EVIDENCE AND SOURCES
What the evidence says and where each figure comes from
Public references on ductwork inspection and cleaning, with what they support and what they do not allow us to claim.
Every figure on this site links to its source. This page brings them together, states what each one supports and makes clear what the evidence does not allow us to claim. Four rules are followed: the source goes in the same sentence, the link leads to the original document, figures are given as the authors publish them and anything that is our own calculation is said so.
Last reviewed: October 20, 2026.
What the evidence supports
| Statement | What supports it | Source |
|---|---|---|
| Ducts can become contaminated and act as a reservoir for microorganisms | A literature review states that, under normal operating conditions, ducts can become contaminated with dust and serve as reservoirs for microorganisms to proliferate. | Zuraimi, 2010 |
| A system that looks clean can be contaminated on the coil and in the pan | In a system that looked clean, 10⁵ to 10⁶ viable bacteria per cm² were counted on the coil fins and 10⁵ to 10⁷ per ml in the pan water. | Hugenholtz and Fuerst, 1992 |
| Condensate pans are a place for possible hidden mold | The EPA lists them among the places where hidden mold may grow and asks for them to be kept clean, flowing and unobstructed. | EPA, chapter 3 and chapter 2 |
| Controlling moisture is what keeps mold from returning | If the cause of the moisture is not corrected, mold returns. The key to mold control is moisture control. | EPA and brief guide |
| Outdoor climate influences what grows on a coil | In 40 commercial units, the microbial ecology of the coils depended on the outdoor climate through the dew point. | Bakker et al., 2020 |
| Particles can increase during a cleaning with inadequate vacuum extraction | In eight homes, particles and bioaerosols were higher during cleaning than before or after. The EPA gives the same warning about inadequate vacuum extraction. | Ahmad et al., 2001 and EPA |
| A cleaning is not permanent in units that run almost continuously | In 17 residential units, the fungal load returned to the previous level one year after high-pressure washing. | Shiraishi et al., 2026 |
| Internal insulation with mold is replaced, not cleaned | Once fiberglass duct liner is contaminated with mold, cleaning is not enough to prevent regrowth. | EPA |
What the evidence does not support, and why this site does not claim it
| Common claim in the industry | What the evidence says | Source |
|---|---|---|
| Cleaning ducts improves the health of occupants | The EPA states that duct cleaning has never been shown to prevent health problems. The 2010 review found insufficient evidence that it relieves occupants’ symptoms. | EPA and Zuraimi, 2010 |
| Cleaning ducts reduces energy consumption by a percentage | The 2010 review found inadequate evidence that cleaning improves airflow and reduces consumption. | Zuraimi, 2010 |
| Dirty ducts are the cause of poor air quality in an office | No field study had related poor indoor air quality to duct contamination. | Zuraimi, 2010 |
| Ducts must be cleaned every set number of months | The EPA does not recommend routine cleaning. It recommends it when there is a reason. | EPA |
| A disinfectant eliminates the microorganisms in the duct | No chemical biocides are registered by the EPA for internally insulated ducts, and there is little research on their effectiveness inside ducts. | EPA |
| The duct is sealed to encapsulate the mold | The EPA states that sealants should never be used on wet duct liner, over actively growing mold or over debris. | EPA |
Bibliography
The dew point on the indoor air quality page is our own calculation with the Magnus formula from the ETESA averages. It is an order of magnitude, not a measurement.