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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.
Among the possible problems with biocide and ozone application
in air ducts:
- Little research has been conducted to demonstrate the
effectiveness of most biocides 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:
- 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.
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, and 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.
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.
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