Mold in a duct system is worth taking seriously for one reason that has nothing to do with the duct: the system distributes whatever is in it to every room it serves, every time it runs. That makes it a building wide problem rather than a local one, and it changes the order in which things have to be done.
Mold in vents, or dust on a grille
Dark deposits around a supply vent are very often dust deposited by air movement rather than growth, particularly on a ceiling grille. Growth tends to be fuzzy rather than smooth, sits on surfaces that are damp rather than merely dusty, and is usually accompanied by a musty smell when the system starts. If the distinction matters, it is settled by looking inside the duct rather than at the grille.
Find the moisture in the system
Ducts grow mold where they carry moisture: an air conditioning coil and drip pan that is not draining, uninsulated ductwork sweating in a humid crawl space or attic, a humidifier, or a return pulling damp air from an unconditioned space. Cleaning without finding which of those is at work produces a clean duct that is dirty again in a season.
What the EPA actually says
The EPA's guidance is that duct cleaning has never been shown to prevent health problems as a routine measure, and that it should be done if there is visible mold growth inside the ducts or on other components of the system. So the rule of thumb is: no routine cleaning, but a real case for it when there is growth, and always with the moisture source corrected.
Before a system with mold in air vents runs again
If a duct system has mold in it, running the system distributes it. Where remediation is underway anywhere in the building, the affected zone is normally isolated or shut down for the duration for exactly this reason. Insulated flexible duct that has grown mold is usually replaced rather than cleaned, because the porous lining cannot be verified clean.
Questions people ask about mold in air ducts
Is it mold on AC vents, or dust?
Frequently it is dust deposited by air movement, especially on a ceiling grille. Growth is fuzzy rather than smooth and is usually accompanied by a musty smell when the system starts. The reliable answer comes from looking inside the duct, not at the grille.
Should I pay for air duct mold cleaning?
The EPA does not recommend routine cleaning, and does say to have it done if there is visible mold growth inside the ducts or on other system components. The case for cleaning is a finding, not a schedule.
Does air duct mold remediation mean cleaning or replacing?
Sheet metal duct is cleanable. Insulated flexible duct and fibrous liner that has grown mold is normally replaced, because a porous lining cannot be cleaned and verified in the way a hard surface can.
What causes mold in AC vents and air ducts?
Moisture in the system: a coil or drip pan that is not draining, duct sweating in a humid unconditioned space, a humidifier, or a return drawing damp air. Correcting that is what makes the cleaning last.
Is mold on air vents dangerous?
It is worth taking seriously because the system distributes whatever is in it to every room it serves, every time it runs, which makes it a building-wide problem rather than a local one. First establish whether it is growth or dust deposited by the air; where it is growth, the EPA says to have the system cleaned, the moisture that fed it has to be found and fixed, and the system should not run through a zone under remediation.
What does early stage black mold in air vents look like?
Usually like dust, which is why it is so often missed or so often imagined. Dark speckling on a grille is settled dust more often than growth, and the question the system answers is whether there is a moisture source inside it.