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Quick answer
Mold on the face of a removable metal vent cover is a hard surface, and EPA says to scrub mold off hard surfaces with detergent and water and dry them completely. [2] Mold inside the ducts, on the cooling coil or in duct insulation is a bigger job. EPA says not to run an HVAC system you know or suspect is contaminated with mold, and that duct insulation that gets wet or moldy cannot be effectively cleaned and should be removed and replaced. [2][1] A musty smell from the vents can point to a clogged AC drain. [6]
Do this first: vent cover, duct or AC unit?
Where the mold is decides who should clean it. A register or grille is the removable cover over a duct opening. The duct behind it, the cooling coil and the drain pan in the indoor unit are part of a system that moves air through the whole house.
Look closely at the vent cover. Note whether the spots are only on its front face or also on the inside of the opening.
Shine a flashlight into the duct as far as you can see. EPA notes that many sections of a heating and cooling system may not be accessible for a visual inspection. [1]
If the growth continues into the duct, the system is part of a known moisture problem, or there is mold near the system's air intake, stop running it. EPA says to consult its duct cleaning guide before taking further action, and not to run the HVAC system if you know or suspect it is contaminated with mold, because it could spread mold throughout the building. [2]
Check the indoor unit's drain pan. EPA says to make sure the condensate pan drains properly, and that substantial standing water or debris there indicates a problem needing immediate attention. [1]
Measure the room's humidity. EPA recommends below 60 percent, ideally 30 to 50 percent, and says a humidity meter costs about $10 to $50. [2]
Skip the mold test for growth you can see. EPA says that in most cases sampling is unnecessary when mold is visible, and CDC says all molds should be treated the same for removal. [2][3]
Who should stay out of the cleanup
CDC says people with allergies, people with weakened immune systems or underlying lung disease, and people with chronic respiratory diseases such as asthma or COPD should not take part in mold cleanup. [4] If anyone in the home has health concerns, talk to a doctor.
What causes mold in air vents?
Moisture. EPA's duct cleaning guide, first published in 1997 and still posted on EPA's site, says moisture should not be present in ducts, and that controlling it is the most effective way to prevent biological growth in them. It says research suggests condensation on or near the AC's cooling coils is a major factor in moisture getting into the system, and that condensation or high humidity is an important indicator of the potential for mold growth on any type of duct. [1]
Condensation on cold metal. EPA says that when warm, humid air contacts a cold surface, condensation may form, and that condensation can be a sign of high humidity. [5][2] A metal register carrying cold air into a humid room can be that kind of cold surface.
Ducts in attics and crawl spaces. EPA says ducts in spaces that are not air conditioned should be properly sealed and insulated to keep condensation from getting into the system. [1]
An oversized AC. EPA says a unit that is too big cycles on and off often and removes moisture poorly, especially in humid areas. [1]
A drain pan that does not drain. EPA's mold course lists poorly draining condensate pans inside air handling units, and porous liners inside ductwork, among places hidden mold grows. [5]
Where the mold is, what it usually points to, and who usually handles it
Where you see it
What it usually points to
Usually
A few spots on the face of a metal vent cover
Condensation or high humidity in that room
DIY: scrub, dry, lower humidity
Inside a sheet metal duct, past the cover
Moisture getting into the system
Pro: EPA says consider duct cleaning for substantial visible growth
Fiberglass-lined duct, duct board or other duct insulation that is wet or moldy
Wet insulation
Pro: EPA says a qualified heating and cooling contractor should remove and replace it
A smell can come before anything you can see. EPA says a moldy odor suggests mold is growing in the building and should be investigated. [7] It also says you may suspect hidden mold when a building smells moldy but you cannot see the source, and lists inside ductwork among the places it hides. [2]
With vents, check the air conditioner first. Carrier lists musty, moldy smells from the indoor unit or the air vents as a symptom of a clogged condensate drain line [6], and Trane lists a musty smell when the AC is running. [8] Trane says that line can clog with mold and mildew that form in standing water in the drain pan. [8] If the pan holds water, the guide to an AC leaking water inside covers how to clear the drain.
If the smell is house-wide rather than from the vents, see the guide to a musty smell with no visible mold. EPA's duct guide says that if no one in the household has allergies or unexplained symptoms and a look inside the ducts shows no musty odor or visible mold, duct cleaning is probably unnecessary. [1]
How to clean mold off a vent cover
EPA's duct guide notes that registers can be easily vacuumed or removed and cleaned. [1] For mold on the cover itself, follow EPA's advice for hard surfaces.
Turn the system off at the thermostat so it is not blowing air while you work.
Put on gear. For mold cleanup, EPA says you may want to wear an N-95 respirator, and it recommends long gloves and goggles without vent holes. [2]
A practical step: unscrew the cover, slip it into a plastic bag, and carry it to a utility sink, tub or outdoor spot.
Scrub the mold off with detergent and water, then dry the cover completely. [2]
Do not reach into the duct with water, steam or sprays. EPA says steam cleaning and other methods involving moisture should not be used on any kind of duct work. [1]
Put the dry cover back, and check it again in a week or two. If the mold returns, the moisture is still there; EPA says that if the conditions causing the mold are not corrected, it will grow back. [1]
Skip the bleach and the duct sprays. EPA says using a biocide such as chlorine bleach is not recommended as a routine part of mold cleanup. [2] For ducts, it says no chemical biocides are currently registered by EPA for use in internally insulated duct systems, and that sealants should never be used on wet duct liner or to cover actively growing mold. [1]
Safe to do yourself
A few spots on a removable metal vent cover, with a clean duct behind it and the room's humidity brought down, is a scrub-and-dry job. EPA's general guideline is that in most cases you can handle a moldy area under about 10 square feet yourself. [2] That fits a vent cover. It does not cover mold inside the ducts or the AC unit, where EPA says not to run a system you suspect is contaminated and to consult its duct guide first. [2] Fix the humidity or the spots will come back.
When the mold is inside the ducts or the AC unit
EPA says you should consider having your ducts cleaned if there is substantial visible mold growth inside hard surface ducts, such as sheet metal, or on other parts of the heating and cooling system. [1] Its guide adds several points:
Something that looks like mold may not be. EPA says a positive determination can be made only by an expert and may require lab analysis, and that for about $50 some labs can check a sample on clear sticky tape. [1]
If insulated ducts get wet or moldy, the insulation cannot be effectively cleaned and should be removed and replaced. EPA, NADCA and NAIMA all recommend replacing wet or moldy fiberglass duct material. [1]
The cause must be corrected before any cleaning or replacing, or the problem will likely come back. [1]
Cleaning has to cover the whole system. EPA says the provider should agree to clean all components, because failing to clean one part of a contaminated system can re-contaminate the entire system. [1]
Before you hire. EPA suggests talking to at least three providers and getting written estimates, asking each one to show you the contamination they say exists, and avoiding anyone who makes sweeping health claims for duct cleaning. Be wary of companies that claim EPA certification, since EPA does not certify duct cleaners. [1]
What it costs, and what about health?
Angi's air duct cleaning cost guide, updated October 9, 2026, puts the national average at $389, with most homeowners paying $271 to $509. It lists mold and mildew remediation in ducts at $1,100 to $3,400. [9] Angi says home size and the number of vents (about $35 each to clean) are the main price factors, and that flex ducts cost more to clean than rigid metal. [9] Replacing wet or moldy duct insulation, or repairing whatever let the moisture in, is extra.
On health, CDC says damp and moldy environments may cause a variety of health effects, or none at all. People with asthma or mold allergies may have severe reactions, and people with weakened immune systems or chronic lung disease may get lung infections from mold. [10] EPA says duct cleaning has never been shown to actually prevent health problems, and that anyone with unusual or unexplained symptoms they think are tied to the home should talk with a doctor. [1]
Insurance. Washington's insurance commissioner says homeowner insurance usually does not cover mold unless it comes from water damage the policy covers, though some insurers provide limited mold coverage. [11] Check your own policy.
Common questions
Is mold in air vents dangerous?
CDC says damp and moldy environments may cause a variety of health effects, or none at all, and that people with asthma or mold allergies may have severe reactions. [10] Do not run a system you suspect is contaminated. [2] For health questions, talk to a doctor.
Can I clean mold out of air vents myself?
You can clean a removable metal vent cover with detergent and water and dry it. [2] Do not clean inside the ducts with water or steam; EPA says moisture methods should not be used on duct work. [1] EPA says wet or moldy duct insulation should be removed and replaced by a qualified heating and cooling contractor, and to consider professional duct cleaning for substantial mold inside hard ducts.
Why does my AC smell like mold when it turns on?
A musty smell when the AC runs is a symptom Trane and Carrier link to a clogged condensate drain, where mold and mildew can form in standing water in the pan. [8][6] Check the drain pan and line.
Should I turn off my AC if there is mold in the vents?
If the mold is inside the system or you suspect it is, yes. EPA says not to run the HVAC system if you know or suspect it is contaminated with mold, because it could spread mold throughout the building. [2]
Does air duct cleaning get rid of mold?
Only if the moisture is fixed first. EPA says mold growth will recur if its cause is not corrected, and that wet or moldy insulated ducts cannot be effectively cleaned and should be replaced. [1]
How do I know if it is mold or just dust on my vents?
EPA says it is normal for return registers to get dusty, and that a positive determination of mold can be made only by an expert and may require lab analysis. For about $50, some labs can check a sample on clear sticky tape. [1]
