Should You Have the Air Ducts in Your Home Cleaned?

Should You Have Your Air Ducts Cleaned Cover
Should You Have the Air Ducts in Your Home Clenaed?
EPA 402-K-97-002, October 1997

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Summary

Knowledge about air duct cleaning is in its early stages, so a blanket recommendation cannot be offered as to whether you should have your air ducts in your home cleaned. The U.S. Environmental Protection Agency (EPA) urges you to read this document in it entirety as it provides important information on the subject.

Duct cleaning has never been shown to actually prevent health problems. Neither do studies conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts. This is because much of the dirt in air ducts adheres to duct surfaces and does not necessarily enter the living space. It is important to keep in mind that dirty air ducts are only one of many possible sources of particles that are present in homes. Pollutants that enter the home both from outdoors and indoor activities such as cooking, cleaning, smoking, or just moving around can cause greater exposure to contaminants than dirty air ducts. Moreover, there is no evidence that a light amount of household dust or other particulate matter in air ducts poses any risk to your health.

You should consider having the air ducts in your home cleaned if:

checkmark There is substantial visible mold growth inside hard surface (e.g., sheet metal) ducts or on other components of your heating and cooling system. There are several important points to understand concerning mold detection in heating and cooling systems:
  • Many sections of your heating and cooling system may not be accessible for a visible inspection, so ask the service provider to show you any mold they say exists.
  • You should be aware that although a substance may look like mold, a positive determination of whether it is mold or not can be made only by an expert and may require laboratory analysis for final confirmation. For about $50, some microbiology laboratories can tell you whether a sample sent to them on a clear strip of sticky household tape is mold or simply a substance that resembles it.
  • If you have insulated air ducts and the insulation gets wet or moldy it cannot be effectively cleaned and should be removed and replaced.
  • If the conditions causing the mold growth in the first place are not corrected, mold growth will recur.
checkmark Ducts are infested with vermin, e.g. (rodents or insects).
checkmark Ducts are clogged with excessive amounts of dust and debris and/or particles are actually released into the home from your supply registers.

If any of the conditions identified above exists, it usually suggests one or more underlying causes. Prior to any cleaning, retrofitting, or replacing of your ducts, the cause or causes must be corrected or else the problem will likely recur.

Some research suggests that cleaning heating and cooling system components (e.g., cooling coils, fans and heat exchangers) may improve the efficiency of your system, resulting in a longer operating life, as well as some energy and maintenance cost savings. However, little evidence exists that cleaning only the ducts will improve the efficiency of the system.

You may consider having your air ducts cleaned simply because it seems logical that air ducts will get dirty over time and should be occasionally cleaned. Provided that the cleaning is done properly, no evidence suggests that such cleaning would be detrimental. EPA does not recommend that the air ducts be cleaned routinely, but only as needed. EPA does, however, recommend that if you have a fuel burning furnace, stove or fireplace, they be inspected for proper functioning and serviced before each heating season to protect against carbon monoxide poisoning.

If you do decide to have your air ducts cleaned, take the same consumer precautions you normally would in assessing the service provider's competence and reliability.

Air duct cleaning service providers may tell you that they need to apply chemical biocide to the inside of your ducts as a means to kill bacteria (germs) and fungi (mold) and prevent future biological growth. They may also propose the application of a "sealant" to prevent dust and dirt particles from being released into the air or to seal air leaks. You should fully understand the pros and cons of permitting application of chemical biocides or sealants. While the targeted use of chemical biocides and sealants may be appropriate under specific circumstances, research has not demonstrated their effectiveness in duct cleaning or their potential adverse health effects. No chemical biocides are currently registered by EPA for use in internally-insulated air duct systems (see Should chemical biocides be applied to the inside of air ducts?).

Whether or not you decide to have the air ducts in your home cleaned, preventing water and dirt from entering the system is the most effective way to prevent contamination (see How to Prevent Duct Contamination).

What is Air Duct Cleaning?

Most people are now aware that indoor air pollution is an issue of growing concern and increased visibility. Many companies are marketing products and services intended to improve the quality of your indoor air. You have probably seen an advertisement, received a coupon in the mail, or been approached directly by a company offering to clean your air ducts as a means of improving your home's indoor air quality. These services typically — but not always — range in cost from $450 to $1,000 per heating and cooling system, depending on:
the services offered

  • the size of the system to be cleaned
  • system accessibility
  • climatic region
  • level of contamination

If you decide to have your heating and cooling system cleaned, it important to make sure the service provider agrees to clean all components of the system and is qualified to do so.

Duct cleaning generally refers to the cleaning of various heating and cooling system components of forced air systems, including the supply and return air ducts and registers, grilles and diffusers, heat exchangers heating and cooling coils, condensate drain pans (drip pans), fan motor and fan housing, and the air handling unit housing (See diagram).

If not properly installed, maintained and operated, these components may become contaminated with particles of dust, pollen or other debris. If moisture is present, the potential for microbiological growth (e.g., mold) is increased and spores from such growth may be released into the home's living space. Some of these contaminants may cause allergic reactions or other symptoms in people if they are exposed to them. If you decide to have your heating and cooling system cleaned, it is important to make sure the service provider agrees to clean all components of the system and is qualified to do so. Failure to clean a component of a contaminated system can result in re-contamination of the entire system, thus negating any potential benefits. Methods of duct cleaning vary, although standards have been established by industry associations concerned with air duct cleaning. Typically, a service provider will use specialized tools to dislodge dirt and other debris in ducts, then vacuum them out with a high-powered vacuum cleaner.

In addition, the service provider may propose applying chemical biocides, designed to kill microbiological contaminants, to the inside of the duct work and to other system components. Some service providers may also suggest applying chemical treatments (sealants or other encapsulants) to encapsulate or cover the inside surfaces of the air ducts and equipment housings because they believe it will control mold growth or prevent the release of dirt particles or fibers from ducts. These practices have yet to be fully researched and you should be fully informed before deciding to permit the use of biocides or chemical treatments in your air ducts. They should only be applied, if at all, after the system has been properly cleaned of all visible dust or debris.

Note: Use of sealants to encapsulate the inside surfaces of ducts is a different practice than sealing duct air leaks. Sealing duct air leaks can help save energy on heating and cooling bills. For more information, see EPA's Energy Star website.

Deciding Whether or Not to Have Your Air Ducts Cleaned

image of air duct components
View a larger version of the graphic

Knowledge about the potential benefits and possible problems of air duct cleaning is limited. Since conditions in every home are different, it is impossible to generalize about whether or not air duct cleaning in your home would be beneficial.

If no one in your household suffers from allergies or unexplained symptoms or illnesses and if, after a visual inspection of the inside of the ducts, you see no indication that your air ducts are contaminated with large deposits of dust or mold (no musty odor or visible mold growth), having your air ducts cleaned is probably unnecessary. It is normal for the return registers to get dusty as dust-laden air is pulled through the grate. This does not indicate that your air ducts are contaminated with heavy deposits of dust or debris; the registers can be easily vacuumed or removed and cleaned.

On the other hand, if family members are experiencing unusual or unexplained symptoms or illnesses that you think might be related to your home environment, you should discuss the situation with your doctor. EPA has published the following publications for guidance on identifying possible indoor air quality problems and ways to prevent or fix them.

You may consider having your air ducts cleaned simply because it seems logical that air ducts will get dirty over time and should occasionally be cleaned. While the debate about the value of periodic duct cleaning continues, no evidence suggests that such cleaning would be detrimental, provided that it is done properly.

On the other hand, if a service provider fails to follow proper duct cleaning procedures, duct cleaning can cause indoor air problems. For example, an inadequate vacuum collection system can release more dust, dirt and other contaminants than if you had left the ducts alone. A careless or inadequately trained service provider can damage your ducts or heating and cooling system, possibly increasing your heating and air conditioning costs or forcing you to undertake difficult and costly repairs or replacements.

You should consider having the air ducts in your home cleaned if:

checklistThere is substantial visible mold growth inside hard surface (e.g., sheet metal) ducts or on other components of your heating and cooling system. There are several important points to understand concerning mold detection in heating and cooling systems:
  • Many sections of your heating and cooling system may not be accessible for a visible inspection, so ask the service provider to show you any mold they say exists.
  • You should be aware that although a substance may look like mold, a positive determination of whether it is mold or not can be made only by an expert and may require laboratory analysis for final confirmation. For about $50, some microbiology laboratories can tell you whether a sample sent to them on a clear strip of sticky household tape is mold or simply a substance that resembles it.
  • If you have insulated air ducts and the insulation gets wet or moldy it cannot be effectively cleaned and should be removed and replaced.
  • If the conditions causing the mold growth in the first place are not corrected, mold growth will recur.
checkmarkDucts are infested with vermin, e.g. (rodents or insects)
checkmarkDucts are clogged with excessive amounts of dust and debris and/or particles are actually released into the home from your supply registers.

Other Important Considerations

Duct cleaning has never been shown to actually prevent health problems. Neither do studies conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts or go down after cleaning. This is because much of the dirt that may accumulate inside air ducts adheres to duct surfaces and does not necessarily enter the living space. It is important to keep in mind that dirty air ducts are only one of many possible sources of particles that are present in homes. Pollutants that enter the home both from outdoors and indoor activities such as cooking, cleaning, smoking, or just moving around can cause greater exposure to contaminants than dirty air ducts. Moreover, there is no evidence that a light amount of household dust or other particulate matter in air ducts poses any risk to health.

EPA does not recommend that air ducts be cleaned except on an as-needed basis because of the continuing uncertainty about the benefits of duct cleaning under most circumstances. EPA does, however, recommend that if you have a fuel burning furnace, stove, or fireplace, they be inspected for proper functioning and serviced before each heating season to protect against carbon monoxide poisoning. Some research also suggests that cleaning dirty cooling coils, fans and heat exchangers can improve the efficiency of heating and cooling systems. However, little evidence exists to indicate that simply cleaning the duct system will increase your system's efficiency.

If you think duct cleaning might be a good idea for your home, but you are not sure, talk to a professional. The company that services your heating and cooling system may be a good source of advice. You may also want to contact professional duct cleaning service providers and ask them about the services they provide. Remember, they are trying to sell you a service, so ask questions and insist on complete and knowledgeable answers.

Suggestions for Choosing a Duct Cleaning Service Provider

To find companies that provide duct cleaning services, check your Yellow Pages under "duct cleaning" or contact the National Air Duct Cleaners Association (NADCA) at the address and phone number in the information section located at the end of this guidance. Do not assume that all duct cleaning service providers are equally knowledgeable and responsible. Talk to at least three different service providers and get written estimates before deciding whether to have your ducts cleaned. When the service providers come to your home, ask them to show you the contamination that would justify having your ducts cleaned.

checkmark Do not hire duct cleaners who make sweeping claims about the health benefits of duct cleaning — such claims are unsubstantiated. Do not hire duct cleaners who recommend duct cleaning as a routine part of your heating and cooling system maintenance. You should also be wary of duct cleaners who claim to be certified by EPA. Note: EPA neither establishes duct cleaning standards nor certifies, endorses, or approves duct cleaning companies.
checkmark Do not allow the use of chemical biocides or chemical treatments unless you fully understand the pros and the cons (See "Unresolved Issues of Duct Cleaning).
checkmark Check references to be sure other customers were satisfied and did not experience any problems with their heating and cooling system after cleaning.
checkmark Contact your county or city office of consumer affairs or local Better Business Bureau to determine if complaints have been lodged against any of the companies you are considering.
checkmark Interview potential service providers to ensure:
  • they are experienced in duct cleaning and have worked on systems like yours;
  • they will use procedures to protect you, your pets and your home from contamination; and
  • they comply with NADCA's air duct cleaning standards and, if your ducts are constructed of fiber glass duct board or insulated internally with fiber glass duct liner, with the North American Insulation Manufacturers Association's (NAIMA) recommendations.
checkmark Ask the service provider whether they hold any relevant state licenses. As of 1996, the following states require air duct cleaners to hold special licenses: Arizona, Arkansas, California, Florida, Georgia, Michigan and Texas. Other states may require them as well.
checkmarkIf the service provider charges by the hour, request an estimate of the number of hours or days the job will take, and find out whether there will be interruptions in the work. Make sure the duct cleaner you choose will provide a written agreement outlining the total cost and scope of the job before work begins.

What to Expect From an Air Duct Cleaning Service Provider

If you choose to have your ducts cleaned, the service provider should:

checkmark Open access ports or doors to allow the entire system to be cleaned and inspected.
checkmark Inspect the system before cleaning to be sure that there are no asbestos-containing materials (e.g., insulation, register boots, etc.) in the heating and cooling system. Asbestos-containing materials require specialized procedures and should not be disturbed or removed except by specially trained and equipped contractors.
checkmark Use vacuum equipment that exhausts particles outside of the home or use only high-efficiency particle air (HEPA) vacuuming equipment if the vacuum exhausts inside the home.
checkmark Protect carpet and household furnishings during cleaning.
checkmark Use well-controlled brushing of duct surfaces in conjunction with contact vacuum cleaning to dislodge dust and other particles.
checkmark Use only soft-bristled brushes for fiberglass duct board and sheet metal ducts internally lined with fiberglass. (Although flex duct can also be cleaned using soft-bristled brushes, it can be more economical to simply replace accessible flex duct.)
checkmark Take care to protect the duct work, including sealing and re-insulating any access holes the service provider may have made or used so they are airtight.
checkmark Follow NADCA's standards for air duct cleaning and NAIMA's recommended practice for ducts containing fiber glass lining or constructed of fiber glass duct board.

How to Determine if the Duct Cleaner Did A Thorough Job

A thorough visual inspection is the best way to verify the cleanliness of your heating and cooling system. Some service providers use remote photography to document conditions inside ducts. All portions of the system should be visibly clean; you should not be able to detect any debris with the naked eye. Show the Post-Cleaning Consumer Checklist to the service provider before the work begins. After completing the job, ask the service provider to show you each component of your system to verify that the job was performed satisfactorily.

If you answer "No" to any of the questions on the checklist, this may indicate a problem with the job. Ask your service provider to correct any deficiencies until you can answer "yes" to all the questions on the checklist.

Post Cleaning Consumer Checklist Yes No
General Did the service provider obtain access to and clean the entire heating and cooling system, including ductwork and all components (drain pans, humidifiers, coils and fans)?
Has the service provider adequately demonstrated that duct work and plenums are clean? (Plenum is a space in which supply or return air is mixed or moves; can be duct, joist space, attic and crawl spaces, or wall cavity.)
Heating Is the heat exchanger surface visibly clean?
Cooling
Components		 Are both sides of the cooling coil visibly clean?
If you point a flashlight into the cooling coil, does light shine through the other side? It should if the coil is clean.
Are the coil fins straight and evenly spaced (as opposed to being bent over and smashed together)?
Is the coil drain pan completely clean and draining properly?
Blower Are the blower blades clean and free of oil and debris?
Is the blower compartment free of visible dust or debris?
Plenums Is the return air plenum free of visible dust or debris?
Do filters fit properly and are they the proper efficiency as recommended by HVAC system manufacturer?
Is the supply air plenum (directly downstream of the air handling unit) free of moisture stains and contaminants?
Metal Ducts Are interior ductwork surfaces free of visible debris? (Select several sites at random in both the return and supply sides of the system.)
Fiber Glass Is all fiber glass material in good condition (i.e., free of tears and abrasions; well adhered to underlying materials)?
Access Doors Are newly installed access doors in sheet metal ducts attached with more than just duct tape (e.g., screws, rivets, mastic, etc.)?
With the system running, is air leakage through access doors or covers very slight or non-existent?
Air Vents Have all registers, grilles and diffusers been firmly reattached to the walls, floors and/or ceilings?
Are the registers, grilles and diffusers visibly clean?
System Operation Does the system function properly in both the heating and cooling modes after cleaning?

How to Prevent Duct Contamination

Whether or not you decide to have the air ducts in your home cleaned, committing to a good preventive maintenance program is essential to minimize duct contamination.

To prevent dirt from entering the system:

checkmark Use the highest efficiency air filter recommended by the manufacturer of your heating and cooling system.
checkmark Change filters regularly.
checkmark If your filters become clogged, change them more frequently.
checkmark Be sure you do not have any missing filters and that air cannot bypass filters through gaps around the filter holder.
checkmark When having your heating and cooling system maintained or checked for other reasons, be sure to ask the service provider to clean cooling coils and drain pans.
checkmark During construction or renovation work that produces dust in your home, seal off supply and return registers and do not operate the heating and cooling system until after cleaning up the dust.
checkmark Remove dust and vacuum your home regularly. (Use a high efficiency vacuum (HEPA) cleaner or the highest efficiency filter bags your vacuum cleaner can take. Vacuuming can increase the amount of dust in the air during and after vacuuming as well as in your ducts).
checkmark If your heating system includes in-duct humidification equipment, be sure to operate and maintain the humidifier strictly as recommended by the manufacturer.

Whether of not you decide to have the air ducts in your home cleaned, committing to a good preventive maintenance program is essential to minimize duct contamination.

To prevent ducts from becoming wet:

Moisture should not be present in ducts. Controlling moisture is the most effective way to prevent biological growth in air ducts.

Moisture can enter the duct system through leaks or if the system has been improperly installed or serviced. Research suggests that condensation (which occurs when a surface temperature is lower than the dew point temperature of the surrounding air) on or near cooling coils of air conditioning units is a major factor in moisture contamination of the system. The presence of condensation or high relative humidity is an important indicator of the potential for mold growth on any type of duct. Controlling moisture can often be difficult, but here are some steps you can take:

checkmark Promptly and properly repair any leaks or water damage.
checkmark Pay particular attention to cooling coils, which are designed to remove water from the air and can be a major source of moisture contamination of the system that can lead to mold growth. Make sure the condensate pan drains properly. The presence of substantial standing water and/or debris indicates a problem requiring immediate attention. Check any insulation near cooling coils for wet spots.
checkmark Make sure ducts are properly sealed and insulated in all non-air-conditioned spaces (e.g., attics and crawl spaces). This will help to prevent moisture due to condensation from entering the system and is important to make the system work as intended. To prevent water condensation, the heating and cooling system must be properly insulated.
checkmark If you are replacing your air conditioning system, make sure that the unit is the proper size for your needs and that all ducts are sealed at the joints. A unit that is too big will cycle on and off frequently, resulting in poor moisture removal, particularly in areas with high humidity. Also make sure that your new system is designed to manage condensation effectively.

Unresolved Issues of Duct Cleaning

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 risks to your health. 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 your air ducts is normal. Duct cleaning is not considered to be a necessary part of yearly maintenance of your heating and cooling system, which consists of regular cleaning of drain pans and heating and cooling coils, regular filter changes and yearly inspections of heating equipment. Research continues in an effort to evaluate the potential benefits of air duct cleaning.

In the meantime

checkmark Educate yourself about duct cleaning by contacting some or all of the sources of information listed at the end of this publication and asking questions of potential service providers.

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 fiber glass duct board or sheet metal ducts that are lined on the inside with fiber glass duct liner. Since the early 1970's, a significant increase in the use of flexible duct, which generally is internally lined with plastic or some other type of material, has occurred.

The use of insulated duct material has increased due:

  • to improved temperature control
  • energy conservation
  • reduced condensation

Internal insulation provides better acoustical (noise) control. Flexible duct is very low cost. 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 supporting significant mold growth. Keeping them reasonably clean and dry is generally adequate. However, there is substantial debate about whether porous insulation materials (e.g., fiber glass) 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, treatment of mold contamination on bare sheet metal is much easier. Cleaning and treatment with an EPA-registered biocide are possible. Once 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. EPA, NADCA and NAIMA all recommend the replacement of wet or moldy fiber glass duct material.

In the meantime

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.

checkmark Correct any water leaks or standing water.
checkmark Remove standing water under cooling coils of air handling units by making sure that drain pans slope toward the drain.
checkmark If humidifiers are used, they must be properly maintained.
checkmark Air handling units should be constructed so that maintenance personnel have easy, direct access to heat exchange components and drain pans for proper cleaning and maintenance.
checkmark Fiber glass, or any other insulation material that is wet or visibly moldy (or if an unacceptable odor is present) should be removed and replaced by a qualified heating and cooling system contractor.
checkmark Steam cleaning and other methods involving moisture should not be used on any kind of duct work.

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

No products are currently registered by EPA as biocides for use on fiberglass duct board or fiberglass lined ducts so it is important to determine if sections of your system contain these materials before permitting the application of any biocide.

Air duct cleaning service providers may tell you that they need to apply a chemical biocide to the inside of your ducts to kill bacteria (germs) and fungi (mold), and prevent future biological growth. Some duct cleaning service providers may propose to introduce ozone 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.

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

  • 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 through the system and released into other areas of your home.
  • Some people may react negatively to the biocide or ozone, causing adverse health reactions.

Chemical biocides are regulated by EPA under Federal pesticide law. A product must be registered by EPA for a specific use before it can be legally used for that purpose. The specific use(s) must appear on the pesticide (e.g., 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 directions.

A small number of products are currently registered by 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 if all label 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.

All of 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 fiber glass duct board or fiber glass lined ducts, so it is important to determine if sections of your system contain these materials before permitting the application of any biocide.

In the meantime

Before allowing a service provider to use a chemical biocide in your duct work, the service provider should:

checkmark Demonstrate visible evidence of microbial growth in your duct work. Some service providers may attempt to convince you that your air ducts are contaminated by demonstrating that the microorganisms found in your home grow on a settling plate (i.e., petri dish). This is inappropriate. Some microorganisms are always present in the air, and some growth on a settling plate is normal. As noted earlier, only an expert can positively identify a substance as biological growth and lab analysis may be required for final confirmation. Other testing methods are not reliable.
checkmark Explain why biological growth cannot be removed by physical means, such as brushing, and further growth prevented by controlling moisture.

If you decide to permit the use of a biocide, the service provider should:

checkmark Show you the biocide label, which will describe its range of approved uses.
checkmark 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.
checkmark Always use the product strictly according to its label instructions.

While some low toxicity products may be legally applied while occupants of the home are present, you may wish to consider leaving the premises while the biocide is being applied as an added precaution.

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. Application of sealants may also affect the acoustical (noise) and fire retarding characteristics of fiber glass lined or constructed ducts and may invalidate the manufacturer's warranty.

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

In the meantime

Most organizations concerned with duct cleaning, including EPA, NADCA, NAIMA and the Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) 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 fiber glass insulation or when combating fire damage within ducts. Sealants should never be used on wet duct liner, to cover actively growing mold, or to cover debris in the ducts, and should only be applied after cleaning according to NADCA or other appropriate guidelines or standards.

To Learn More About Indoor Air Quality

U.S. Environmental Protection Agency

Office of Radiation and Indoor Air
Indoor Environments Division (6609J)
1200 Pennsylvania Avenue, N.W.
Washington, DC 20460

The following EPA publications are available on this web site, some can be ordered from NSCEP. (see also: Publications and Resources)

To Learn More About Air Duct Cleaning

National Air Duct Cleaners Association (NADCA)

1120 Route 73, Suite 200
Mt. Laurel, NJ  08054
Phone: (855) GO-NADCA • (856) 380-6810
E-mail: [email protected]
Website: NADCA

Find a NADCA professional duct cleaner near you.

North American Insulation Manufacturers Association (NAIMA)

44 Canal Center Plaza, Suite 310, Alexandria, VA 22314
Phone: (703) 684-0084
Website: NAIMA
Website: NAIMA Member Company Listing

"Cleaning Fibrous Glass Insulated Air Duct Systems; Recommended Practice", NAIMA Pub. No. AH122, 40 pages (Cost is $7.50 for a printed version, no free copies available.)

Other Useful Resources

For a free list of state consumer protection offices:

State Consumer Protection Offices

For more information on biocides:

Antimicrobial Information Hotline
Phone: (703) 308-0127 / Fax: (703) 308-6467
Monday-Friday 8:00 AM - 5:00 PM EST
E-mail: [email protected] 
Website: Regulating Antimicrobial Pesticides

The Antimicrobials Information Hotline provides answers to questions concerning current antimicrobial issues (disinfectants, fungicides, others) regulated by the pesticide law, rules and regulations. These cover interpretation laws, rules and regulations, and registration and re-registration of antimicrobial chemicals and products. The Hotline also provide information health & safety issues on registered antimicrobial products, product label and the proper and safe use of these antimicrobial products.

Consumer Checklist

checkmark Learn as much as possible about air duct cleaning before you decide to have your ducts cleaned by reading this guidance and contacting the sources of information provided.
checkmark Consider other possible sources of indoor air pollution first if you suspect an indoor air quality problem exists in your home.
checkmark Have your air ducts cleaned if they are visibly contaminated with substantial mold growth, pests or vermin, or are clogged with substantial deposits of dust or debris.
checkmark Ask the service provider to show you any mold or other biological contamination they say exists. Get laboratory confirmation of mold growth or decide to rely on your own judgment and common sense in evaluating apparent mold growth.
checkmark Get estimates from at least three service providers.
checkmark Check references.
checkmark Ask the service provider whether he/she holds any relevant state licenses. As of 1996, the following states require air duct cleaners to hold special licenses: Arizona, Arkansas, California, Florida, Georgia, Michigan and Texas. Other states may also require licenses.
checkmark Insist that the service provider give you knowledgeable and complete answers to your questions.
checkmark Find out whether your ducts are made of sheet metal, flex duct, or constructed of fiber glass duct board or lined with fiber glass since the methods of cleaning vary depending on duct type. Remember, a combination of these elements may be present.
checkmark Permit the application of biocides in your ducts only if necessary to control mold growth and only after assuring yourself that the product will be applied strictly according to label directions. As a precaution, you and your pets should leave the premises during application.
checkmark Do not permit the use of sealants except under unusual circumstances where other alternatives are not feasible.
checkmark Make sure the service provider follows the National Air Duct Cleaning Association's (NADCA) standards and, if the ducts are constructed of flex duct, duct board, or lined with fiber glass, it follows the guidelines of the North American Insulation Manufacturers Association (NAIMA)
checkmark Commit to a preventive maintenance program of yearly inspections of your heating and cooling system, regular filter changes, and steps to prevent moisture contamination.

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Last updated on February 13, 2024