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, and level of
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.
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
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 seal or
cover the inside surfaces of the air ducts and equipment housings because
they believe the sealant 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 sealants in your air ducts. They should only be applied,
if at all, after the system has been properly cleaned of all visible dust or
Deciding Whether or Not to Have Your Air Ducts Cleaned
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.
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:
Ducts are infested with vermin, e.g. (rodents or insects); or
Ducts are clogged with excessive amounts of dust and debris and/or
particles are actually released into the home from your supply registers.
- 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.
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.
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". 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
- 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.
- Do not allow the use of chemical biocides or sealants unless you fully
understand the pros and the cons.
- Check references to be sure other customers were satisfied and did not
experience any problems with their heating and cooling system after
- Contact your local 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
- they will use procedures to protect you, your pets, and your home
from contamination; and
- they comply with 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.
- 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.
What to Expect From an Air Duct Cleaning Service Provider
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
- 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 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. After completing the job, ask the service provider to show
you each component of your system to verify that the job was performed
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
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
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
- 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-airconditioned 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.
Should chemical biocides be applied to the inside of air ducts?
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.
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.
In the meantime...
Before allowing a service provider to use a chemical biocide in your duct
work, the service provider should:
1. 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.
2. 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
1. Show you the biocide label, which will describe its range of approved
2. Apply the biocide only to uninsulated areas of the duct system after
proper cleaning, if necessary to reduce the chances for regrowth of mold.
3. 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 seal 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
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, do not currently
recommend the routine use of sealants in any type of duct. Instances when
the use of sealants 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
appropriate guidelines or standards.