Antimicrobial Treatments on HVAC Air Filters

Product claim: "Destroys mold, mildew and bacteria... prevents growth of microbes in the dirty filter."

What we've found: Antimicrobial treatments sometimes produce dramatic results under artificial test conditions. However, most studies suggest that these treatments work marginally, if at all, in real world air filter applications.

Product claim: "Will not off gas... will not migrate from the filter, will not leach into the environment."

What we've found: While in many cases these claims are true, they also help to explain why antimicrobial treatments on air filters are unlikely to be effective. Direct contact is necessary for the treatment to penetrate and destroy a microorganism. This may occur briefly on clean treated filters. But as soon as dust builds up on the filter surface, direct contact can no longer be achieved-and precisely because there is no off gassing, the treatment is no longer effective.

Product claim: "Significantly reduces airborne microorganisms."

What we've found: A well-designed, high efficiency air filter will significantly reduce airborne microorganisms by trapping them in the filter matrix. There is no conclusive evidence, however, that antimicrobial treatments cause any further reduction in the level of airborne microorganisms.

Product claim. "Increases filter life... improves efficiency... reduces energy costs... lowers static pressure?"

What we've found: We have seen no conclusive test results to support any of these claims... nor have we found a reasonable explanation as to how antimicrobial treatments could possibly offer these advantages.

A properly designed, high efficiency air filter can greatly reduce airborne concentrations of these harmful respirable particulates. As a filter collects dust, however, more and more of these microorganisms become concentrated in the media matrix over time.

Antimicrobial treatments for air filtration products have recently become a topic of considerable interest. These antimicrobial agents, which are intended to destroy or inhibit the growth of microorganisms, have been used in commercial and industrial surface applications (i.e. walls, carpets, etc.) for several years. Applied to air filter media, the treatments purport to improve indoor air quality by destroying a broad range of microorganisms and preventing the growth of microbes in the dirty filter.

Are antimicrobial treatments really effective when used in air filters? Camfil Farr's Research and Development group first studied this question as far back as the 1960s. In recent years, the company has resumed its research on antimicrobial treatments for air filters. The conclusion, both 30 years ago and today are the same. Based on existing evidence, antimicrobial treatments appear to work marginally, if at all, when applied to air filter media.

The following survey of recent studies strongly supports this position.

An ASHRAE sponsored report by Foarde, Karin K. et al, describes numerous investigations of the penetration of challenge organisms through test filters. In two cases, disinfectant or bacteriostatic treatments were applied to the filter media. Viable organism penetration was unchanged by the filter treatment, and bacteriostatic action could not be verified.

Another ASHRAE sponsored study by Kuehn, T.H. et al, found that at high humidity (90% RH and above), with intermittent wetting and a significant dust load, fungi and bacteria would grow at a very slow rate. Rapid growth could be affected by loading filters with an artificial nutrient. But at humidity of 70% RH and below, without significant wetting or dust loading, no organism growth could be found on a polymer media, and only minor fungal growth could be found on the glass fiber filters. After filters were exposed to spray wetting, increased fungal growth and spores could be found on the downstream side of the filters. It could not be clearly determined whether these microorganisms grew through the filter media or were washed through.

Price, D., et al, compared untreated filters and media with those treated with an antimicrobial agent (specifically, a water-soluble, phosphated quaternary amine biocide compound). This is the only study to find that an antimicrobial agent markedly inhibited the growth of organisms. However, the findings are questionable in that the most dramatic results were achieved on dust-laden media placed in an agar culture that was more than 80& water. Placed in this moist environment, the water-soluble antimicrobial agent could easily infiltrate the culture medium and promote growth on the media. This effect would not occur in the dry state and will not necessarily kill fungal spores in the wet state. (The production of spores is a survival mode used by many organisms to cope with adverse conditions. The spore is highly resistant to almost every destructive method known except fire or superheated steam under pressure.)

Ohgke, H., et al, found that fungal growth on filters could only be found under unrealistic conditions. Polycarbonate fiber media filters did not exhibit fungal growth, even after wetting and inoculation of nutrients. This paper denies empiric observations of microbial growth on filters observed by many others.

Kalliokaski, P., et al, studied fungal growth on a variety of building materials. The study determined that moisture is the most significant factor affecting fungal growth. It was found that relative humidity below 70% with no condensation fostered minimum growth. Above 96% RH, mold growth increased dramatically.

Additional studies by Crow, S.A., et al, and Moore, R., gave further support to the critical role of moisture control.

Based on these studies, it appears doubtful that antimicrobial treatments accomplish anything more than a very minor reduction in microbiological growth on filters, and do nothing to prevent viable organisms (i.e. those unaffected by the antimicrobial agent) from growing downstream of the filters if the right conditions exist. Also, none of the studies was able to determine whether microorganisms downstream of the filters grew through or were washed through the fitters by excessive water in the system. Also, the studies did not determine what percentage of microorganisms found on the downstream side of treated filters were still viable as compared with untreated filters.

Conclusions

The effectiveness of antimicrobial treatments on air filter media is marginal at best. The primary reason; direct contact is required for an antimicrobial agent to penetrate and destroy microorganisms. While this may occur for a brief time on clean treated filters, direct contact is prevented as soon as a minimal dust cake builds up on the filter surface. Once this buildup occurs, there is no evidence that treated media prevents the growth of microorganisms.

There is little evidence that viable microorganisms which are not captured but pass through an antimicrobial filter are killed or Inhibited in any way after arriving downstream of the filter.

There is conflicting evidence as to whether microorganism growth on the upstream side of the filter leads to penetration and/or grow-through to the downstream side of the filter, followed by shedding... unless significant water wetting of the filter occurs.

There is also no evidence that antimicrobial filters prevent any of the aforementioned problems.

Several studies agree that the best way to inhibit biological proliferation is to keep relative humidity below 70% and prevent water from entering the HVAC system, and its filters.

Future Outlook

A proven antimicrobial treated air filter has not yet been achieved. Camfil Farr is actively pursuing treatments and application methods, which may eventually provide an effective way to reduce proliferation of microorganisms on filter media, and, at the same time, significantly restrict, filter pass-through of viable microorganisms.

The investigative arms of several professional engineering societies are also studying these matters. It is anticipated that some positive information will be forthcoming in the near future.

To prevent microbial growth:

• Operate your HVAC system within ASHRAE humidity design criteria, under 60%
• Operate HVAC system to provide design temperatures
• Eliminate areas of free standing water.

To obtain a print copy of this document please send an email with your name,  address, and requested documant  to literature@camfilfarr.com. For a PDF copy of this document CLICK HERE.