The short answer
Mold in air ducts is a fungus growing on whatever part of your heating and cooling system stays wet. Often that part is not a duct. Mold turns up in five places, and three of them sit inside the air handler rather than in the runs. The species almost never changes what you do. What changes it is how much there is, and what the surface is made of, because metal can be cleaned and porous material cannot.
I train SafeAir’s new technicians and leads. The photographs below come from one of our mold jobs, in Hillsboro in May 2024. Below is where it actually turns up, how to tell it from dust, and what air duct mold cleaning can and cannot fix.
Where mold in an HVAC system usually is
Air moves through a duct rather than sitting in it, so growth turns up where the water collects and the air slows. That is a short list.
Three of the five sit inside the air handler, so open its door before you look down a duct.
- The evaporator coil. It is cold and wet every time the system runs in summer. Condensation is its job.
- The drain pan under it. A blocked drain line leaves standing water directly under the airstream.
- The blower housing and the air handler cabinet. Wet air passes it all day, and the cabinet is dark.
- Insulation that has been damp. Internal liner, duct board, and anything near a leak.
- Panned joist returns. The duct walls are the floor joists and the subfloor, so the surface is bare wood.
The Frahn home in Hillsboro is a 3,339 square foot house, booked in May 2024 for a standard cleaning of its 16 ducts. Our technicians found the usual dirt through the runs, then mold in dark and yellow-green patches across the blower housing, heavy enough to bury the manufacturer’s model plate.
Nothing was growing in the ducts. The ductwork was carrying what the air handler was making, which is why cleaning only the runs leaves the source in place. It does happen in the runs too. At the Drewes home in Imperial, a 2,500 square foot house with 15 ducts and a household with persistent allergy symptoms, we found mold in May 2024 alongside packed sediment, dust and pet hair on the trunk line floor. In a 34-duct home in Arnold, construction debris left from the build was still sitting in the ductwork.
Telling mold from dust, soot and rust
A return holds skin cells, carpet fibers, pet dander, pollen, drywall dust after a remodel, sometimes soot. Four tests separate mold from the rest.
- Smell, at the moment the fan starts. A musty smell that arrives with the airflow and fades after a few minutes points at the system rather than the room.
- Color and pattern. Dust is gray and even, while mold comes in patches, usually black, olive-green, yellow-green or white.
- Texture. Soot smears, and leaves a mark on the cloth. Rust is orange, and stays put. Mold sits proud of the surface, fuzzy or powdery under a fingertip.
- Where you find it. Growth on a register face is condensation on that register. Growth at the coil or the drain pan is the system.
A register that grows a black ring every summer is a condensation problem, not a duct problem. Clean it and it returns.
What the EPA’s 10 square feet looks like inside a duct
Ten square feet is the number everybody quotes. Under that, says the EPA, a homeowner can usually handle the cleanup themselves: roughly a 3 foot by 3 foot patch. Fine for a bathroom wall. Nobody converts it for a duct, so here it is.
Inside surface area is perimeter times length, so:
- A 20 by 8 inch trunk line has about 4.7 square feet of inside surface for every foot of run. Ten square feet is about 2 feet of that trunk.
- A 4 by 10 inch branch run has about 2.3 square feet per foot. Ten square feet is about 4 feet of branch.
- An 8 inch round pipe has about 2.1 square feet per foot. Ten square feet is roughly 5 feet of pipe.
Two feet of trunk is not much duct. Growth that looks small in a phone photograph can already be past the line the EPA draws. Above 10 square feet, the EPA sends readers to its guide for professional mold remediation rather than a cleanup at home.
The parts that get replaced, not cleaned
Sheet metal can be cleaned. Porous material cannot, and the EPA does not hedge on its duct page: soaked or moldy insulation has to come out, and new insulation goes in.
Four parts of a system are porous, and they are the ones people scrub hardest:
- Internal duct liner. Fiberglass bonded to the inside of metal duct for noise control. Mold grows into it.
- Duct board. The whole duct is fiberglass board with a foil face. There is no hard surface underneath.
- Flexible duct. A thin liner over a wire coil, replaced rather than repaired once it is torn or moldy.
- Panned joist returns. The duct wall is the wood framing of the house, and wood holds what lands on it.
On wood or fiberglass rather than metal, this stops being duct cleaning and becomes mold remediation, a different trade with different equipment, and the inspection tells you which one you have.
Five molds people ask about
The CDC names Cladosporium, Penicillium and Aspergillus as the most common indoor molds. Alternaria and Stachybotrys, the one people call black mold, complete the chart. Read it as background, not as a diagnosis.
On a phone, swipe the table sideways to see every column.
| Mold | Color | Texture | Where it grows | Why people notice it |
|---|---|---|---|---|
| Cladosporium | Black, brown or olive-green | Powdery or fuzzy | Fabric, wood and paper in cool damp spots. Basements, bathrooms, window frames, HVAC vents | A known allergen. Sneezing, runny nose, eye irritation and sinus symptoms. |
| Penicillium | Blue-green | Fuzzy, spreads fast | Water-damaged carpet, wallpaper, insulation, mattresses, rotting wood | Coughing, sneezing, wheezing, itchy eyes. Some species produce mycotoxins. |
| Aspergillus | Green, yellow, black or white | Powdery or fluffy | Dust, building materials, wall cavities, insulation, supply vents | Allergy and asthma flare-ups. Serious lung infection risk if immunocompromised. |
| Alternaria | Dark green to brown | Velvety | Bathrooms, under sinks, around windows, anywhere a leak went unnoticed | Strongly associated with seasonal allergy and asthma attacks. |
| Stachybotrys chartarum the one people call black mold | Dark greenish-black | Slimy, not powdery | Soaked drywall, wood framing, ceiling tiles and paper, after a flood or roof leak | Can produce mycotoxins. Needs cellulose and a genuinely wet surface, not just humidity. |
Cladosporium
Black, brown or olive-green, with a powdery or fuzzy texture.
Where it grows: fabric, wood and paper in cool damp spots. Basements, bathrooms, around windows, and inside HVAC vents.
Why people notice it: it is a known allergen. Sneezing, runny nose, eye irritation, coughing and sinus symptoms, particularly for anyone with asthma or a mold allergy.
Penicillium
Blue-green and fuzzy. It spreads fast across damp surfaces.
Where it grows: water-damaged carpet, wallpaper, insulation, mattresses and rotting wood.
Why people notice it: coughing, sneezing, wheezing, itchy eyes. Some species produce mycotoxins, though allergy is far and away the usual complaint.
Aspergillus
A large group. Green, yellow, black or white depending on the species, usually powdery or fluffy.
Where it grows: dust, building materials, wall cavities and insulation.
Why people notice it: allergic reactions and asthma flare-ups. In people who are immunocompromised it can cause invasive aspergillosis, a serious lung infection.
Alternaria
Fast-growing, dark green to brown, with a velvety texture.
Where it grows: bathrooms, under sinks, around windows, and anywhere a leak has gone unnoticed.
Why people notice it: a significant allergen, strongly associated with seasonal allergy and asthma attacks.
Stachybotrys chartarum, the one people call black mold
Dark greenish-black and slimy rather than powdery. It needs cellulose, and it needs the surface genuinely wet rather than humid.
Where it grows: soaked drywall, wood framing, ceiling tiles and paper, usually after a flood, a roof leak or long-term water damage.
It is one of the molds that can make mycotoxins, which is why the word “toxic” gets attached to it. Be wary of any price built on that word.
Why the species rarely changes what you do
All five get the same treatment in an HVAC system. Find the water, remove the growth mechanically, replace whatever is porous. The CDC takes the same line for a home, and says the type does not change the response.
Mold spores are in every house and in the outdoor air, and only matter where they land wet. Two exceptions count. If somebody in the house is immunocompromised, the answer moves to a doctor rather than to a cleaner. And a lab result changes things when it is going to be used in an insurance claim or a dispute with a landlord.
Is mold testing worth paying for?
The CDC does not recommend mold testing. The EPA says sampling is usually unnecessary once you can see the growth. Neither agency has set a standard for a normal spore count, so the number you get back cannot be compared to anything at all.
If a company says it found mold, the EPA’s duct page says to ask them to show you, because much of a system cannot be seen without opening it up. Some labs can check a sample lifted on clear sticky tape and tell you whether it is mold at all.
Spend it on finding the water instead. The EPA prices a humidity meter at $10 to $50 in a hardware store, and it tells you something a spore count will not.
Find the water, not the mold
Mold is a symptom. The EPA gives two numbers worth taping to the furnace: keep indoor humidity under 60 percent, and between 30 and 50 percent if you can, and dry anything wet within 24 to 48 hours. Dry it inside that window and, in most cases, the EPA says no mold grows. The CDC draws its humidity line at 50 percent.
The water itself usually comes from one of these five. Work down the list first, because cleaning a system that is still getting wet buys you a few months at most.
- A blocked condensate drain line, leaving the drain pan standing full of water all summer.
- A coil running cold enough to sweat onto the cabinet floor below it.
- Duct running through a hot attic or a damp crawl space with insulation that has come loose.
- A furnace humidifier still running in June.
- A roof or plumbing leak above the run, which is a building repair and not ours.
What removing it actually involves
A proper job is mechanical first and chemical second, in that order, and never the other way around. Two to three technicians, 2 to 4 hours on most single-system homes.
- The system comes under continuous negative pressure, through two HyperVac units at 5,000 CFM each, 10,000 together, with 89 inches of static lift, running MERV 15 and HEPA main conical filters, so what comes loose leaves the building instead of the room.
- Every run is agitated with 175 PSI air whips, and the debris is pulled out to the machine.
- Both trunk lines, supply and return, are cleaned, with every accessible vent and register. Growth on the blower housing is treated on the same visit: in Hillsboro that meant mold remediation on the housing alongside the standard clean of all 16 ducts. Blower cleaning and furnace cleaning are separate services, and either can go on the same visit.
- Moldy porous material has to come out and be replaced, not sprayed.
- Only then does an EPA-registered product go on, and only where there is a reason for it. The heat exchanger and combustion chamber stay shut: furnace technician’s work.
Duct sanitizing is $5 per opening on our what to expect page and follows the cleaning rather than replacing it. What a full system costs follows your opening count. Where growth returns to the coil after a proper clean, the usual next step is a UV-C lamp at the coil.
SafeAir cleans HVAC systems and has since 2002. If the water is coming from a drain line, a coil or the structure, you need an HVAC contractor or a remediation contractor first, and probably not us at all until that is fixed. What a dirty system does to the air is worth reading either way.
When this is a doctor’s question, not ours
No duct cleaner can tell you what is making somebody ill. The CDC lists the common reactions: stuffy nose, wheezing, red or itchy eyes and skin. Asthma, allergies and a weakened immune system all make those reactions harder and faster.
If somebody in the house has been ill for weeks, start with their doctor and clean the system in parallel. Do not wait on one to do the other.
Two limits on all of the above. Cleaning does not fix a moisture problem, and porous material needs replacing rather than washing, so a clean system that is still getting wet grows the same thing again. Nobody can confirm mold from a photograph, either. And if the smell sits in one room rather than at the vents when the fan runs, the damp is in that room: you probably do not need us, you need a dehumidifier and a look behind the baseboard.
More questions about mold in air ducts
Your next step
Open the air handler door and look at the coil, the drain pan and the blower housing before you look at a duct.
Find growth and the next step is someone looking at the whole system, with the price agreed before a machine is switched on. A visit where you decide against the work carries a $25 travel fee. Book the work the same day and that fee comes off. Call 314-900-3828 or request an estimate. You can check us first in the NADCA directory, which has carried SafeAir since February 2020, with Dustin Morter holding the ASCS credential, and you can check that entry yourself in under a minute. Our BBB accreditation dates to June 28, 2012. Jefferson County is home, and the vans run north into St. Louis County and St. Charles County.
