The Home Inspector's Guide to Air Duct Cleaning, Part 6: Unresolved Issues of Duct Cleaning

by Nick Gromicko, CMI® and Ben Gromicko



Does duct cleaning prevent health problems?

The bottom line is: no one knows. There are examples of ducts that have become badly contaminated with a variety of materials that may pose a health risk. The duct system can serve as a means to distribute these contaminants throughout a home. In these cases, duct cleaning may make sense. However, a light amount of household dust in the air ducts is normal. Duct cleaning is not considered to be a necessary part of yearly maintenance of the HVAC system, which consists of regular cleaning of drain pans and the heating and cooling coils, regular filter changes, and yearly inspections of the heating equipment. Research continues in an effort to evaluate the potential benefits of air duct cleaning.

In the meantime, educate yourself about duct cleaning by conducting research on the topic. Consider taking InterNACHI’s free, online Indoor Air Quality for Inspectors course.

Are duct materials, other than bare sheet metal ducts, more likely to be contaminated with mold and other biological contaminants?

You may be familiar with air ducts that are constructed of sheet metal. However, many modern residential air duct systems are constructed of flexible insulated duct pipes, fiberglass duct board, or sheet metal ducts that are lined on the inside with a fiberglass duct liner. Since the early 1970s, there's been a significant increase in the use of flexible duct, which generally is internally lined with plastic or some other type of material.

The use of insulated duct material has increased due to improved temperature control, energy conservation, and reduced condensation.  Internal insulation provides better acoustical (noise) control. Flexible duct is inexpensive. These products are engineered specifically for use in ducts or as ducts themselves, and are tested in accordance with standards established by Underwriters Laboratories (UL), the American Society for Testing and Materials (ASTM), and the National Fire Protection Association (NFPA). Many insulated duct systems have operated for years without contributing to significant mold growth. Keeping them reasonably clean and dry is generally adequate. 

However, there is substantial debate about whether porous insulation materials (such as fiberglass) are more prone to microbial contamination than bare sheet metal ducts. If enough dirt and moisture are permitted to enter the duct system, there may be no significant difference in the rate or extent of microbial growth in internally lined or bare sheet metal ducts. However, treating mold contamination on bare sheet metal is much easier. Cleaning and treatment with an EPA-registered biocide are possible. Once a fiberglass duct liner is contaminated with mold, cleaning is not sufficient to prevent re-growth, and there are no EPA-registered biocides for the treatment of porous duct materials. The EPA, NADCA and NAIMA all recommend replacing wet or moldy fiberglass duct material.

Experts do agree that moisture should not be present in ducts, and if moisture and dirt are present, the potential exists for biological contaminants to grow and be distributed throughout the home. 

Controlling moisture is the most effective way to prevent biological growth in all types of air ducts. which is why:

  • Any water leaks or standing water should be corrected.

  • Standing water under the cooling coil of the air-handling unit should be removed by making sure that the drain pan slopes toward the drain.

  • If a humidifier is used, it must be properly maintained.

  • The air-handling unit should be constructed such that maintenance personnel have easy and direct access to the heat exchanger's components and drain pans for proper cleaning and maintenance.

  • Fiberglass or any other insulation material that is wet or visibly moldy should be removed and replaced by a qualified HVAC system contractor.

  • The same action should be taken if an unacceptable odor is present.

  • Steam cleaning, or other methods involving moisture, should not be used on any kind of ductwork.

Should chemical biocides be applied to the interior of air ducts?

Air duct cleaning service providers may tell you that they need to apply a chemical biocide to the inside of the ducts to kill bacteria (germs) and fungi (mold), and to prevent future biological growth. Some duct cleaning service providers may propose introducing ozone in the system to kill biological contaminants. Ozone is a highly reactive gas that is regulated in the outside air as a lung irritant.  However, there remains considerable controversy over the necessity and wisdom of introducing chemical biocides or ozone into the duct work.

No products are currently registered by the EPA as biocides for use on fiberglass duct board or fiberglass-lined ducts, so it's important to determine whether sections of the HVAC system contain these materials before the homeowner permits the application of any biocide.

Among the possible problems with biocide and ozone application in air ducts are:

  • Little research has been conducted to demonstrate the effectiveness of most biocides and ozone when used inside ducts. Simply spraying or otherwise introducing these materials into the operating duct system may cause much of the material to be transported throughout the system and released into other areas of a home.
  • Some people may experience a negative health reaction to the biocide or ozone.

Chemical biocides are regulated by the EPA under federal pesticide laws. A product must be registered by the EPA for a specific use before it can be legally used for that purpose. The specific use(s) must appear on the pesticide (biocide) label, along with other important information. It is a violation of federal law to use a pesticide product in any manner inconsistent with the label's directions.

A small number of products are currently registered by the EPA specifically for use on the inside of bare sheet metal air ducts. A number of products are also registered for use as sanitizers on hard surfaces, which could include the interior of bare sheet metal ducts. While many such products may be used legally inside of unlined ducts (as long as all of the label's directions are followed), some of the directions on the label may be inappropriate for use in ducts. For example, if the directions indicate "rinse with water," the added moisture could stimulate mold growth.

The products discussed above are registered solely for the purpose of sanitizing the smooth surfaces of unlined (bare) sheet metal ducts. No products are currently registered as biocides for use on fiberglass duct board or fiberglass-lined ducts, so it's important to determine whether sections of the HVAC system contain these materials before the homeowner permits the application of any biocide.

Before the homeowner allows a service provider to use a chemical biocide in their home's ductwork, he or she should:

  • Provide visible evidence of microbial growth in the ductwork. Some service providers may attempt to convince the homeowner that the air ducts are contaminated by demonstrating that micro-organisms found in the home can grow on a settling plate (i.e., petri dish). This is inappropriate. Some micro-organisms are always present in the air, and some growth on a settling plate is normal. As noted earlier, only an certified mold inspector can positively identify a substance as biological growth, and lab analysis is typically required for final confirmation. Other testing methods are not as reliable or conclusive.
  • Explain why biological growth cannot be removed by physical means, such as brushing, with further growth prevented by controlling moisture.

If a homeowner decides to permit the use of a biocide, the service provider should:

  • Show the homeowner the biocide label, which will describe its range of approved uses.

  • Apply the biocide only to un-insulated areas of the duct system after proper cleaning, if necessary, to reduce the chances for re-growth of mold.

  • Always use the product in strict accordance with the label's instructions.

As an added precaution, the homeowner should consider leaving the premises while the biocide is being applied.

Do sealants prevent the release of dust and dirt particles into the air?

Manufacturers of products marketed to coat and encapsulate duct surfaces claim that these sealants prevent dust and dirt particles inside air ducts from being released into the air. As with biocides, a sealant is often applied by spraying it into the operating duct system. Laboratory tests indicate that materials introduced in this manner tend not to completely coat the duct surface. The application of sealants may also affect the acoustical (noise) and fire-retarding characteristics of fiberglass-lined or constructed ducts and may void the manufacturer's warranty.

Questions remain about the safety, effectiveness, and overall desirability of sealants. For example, little is known about the potential toxicity of these products under typical-use conditions or in the event that they catch fire.

In addition, sealants have yet to be evaluated for their resistance to deterioration over time, which could add particles to the duct air.

Most organizations concerned with duct cleaning, including the EPA, do not currently recommend the routine use of sealants to encapsulate contaminants in any type of duct. Instances when the use of sealants to encapsulate the duct surfaces may be appropriate include the repair of damaged fiberglass insulation, or when mitigating fire damage within the ducts. Sealants should never be used on a wet duct liner, to cover actively growing mold, or to cover debris in the ducts, and should only be applied after cleaning according to appropriate guidelines or standards.


Whether or not a home inspector should recommend that his client have the air ducts in their house professionally cleaned is a decision that's based on several important factors. A home inspector is not required to determine the home's air quality, the presence of airborne hazards (including mold), or the presence of rodents or insects. Determining the need for or recommending air duct cleaning is also beyond the scope of a general home inspection. That's why InterNACHI® encourages home inspectors to continue educating themselves on the topic of indoor air quality and the subject of duct cleaning, and to become an InterNACHI® Certified Indoor Air Quality Inspector.


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