Duct Cleaning and Indoor Air Quality

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With concern about secondary smoke, dust mites, formaldehyde emissions and bioaerosols, the public has become more aware of indoor air quality problems. Heating, air conditioning and ventilation units, as well as associated ductwork, can be the sources of mold, fungi and other microbial pollutants as well as particulates of dust, secondary smoke and pieces of dead dust mites. Along with the public's concern has been the development of businesses directly associated with indoor air quality. Some of these businesses are reputable and supply effective indoor air quality services; others, on the other hand, offer little more than technical jargon and will take advantage of the unwary consumer. Duct cleaning has been an area that has been attracted by both reputable and unscrupulous businesses.

Florida International University study was undertaken to determine the effectiveness of three commercial HVAC duct cleaning processes in reducing the level of airborne particulate matter and certain bioaerosols in fiberglass board duct systems. They investigated the following three procedures:

1. The contact method in which conventional vacuum cleaning of the interior ducts was performed.
2. The air sweep method in which compressed air is introduced to the duct for dislodging dirt and debris and are carried downstream through the duct and out of the system by vacuum collection equipment.
3. The mechanical brush method in which a rubbery brush is inserted into the ductwork to agitate or dislodge debris and, as with the air sweep method, are drawn through the duct out of the system by vacuum collection equipment.

Their conclusion was that duct cleaning using the air sweep and mechanical brush methods was only modestly effective in the short term for indoor air quality improvement. The contact method was least effective. The effectiveness of the contact method as inconclusive in that one of the experimental homes showed reduction while the other experienced an increase of particulates at the one micro level and above. The air sweep and mechanical brush methods indicated a reduction at the one micron level when readings were taken two days after the cleaning.

With respect to bioaerosol reductions, homes cleaned with the air sweep method showed the greatest reduction two days after cleaning. Based upon FIU's study, it appears that the air sweep method is most effective at reducing both particulates at the one micro level and above and bioaerosols.

FIU found that both particulate count readings and bioaerosol concentrations were higher when the cleaning was in progress than before or after. This may be due to disturbances caused by the cleaning process. The general conclusion was that at the 0.3 micro level, no significant changes occurred between the pre-cleaning and post-cleaning readings. Particulates less than one micron cause the most health problems and respiratory damage. One micron is about 1/10 the thickness of a sheet of paper. Cigarette smoking produces 0.3 micron particles, which were at elevated levels in homes that had smokers.

The air sweep and mechanical brush methods reduced the bioaerosol contaminants by approximately 85%. This measurement is expressed in terms of cfu/m3 and was taken two days after cleaning. However, the remaining concentration will quickly reproduce to previous levels unless the underlying condition of high humidity is reduced.

The shortcoming of the FIU study was that readings were only taken immediately prior to cleaning, during cleaning and two days after cleaning. A follow-up study is currently underway to determine the long-term effectiveness of duct cleaning. It is the conclusion of other indoor air quality experts such as T. Brennan, J. Lstiburek and J. Bower, that generally, between one and three months after duct cleaning has taken place, the bioaerosol counts and particulate levels will increase to near their pre-cleaning levels. Studies also suggest that cleaning techniques that agitate the duct systems, particularly fiberglass ducts, may pose a greater problem by loosening fiberglass particles from the ductboard. Fiberglass now is a suspected carcinogen or cancer-causing agent.

The problems originally associated with how the air ducts got dirty in the first place are generally not addressed by duct cleaning methods. It is more important to fix leaking ducts and ensure cooling coil condensate pans are clean and draining properly. Often, these pans will become clogged due to debris, dust and microbial growths. If the drip pan becomes clogged, moisture carryover can occur, wetting adjacent duct in the process. If these are made of uncoated fiberglass duct board, they will become wet and serve as a media for dust mites, mold and other microbial growth that will affect the air quality downstream.

Dr. Virginia Peart (Housing Specialist with the University of Florida), in her internal unpublished fact sheet "Is Your House A Sick House?" (HeadStart-2001) on cleaning of fiberglass ducts, quotes Dr. Thad Godish and Dr. Harriet Burge. These comments were made at the 1990 Indoor Air Quality In Homes: Synthesizing the Issues and Educating Consumers Symposium. Dr. Godish stated:

"A lot of fiberglass ducts are used, and in high humidity environments, they are going to trap organic dust which can result in mold and bacteria growing on them. In response to that, a number of companies are selling a service which really is not going to do much good simply because the ducts are recontaminated so quickly. It is not possible for the cleaning to be as effective as it needs to be. One of the problems with fiberglass ducts is that they have a very rough surface which makes an excellent trap for dust. A lot of dust is organic dust, and if you have organic dust, you are going to get the growth of mold and bacteria."

Dr. Harriet Burge answered the question, "Can fiberglass-lined ductwork be cleaned?", as follows:

"Fiberglass-lined ductwork that is merely dirty (i.e., that has accumulated dust and dirt) can be cleaned if care is taken not to damage the fiberglass. However, if such ductwork has become wet (that is, if mold is actually growing on the fiberglass), it cannot be cleaned. The mold penetrates into the fiberglass and will begin to grow again as soon as surface growth is removed. The use of biocides (agents designed to kill molds and bacteria) is not recommended in ductwork because of the danger to the building occupants of exposure to the biocide."

Dr. Peart recommends certain precautions before engaging duct cleaning services. What do you know about the company? What questions should you ask? Consider these:

How long has the company been in business?

Written testimonials can be impressive, but it can be more important to talk with company clients of a year ago or longer. Most treatments cannot be guaranteed for long periods of time. Test results of mold or other micro-organisms before and soon after cleaning do not tell you if molds will return in a few months.

What chemicals are used in the treatment?

The best duct systems are made of galvanized sheet metal with outside insulation covered by a vapor retarder. The smooth surfaces in these ducts facilitate cleaning and the zinc coating serves as a fungicide that prevents mold and mildew growth.

From an energy conservation and indoor air quality point of view, controlling the pollution source seems to be the best way of keeping duct systems clean and free from trouble. It is important to change filters regularly and to have the air conditioning system serviced by an HVAC contractor once a year. During this process, it is important that they vacuum the heat exchanger, clean coils and assure that the drip pan is clean and draining properly.

The Use of Ozone in Duct Systems

Some businesses are promoting a duct cleaning method using high concentrations of ozone in the duct system. EPA points out that ozone is a very corrosive and toxic gas that is harmful to humans in any concentration. So, it is imperative (if this method is used) that occupants be cleared and remain away from the house until the ozone levels are back to normal. Generally, ozone concentrations of five to seven parts per million are needed to kill microbial and other bioaerosol contaminants that occur in the ducts. While it has been shown to be effective at killing dust mites, bacteria and germs, the EPA points out that dead toxins such as dust mite carcasses and incapacitated mold and other bioaerosol agents can be just as allergenic as the live ones, if not more so. In addition, if the ductwork is wet, only the surface layer containing contaminants will be affected. Ozone, being very corrosive, has the capability of fading carpets and draperies and rusting or oxidizing metals if not controlled properly.

EPA and most experts do not generally recommend the use of biocides, air fresheners and cleaning agents in the filters or the duct systems until they have been specifically approved for that purpose by EPA. Some harsh cleaning agents that kill germs and bacteria may also harm the delicate lining in human lung tissue and in itself be a carcinogen or allergen.

The overall conclusion about duct cleaning is that the air sweep and mechanical brush methods appear to be only modestly effective in the short-term and in the long-term, it may be effective at reducing air pollutants. A more effective approach is to properly select ducts prior to construction and to keep the air conditioning system clean with proper filtration and servicing of the drip pan and cooling and heating coils once a year by a qualified service contractor. Ozone cleaning methods and the use of biocides, germicides, cleaning agents and deodorants should only be used with great caution and only if approved for that purpose by EPA.