Heather N. Kolich, ANR Agent, UGA Extension Forsyth County
Ventilation is important for a healthy home, and it is one of the functions of the HVAC system: Heating, Ventilation, and Air Conditioning. This forced air system pulls air into the heating or cooling unit, changes its temperature to reach the setting of the thermostat, then blows the air to different rooms through ducts. Filters at the air intake locations trap airborne particles before the air enters the HVAC system.

The main purpose of the filter is to keep dirt, dust, and particles from damaging HVAC system components. Particles entering the heating or cooling unit could damage the blower fan, and a buildup of dirt inside the units can decrease efficiency or shorten their functional life. An added benefit of the filter is that it keeps dirt, dust, and particles from circulating through the house and getting into the air we breathe.
So, if HVAC filters trap dust and dirt before it enters the duct system, why would the ducts need cleaning?
The effectiveness of filters in capturing particles varies widely and is described by effectiveness ratings such as Merv (Minimum Efficiency Reporting Value), MPR (Microparticle Performance Rating), and FPR (Filter Performance Rating). The lowest rated filters (MERV 1-4) are less than 20% effective at capturing “large” particles of dust and pollen measuring 3-10 microns in size. At MERV 13 and higher (MPR 1500-2000; FPR 10) filters capture particles as small as 0.3 microns, including mold, pet dander, bacteria, and viruses, with 50-95 percent efficiency.

Health concerns, a desire to reduce dust levels, and enhancing efficiency of the HVAC system seem to be the top reasons people consider having their air ducts cleaned.
A review of 48 research studies (Zuraimi, 2010) indicates that duct cleaning does not prevent health problems. Duct cleaning can, in fact, release particles that have adhered to ducts (sticking to oil residues from the duct manufacturing process and/or oil applied to galvanized sheet metal components to prevent corrosion), resulting in higher levels of contaminants circulating in the air. In studies evaluating airflow through ducts, cleaning heavily contaminated return ducts – the ducts that bring previously warmed or cooled air back through the system for recirculation – resulted in only an 8 percent increase of airflow as compared to before the cleaning.
Duct cleaning may be needed after initial construction of the building, after major renovations, and to remove dust and debris that has built up over time. Studies have measured annual dust accumulation in ductwork at 1 gram per square meter up to 5.1 grams per square meter, it can be higher in bends in the ducts where air flow is obstructed.

Because research studies show no conclusive benefits from duct cleaning, the U.S. Environmental Protection Agency does not recommend it as a routine practice. Any of the following conditions, however, could warrant duct cleaning:
- Visible mold growth on the sheet metal inside ducts or on other components of the heating or cooling unit.
- An infestation of insects or rodents inside ductwork.
- Dust and debris particles blowing into rooms from the registers.
If duct cleaning is necessary, it’s important to address the issues that caused the ducts to become dirty or clogged. Having the heating and cooling system components cleaned at the same time should be part of the duct cleaning exercise.
Duct cleaning service providers may recommend applying chemical biocides inside ducts to kill bacteria or mold. These are pesticides that may not be approved or safe for use indoors or inside an HVAC system, and they may produce secondary pollutants detrimental to indoor air quality.

To keep ventilation ducts clean, use the highest efficiency air filter recommended by the HVAC system manufacturer and change them regularly. A dirty air filter restricts airflow into the HVAC system and makes it work harder, reducing energy efficiency and potentially causing system failure.