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Indoor Air Quality – The Future

In the last issue, we discussed the July 21, 22, and 23, meeting held in Research Triangle Park, North Carolina, ‘Engineering Solutions to Indoor Air Quality Problems’. The Air & Waste Management Association and the U.S. Environmental Protection Agency sponsored this. As we pointed out in the last issue, this conference, although small, has great impact on future trends in both the government and industry. This issue we will complete our discussion by reporting on presentations given the last day and during the poster sessions dealing with bacterial and fungal growth in air ducts and other parts of the system.

Microbial Contamination of Ventilation Systems

Jerry Tulis of Duke University Medical Center reported a study of ventilation systems in buildings on the campus. Reports of growth in air ducts and associated parts of HVAC systems are often discounted. This is because the sampling techniques commonly used such as swabs and contact plates collect large number of spores as well as any organisms that might be growing. Experts have pointed out that spores are present everywhere and will grow when analyzed in the laboratory giving a false reading. They argue that there may be spores present but they do not grow in the HVAC system so do not cause problems. As a result, they argue that nothing needs to be done to control growth in systems.

In this study, a sampling method was used that would collect only growing organisms. Thus, if only spores were present, there would not be false results. The study demonstrated that fungal growth could occur in any building ventilation system, with the most likely locations being the air handling unit components in close proximity to the conditioning coils. This study offers strong evidence that some form of antimicrobial treatment is needed to avoid HVAC system contamination from growth.

Does Cleaning Control Growth?

Another question that we deal with constantly is how best to control growth in air ducts. Many have argued that keeping system components clean will prevent growth. They suggest that when growth is present, you can remove the built up contamination and it will not re-grow. Karin Foarde of the Research Triangle Institute presented the results of a study that investigated the effectiveness of cleaning as a growth control method. She tested two fiberglass duct liners and sheet metal duct stock. All were artificially soiled. The results showed that notable amounts of surface dust and surface spores were removed on all materials by vacuuming. However fungal re-growth occurred within 6-12 weeks. Interestingly, one of the duct liners tested was factory treated to resist growth. There was no difference observed in the ability of that sample to resist growth and the others. This study would seem to disprove the theory that cleaning alone is sufficient to control growth. Therefore, preventing microbial contamination requires something more than keeping the system clean.

Does Growth in Ducts Hurt Indoor Air Quality?

To most this may seem like a silly question. The common sense answer to most people is, ‘of course.’ It just seems logical that if there were a high level of growth in air ducts that the air passing through those ducts would become contaminated and in turn lead to poor air quality in the conditioned space. In spite of this, many experts insist that there is little harm from fungi and bacteria that are growing inside of an air conditioning system. This is because studies have been done where systems were highly contaminated and yet levels of spores in the conditioned space do not appear to be elevated.

This study was done to investigate under controlled conditions the amount of contamination given off by active growth inside of an air conditioning system. Previous data was from actual buildings. In operating buildings, data collection is difficult. People visiting and working the building plus other activities such as cleaning can effect the level of contamination measured as much as or more than the operation of the system. In this study, air was passed through ducts where high levels of active growth were present. So that growth would be high, a level of 95% RH was maintained inside the ducts. The air coming out of the ducts was sampled regularly over a period of six weeks. Very low levels of spores and microbes were identified in the air supply. This appeared to confirm the theory that growth does not matter.

The researchers then began lowering the humidity and continued sampling. There still was little biological matter given off until the humidity level reached to between 60 and 65%. At these levels, emissions began to increase significantly. They then compared flows at 55% and 95% RH. The emission rates were very different. It appears that at the higher levels of RH the organisms grew but did not give off emissions. At the lower relative humidity, the organisms ceased active growth and generated massive amounts of spores, resulting in high contamination. This would seem to account for the IAQ problems often experienced by buildings in humid climates. The air conditioning system will be off during the night or over the weekend. Extensive growth will occur. During normal operating hours, the system operates and brings the humidity level down. At this point, fungi stop growing and produce spores, which is the way they reproduce. These spores travel throughout the conditioned space and lead to air quality complaints.

Attitudes Will Change

As these research findings are discussed, debated, and additional studies are done, policies, practices, and eventually codes will change. Where the control of microbial contamination may have been seen as un-needed in the past, it will become mandatory. Contractors who have been recommending preventative maintenance practices to their clients all along will be seen as innovative and proactive. Those who do not concern themselves with system hygiene will be considered out of date.

Bob Baker is a member of ASHRAE and Chairman and CEO of BBJ Environmental Solutions, Inc., a manufacturing company specializing in providing clean air through environmentally responsible products, such as BBJ MicroBiocideâ„¢, BBJ Micro Coil Cleanâ„¢, BioSoft Spray Disinfectant/Cleanerâ„¢, and Fresh Ductâ„¢, as well as The Indoor Air First Aid Kitâ„¢. For additional information, Mr. Baker can be reached at (800) 889-2251 or through the company web site at