Mold in Air Ducts and Vents: What You Can See, What EPA Says About Cleaning, and When Ducts Are Replaced (2026)

Grille, Duct, or the Thing That Feeds Them

"Mold in the vents" describes three different findings.

On the grille and the ceiling around it — dark speckling on the register and the paint next to it. That is condensation: cold air from the duct chills the metal, humid room air condenses on it, mold grows on the film. Common in bathrooms and humid climates. It cleans off with detergent and water, and the fix is the room's humidity, not the ducts.

Inside the duct — growth on the duct wall visible when a register is removed, or on the inside of flexible insulated duct. This is the case EPA's duct-cleaning guidance is about.

Upstream — the evaporator coil, the drain pan and the blower in the air handler, which are wet whenever the system runs and are the most common source of the musty blast when it starts. The air conditioner guide covers those; a moldy duct is very often a moldy coil's downstream.

How to Look

1. 1Remove a supply register (two screws) and shine a flashlight down the duct. Dust is normal; fuzzy or slimy growth is not.
2. 2Check the return grille and the filter — a moldy filter alone can cause the smell.
3. 3Note the smell's timing. Musty only when the system starts: coil or pan. Musty constantly from one room's vent: that duct run, often where it passes through a hot attic or a damp crawl space.
4. 4Have a technician open the air handler and show you the coil and the plenum. This is worth more than any duct-cleaning company's camera, because the coil is where the water is.

What EPA Says About Cleaning

The guidance is narrower than the advertising:

• Duct cleaning is not routinely necessary, and it has not been shown to prevent health problems.
• It is reasonable when there is visible mold growth inside hard-surface ducts or on other components, a pest infestation, or ducts clogged with debris to the point of releasing it.
• Moldy insulated ducts and duct insulation cannot be cleaned and should be replaced. Flexible duct and duct board with growth are replaced, not scrubbed.
• The moisture problem that caused the mold must be corrected, or the growth returns after cleaning.
• EPA does not recommend chemical biocides or sealants inside ducts as routine practice.

So the sequence is: look; if growth is visible on metal, clean it properly and fix the moisture; if growth is on insulated duct, replace that run; if nothing is visible and the smell is at start-up, the coil and drain are the job.

The Moisture Underneath

Ducts grow mold where they are wet or where the air in them is humid enough to condense:

• A dirty or slow-draining coil and pan — the most common source; clearing the condensate line is a homeowner job.
• Duct runs through an unconditioned attic or crawl space — cold supply air in a hot, humid space sweats on the outside of the duct and, with leaky joints, inside it. Sealed joints and insulated duct are the fix; attic moisture and crawl-space moisture are the environments.
• Oversized equipment that short-cycles and never dehumidifies; the fan left ON re-evaporating water off the wet coil.
• Indoor humidity over 60% — under 50% is EPA's range.

What a Legitimate Duct Job Includes

If cleaning is warranted: source removal with agitation and HEPA-filtered negative-air collection of the whole system (supply and return, plenum, coil, blower), not a vacuum hose at each register; replacement of moldy flexible duct and insulation; sealing of leaky joints; and a report with before-and-after photographs. NADCA-member companies work to a published standard. A "$99 whole-house duct cleaning" is a vacuum hose and a sales visit.

Health, Briefly

Ducts distribute whatever grows in them to every room, which is why symptoms tied to the system running — the recognized pattern — deserve the coil-and-duct look, and why anyone with asthma or mold allergy should not be the one cleaning them. A HEPA purifier in the bedroom helps with the air while the source is fixed, and does nothing about the source.

Bottom Line

Mold on the grille: wipe it and lower the room's humidity. Mold inside metal duct: clean it properly, once, after the moisture is fixed. Mold on insulated duct: replace that run. And before any of it, the coil and the drain pan — because in most houses the ducts are moldy for the same reason the air handler is wet.