Should You Have the Air Ducts in Your Home Cleaned?

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:

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.

Ducts are infested with vermin, e.g. (rodents or insects).

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).

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.

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.

• Indoor Air Quality: An Introduction for Health Professionals
• The Inside Story: A Guide to Indoor Air Quality

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:

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.

Ducts are infested with vermin, e.g. (rodents or insects)

Ducts are clogged with excessive amounts of dust and debris and/or  particles are actually released into the home from your supply  registers.

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.

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.

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.

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).

Check references to be sure other customers were satisfied and did  not experience any problems with their heating and cooling system after  cleaning.

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.

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.

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.

If 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.

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

Open access ports or doors to allow the entire system to be cleaned and inspected.

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.

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.

Protect carpet and household furnishings during cleaning.

Use well-controlled brushing of duct surfaces in conjunction with contact vacuum cleaning to dislodge dust and other particles.

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.)

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.

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.

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?

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:

Use the highest efficiency air filter recommended by the manufacturer of your heating and cooling system.

Change filters regularly.

If your filters become clogged, change them more frequently.

Be sure you do not have any missing filters and that air cannot bypass filters through gaps around the filter holder.

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.

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.

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).

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:

Promptly and properly repair any leaks or water damage.

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.

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.

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.

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.

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.

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.

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.

 

Correct any water leaks or standing water.

 

Remove standing water under cooling coils of air handling units by making sure that drain pans slope toward the drain.

 

If humidifiers are used, they must be properly maintained.

 

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.

 

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.

 

Steam cleaning and other methods involving moisture should not be used on any kind of duct work.

 

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.

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

 

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.

 

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:

 

Show you 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 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.

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.

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

This article originally appeared on EPA.com