Just how effective is duct cleaning? The answer is ....

May 2, 2000
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After enduring years of advertising claims about the effectiveness of air duct cleaning — some of them outrageous and almost all of them unfounded in actual science — and after seeing it grow from a fledgling small business opportunity to a multi-million dollar industry, the Environmental Protection Agency (EPA) became interested in 1997.

The EPA decided to conduct a study on the effectiveness of air duct cleaning. The National Air Duct Cleaners Association (NADCA) had the opportunity to get involved in the “Pilot Field Study to Evaluate the Effectiveness of Cleaning Residential Heating and Air-conditioning systems and the Impact on Indoor Air Quality and System Performance.”

An article about that study was published in the April 1999 issue of Better Homes and Gardens. Titled “Duct Cleaning: the Inside Dirt,” it stated that, “…a recent study by the Environmental Protection Agency (EPA) found little reason to recommend duct cleaning. The Agency says there is no proof that having your ducts cleaned will improve the quality of the air in your home, and that duct cleaning has never been shown to improve health problems.”

Industry's defense

That statement is a blow to duct cleaners everywhere — or would be if it were true.

The actual project summary as released by the EPA states, “The results…have demonstrated that mechanical cleaning methods… effectively removed particulate and fibrous contamination.”

It went on to say that duct cleaning’s effects on bacteria, molds, and fungi “could not be fully evaluated because chemical biocides were not used in this study.”

The EPA summary also admits that some of its conclusions were invalidated because some results “may have been attributable to minor repairs of leaks in the ducts and loose floor boots of supply registers” made during the cleaning.

According to Bob Allen, president of Video-Aire International, Fort Worth, TX, the impact of the positive result was weakened by non-compliance with the scientific method.

Plans for further EPA duct cleaning studies have been shelved. However, a scientific study proving the effects of hvac sanitation on indoor mold and fungal populations does exist.

“Effect of heating-ventilation-air conditioning sanitation on airborne fungal populations in residential environments,” was published in the Annals of Allergy, Vol. 71. Commissioned by an air duct-cleaning company in Fort Worth, an independent lab (Mycotech Biological, Inc.) performed this study, which demonstrated a definite decrease in biocontaminants after cleaning.

Different study, different results

Allen and Jerry Betsill, owners of Abbey Road Clean-Aire (predecessor to Video-Aire), wanted to know if the duct-cleaning process they were using really worked.

They had read claims made by another local duct-cleaning company and realized the great tool they would have if they could show quantifiable results.

“Anecdotal evidence, reports from customers, etc., supported our conviction that what we were doing was effective, and we came to believe our process could withstand the scrutiny of a formal test,” recall Allen and Betsill. (Betsill later left Abbey Road to become an attorney.)

They commissioned Robert Garrison and Larry Robertson of Mycotech Biological, Inc., to design a protocol for an independent, controlled study of their duct-cleaning process.

Conducted in 1990 and 91, the study underwent an extensive, two-and-a-half year peer review process before its publication in the Annals of Allergy Medicine in 1993.

Assigned by editorial advisors to an anonymous panel of Ph.D.s and M.D.s, all questions and criticisms had to be answered and addressed to the satisfaction of all members of the panel.

“We had invented a whole new sampling method, so it was the method itself, as well as the results, that were under review,” said Robertson.

This study was later endorsed as “recommended reading” by the American College of Allergy, Asthma, and Immunology during its literature review course in Charleston, SC in 1995.

Study scope

The scope of the Mycotech study was more narrowly focused than that of the EPA study.

The EPA tested for dust levels, hvac efficiency, and effects on IAQ in general. Mycotech teamed with medical doctors specializing in allergy medicine.

Citing a body of research on the many molds and fungi that find fertile ground inside hvac systems and their negative consequences for allergy sufferers, the company tested specifically for hvac sanitation’s effects on these airborne fungal populations.

Robertson and Garrison tested air in its flow stream from supply vents and into return vents, rather than testing physical levels of contaminants inside the ducts.

The idea was to duplicate subjects’ real exposure to airborne contaminants — the air they breathe when their hvac system cycles on.

Roughly the same number of homes (11) and hvac units were tested in both studies. However, while the EPA study tested only one week, with hvac systems in cooling mode, the Mycotech study was performed over the course of one year. The homes were tested in winter heating mode and summer cooling mode.

“We took a chance,” said Allen. “We understood that Mycotech intended to publish their study no matter what their findings, but we believed in our process enough to take the risk and it paid off.”

The results of the study demonstrated a 98% reduction in airborne contaminants from supply vents in summer phase; a 91% reduction from supply vents in winter phase; and an 84% reduction in regard to return air ducts only, in summer and winter phases.

According to Louise Bethea, M.D., P.A., Board Certified Allergist and Immunologist from Houston, TX, this is good news for allergy sufferers.

“Of course, one of the first-line therapies in the treatment of allergies and asthma is the avoidance of offending allergens,” said Bethea. “Heating and air conditioning systems have been established as harbors for molds, and certainly with vents in every room this same system is an efficient distribution system.

“This [study result] indicates dramatically that air duct cleaning by a qualified commercial firm does indeed provide relief from airborne molds, one of the most common allergens.”

What it means

No single company or researcher has all of the answers, and no one involved argues that these studies or their results are definitive.

The bulk of evidence so far indicates that the contamination of air distribution systems in today’s sealed buildings is a real problem, and that hvac system cleaning, if done properly, can be an effective tool in remedying this problem.

More research is needed. Corporations, trade organizations, and government agencies must be willing to initiate new research projects to study a variety of methods for both cleaning and testing.

The challenge lies in defining the terms and ensuring good scientific methods. This will require focusing parameters, establishing clear controls and sticking to them, and being willing to be held up to close scrutiny by the scientific community.

The payoff is that the more information that becomes available, the more effective this industry can be in providing a high-quality, justifiable service to consumers.

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