Air Quality and Duct Cleaning

/ / Healthy Building Inspections & Testing
NADCA ACR Duct Cleaning Standard

Air quality and duct cleaning are big concerns for some… and big business for others. It seems like every cheap mailer we receive has a scary picture of a rat infested attic and they’re selling duct cleaning services as a way to improve indoor air quality.

Is air quality affected by duct cleaning?

The simple answer is yes. No matter what the condition of the ducts, having a crew of guys lugging around hoses and pumping air throughout the building and mechanical system will affect air quality. The workers bring in their own skin cells, fragranced personal products, and “emissions” (gases). And when they walk on carpet, physically jar loose a supply register or faceplate to the furnace/AC, or turn on their duct “agitator”… there will be an immediate change in indoor air quality. The short-term change is a spike in aerosolized fine particulates.

IF the ducts are significantly dirty… AND IF the duct cleaning company knows their business and follows best practices… THEN the long-term benefits of duct cleaning may justify the short-term disruption in your schedule and spike in aerosolized fine particulates. Which leads us to the real question…

When is duct testing a good idea for air quality?

There are several schools of thought on this matter, and I’m of the school that relies on official unbiased guidelines for determining when ducts should be cleaned. The National Air Duct Cleaners Association (NADCA) has a guideline, NADCA ACR Assessment, Cleaning and Restoration of HVAC Systems. This guideline is available for free from the NADCA website.


It is highly recommended that HVAC systems be cleaned when an HVAC cleanliness inspection indicates that the system is contaminated with a significant accumulation of particulate or if microbial contamination conditions have reached either Condition 2 or Condition 3. If the preliminary inspection shows that HVAC system performance is compromised due to contamination buildup, cleaning is highly recommended.

Often HVAC system components collect significant amounts of debris and particulate during construction activities within a building. It is highly recommended that newly installed HVAC systems or HVAC systems undergoing renovation be verified clean, and protected before the system is permitted to operate. It is highly recommended that consistent HVAC system inspections be part of a building’s overall indoor air quality management program.

NADCA advises a competent inspector carefully evaluate the duct system and major HVAC components on according to Table 1:

HVAC Cleanliness Inspection Schedule - NADCA ACR Table 1

HVAC Cleanliness Inspection Schedule – NADCA ACR Table 1

Air-Handling Units should be inspected a minimum of once per year, and supply and return ductwork should be inspected a minimum of once per one or two years – depending on the building type. This is a good guidance document intended to provide standardization to how ducts are inspected, and what justifies duct cleaning. According to NADCA, a little visible dust in the duct system is insufficient justification for duct cleaning.

The US EPA has also come out with guidelines to help homeowners decide when to have their ducts cleaned. Here are some excerpts from Should You Have the Air Ducts in Your Home Cleanedby the EPA:

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

It may come as a surprise to many that the EPA and NADCA do not recommend routine duct cleaning unless rather significant problems are discovered during an inspection. If there is active mold growth, infestations, or dirt accumulation hindering the performance of the HVAC… you’ve got problems!

Controlling Indoor Air Quality and Duct Cleaning:

So if you’ve really got a problem… will duct cleaning help improve IAQ? The sad truth is duct cleaning – done the wrong way – can cause more problems than it solves. There really is a “right way” of duct cleaning, and the control measures spelled out in the NADCA ACR standard minimize the risk of cross-contamination. The last thing you want is for the contaminants (mold, animal feces, excessive dirt) leave the ducts and contaminate the rest of your living space. The control measures are all about limiting the use of biocides and controlling fine particulate.

The EPA and NADCA guidelines speak at length about the use of biocides in duct cleaning.

EPA Excerpts:

“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.”

“Do not allow the use of chemical biocides or chemical treatments unless you fully understand the pros and the cons.”

“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.”

“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.”

Air quality and duct cleaning are inextricably entwined. So duct cleaning is not without its own risks. Be sure your ducts are inspected by a qualified professional before deciding to have them cleaned. If they do need to be cleaned, refer to the EPA and NADCA guidelines to ensure that air quality isn’t worsened by your well-intentioned duct cleaning.


Healthy Building Science is an environmental consulting firm which provides indoor air quality testing services for commercial, multi-family buildings, offices, industrial and manufacturing workplaces, hospitals and medical facilities, and single-family homes in the greater San Francisco Bay Area and all of Northern California including the cities of San Francisco, San Jose, Oakland, Sacramento.