental Chamber, was developed by Foarde and her colleagues at RTI. “We expect that ASTM Method 6329-98 will be widely used in future studies related to mold growth and the indoor environment,” commented Veeck.