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Duct Cleaning for Quackers?

Caoimhín P. Connell
Forensic Industrial Hygienist

Although popular marketing organizations promote the idea that clean air ducts equate to clean air, there is actually very little scientific data to support those claims.

According to the US Environmental Protection Agency: 4

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.

Indeed, the reality is that new and freshly cleaned ducts generally result in higher particulate concentrations in the air, all other factors being equal.

To begin with, the absolute statement that cleaning ducts results in cleaner air is inappropriate if for no other reason than it fails to recognize that a duct system in, say, Honolulu will be significantly different than the exact same duct system in, say, Toronto. This statement is true for residential as well as industrial duct/HVAC systems.

In situations where the air stream in ductwork has the possibility of dropping below the dew point or where ducts are actually damp and organisms are actively growing, one would of course clean the ducts, but only after the moisture problem had been corrected.

Let’s use a case example of a typical duct system located in a home in a more temperate zone, say, from 35° north on up. (The 35th north parallel would roughly pass through Albuquerque, Nashville, Oklahoma City, Seville Spain, Nicosia Cypress, Athens Greece, Rabat Morocco, Algiers Algeria, etc.) First of all, one should stop and ask “How did the ducts get dirty in the first place?” Some people seem to think dirt in a duct system is somehow there because of spontaneous generation, and as it “grows,” the threat for entering the air-stream increases; but such is not the case. The ducts got dirty because as air moved through the duct, some of the particulate loading in that air-stream dropped out of the air-stream and remained in the duct. That is, the air coming out of the duct is slightly cleaner than the air going in; the balance of material is that found in the duct. As the duct gets dirtier, turbulence in the duct increases, and, through a variety of mechanisms, a greater proportion of particulates drop out of the air-stream. That is, the air coming out of the duct is even cleaner than before.

The dirt is in the duct because the duct work filtered that junk out of the air, and it remains in the duct because it is stable there. Clean ducts, by virtue of the fact that they are clean, are less efficient filters. The Indoor Environments Division of the U.S. Environmental Protection Agency states 1 there is no evidence to show that cleaning air ducts improves air quality. And The American Lung Association states:

With increasing attention focused on health concerns from biological contaminants and dust in the indoor environment, the ALA offers the following recommendations regarding the use of duct cleaning:

Duct cleaning has not been shown to prevent health problems, nor is scientific evidence currently available to conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts. The U.S. Environmental Protection Agency does not recommend that air ducts be cleaned routinely, but only as needed.

When health problems are believed to be the result of biological contaminants or dust in indoor air, it is important to first determine that contaminated ducts are the cause of the health problems and verify that the ducts are, in fact, contaminated. The source of the problem may lie elsewhere, so cleaning ducts may not permanently solve the problem.

People who have their ducts cleaned should verify that the service provider takes steps to protect individuals from exposure to dislodged pollutants and chemicals used during the cleaning process. This may involve using HEPA filtration when cleaning, providing respirators for workers and having occupants vacate during cleaning."

In a recent study,1a the authors found not only that: Duct cleaning had no measurable effect on supply air quality. but in some of their study buildings, the particulate loading in the air increased following duct cleaning. Not all of the information out there is scientific; even your Mum’s favourite magazine, Better Homes and Gardens concluded: 2

… knowledge about the potential benefits and possible problems of air-duct cleaning is limited because conditions in every home are different. The quality of air inside your home can be affected by cigarette smoking, cooking, and open windows. Even vacuuming the carpets can kick up household pollutants. With so many sources of indoor pollution, it's difficult to prove any role that ducts play in adding to the problem.

One recent study3 is often cited as support for the argument that duct cleaning reduces airborne fungi in residences. However, the conclusions by the authors are not well supported by their reported data. In that study, the authors report that the airborne fungi in a home was reduced after the ducts were cleaned. However, not only did the researchers clean the ductwork, but they also installed high efficiency electrostatic filters in the ventilation units. In their conclusions, the authors attribute their results exclusively to duct cleaning and seem to entirely forget that they also installed the ESP filters. I believe that their data demonstrate that the installation of the high efficiency filters alone was sufficient to produce the reduction of airborne fungal entities seen in their study homes and the duct cleaning had very little if anything to do with the reduction in some airborne fungi. I say “some” airborne fungi because in the study (and not usually reported by those who quote the study to support duct cleaning) the concentrations of several genera of fungi actually increased after duct cleaning. In general, the study performed by Garrison and Robinson has several fundamental deficiencies that reduce the confidence in the results and conclusions.

Cleaning the duct does two things 1) reduces the filtering efficiency of the duct system (i.e. the air coming out of the duct after cleaning is dirtier than before cleaning) and 2) disturbs the material such that any remaining dirt really does have an increased possibility to re-enter the air stream.

Since the relative humidities we see in most properly operating systems are simply not sufficiently elevated to exclusively support mite or fungal growth, these organisms do not normally colonize ducts. Therefore, unless there are very unusual circumstances, at this time, I have not found any compelling reason to recommend the cleaning of the ducts in systems that have not suffered water loss or water intrusion problems.

Our Industrial Hygienist, Caoimhín P. Connell, was recently the featured guest on a trade-oriented radio program, and that interview can be heard here.

1 Indoor Environments Division (6609J) Office of Air and Radiation (OAR) EPA-402-K-97-002, October 1997

1a Kolari S, Luoma M, Ikäheimo M, Pasanen P The Effect Of Duct Cleaning On Indoor Air Quality In Office Buildings Proceedings: Indoor Air 2002

2 Duct Cleaning: The Inside Dirt Better Homes and Gardens April, 1999

3 Garrison RA, Robertson LD, Koehn RD, Wynn SR. Effect of heating-ventilation-air conditioning system sanitation on airborne fungal populations in residential environments. Ann Allergy 1993;71:548- 56.

4 Should You Have the Air Ducts in Your Home Cleaned? EPA 402-K-97-002, October 1997, Last updated, Thursday, November 08, 2012

This page was created on December 12, 2002, and was updated on October 10, 2013. Since then there have been visitors. The page will be updated from time-to-time

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