Cladosporium Mold: The Most Common Indoor Species
Cladosporium is the mold I recover most often from ordinary houses, and it is an allergen story more than a “toxic mold” story. It looks olive-green, brown, or almost black. It likes window sashes, bathroom ceilings, fabrics, dusty cold walls, and HVAC coils. Color still is not a diagnosis. A lab names the genus. Indoor growth still means moisture, even when the moisture is only condensation. CDC’s indoor-mold language is about damp buildings, allergy, and asthma, not a special poison attached to this species. Small patches on hard surfaces are usually a DIY job under EPA’s 10 square foot rule. Growth in ducts, on drywall, or behind a stain that keeps returning is a remediation job.
I am a licensed mold assessor. Homeowners hear “black spots on the window” and jump to Stachybotrys. More often the lab result is Cladosporium. That does not make it harmless if someone in the house has asthma. It also does not make it a toxin emergency.
Color is not a diagnosis
Olive-green, brown, gray, and black all show up on Cladosporium colonies. Alternaria, Aspergillus niger, and soot can look the same from three feet away. CDC does not recommend routine sampling just to put a Latin name on household mold, and CDC has said it is generally not necessary to identify the species growing in a residence. You still sample when you need a written record, when you cannot see the edges of the problem, or when you are clearing a contractor’s work.
Species ID is lab work: tape lift, swab, or a cultured bulk piece, analyzed by a mycologist. A flashlight photo is not that.
EPA’s rule does not change with the genus: “The key to mold control is moisture control.” Clean without fixing the water, “then, most likely, the mold problem will come back.”
What Cladosporium looks like
On a window track it looks like a suede or powdery film, olive to dark brown, sometimes in speckles that smear when you wipe them. On a bathroom ceiling it can look like a field of small dark dots. On fabric and leather it looks dusty or sooty. On HVAC coils and blower wheels it can look like a dark fur over the fins.
| Surface | Typical look | Often confused with |
|---|---|---|
| Vinyl or aluminum window frames | Olive-brown film in the track and on the sash | Dirt, Alternaria |
| Painted drywall or ceiling | Speckled dark spots, sometimes in a cold corner | Aspergillus, soot from candles |
| Fabric, leather, closet contents | Dusty or sooty coating | Age, mildew |
| AC coils, vents, drip pans | Dark growth on fins and in the pan | Dust caked on condensate |
It is usually dry and powdery, not slimy. Slimy growth on soaked drywall after a long leak is a different moisture history. Compare Alternaria on showers and window frames, Aspergillus on dust and HVAC, and black mold if you are trying to rule out Stachybotrys. Our types of mold hub is organized by color because that is how people search. Color is still only a clue.
Where it grows
Cladosporium is abundant outdoors on plants and in soil. Spores come in on shoes, through windows, and on outdoor air. Indoors it colonizes surfaces that stay cool and slightly damp more readily than species that need a soaked wall.
Windows and exterior walls
Condensation on single-pane glass, metal frames, and poorly insulated corners is enough. I see it in the window track, on the rubber gasket, and on the drywall return next to the glass. Wipe the track and you have not fixed the cold surface or the indoor humidity.
Bathrooms, kitchens, and laundry
Steam that is not exhausted, a bath fan that vents into the attic, and a dryer that is not ducted outside keep surfaces in the dew-point range. Tile is cleanable. The painted ceiling and the drywall behind the shower are not, once the paper is colonized.
Fabrics, leather, and stored clothes
Closets on exterior walls, shoes in a damp garage, and upholstered furniture in a basement pick it up. Porous goods that stay musty after cleaning are discard candidates under EPA’s porous-material guidance.
HVAC
This is the indoor location that turns a common fungus into a whole-house complaint. Cooling coils, drain pans, blower wheels, and internally insulated ducts collect condensate and dust. Cladosporium is one of the genera we recover there. EPA: “Do not run the HVAC system if you know or suspect that it is contaminated with mold - it could spread mold throughout the building.” If duct insulation is wet or moldy, EPA says replace it. See air duct cleaning for how a real cleaning is scoped.
A dark film on a supply register can be growth, or it can be spores and dust blowing out of a dirty system. You do not know which until someone looks at the coil and the lining, not just the vent cover.
Health: allergen, not a toxin headline
Mayo Clinic lists Cladosporium with Alternaria, Aspergillus, and Penicillium among the molds that commonly cause allergies. That matches what I hear from occupants: sneezing, stuffy nose, itchy eyes, cough, and asthma that gets worse in the house and eases outdoors.
CDC’s mold pages do not single out Cladosporium as a poison. They describe damp indoor environments. “Exposure to damp and moldy environments may cause a variety of health effects, or none at all.” For some people: stuffy nose, sore throat, coughing or wheezing, burning eyes, or skin rash. “People with asthma or who are allergic to mold may have severe reactions. Immune-compromised people and people with chronic lung disease may get infections in their lungs from mold.”
EPA: “Molds produce allergens (substances that can cause allergic reactions), irritants, and in some cases, potentially toxic substances (mycotoxins).” Allergic reactions to mold are common. “Molds can also cause asthma attacks in people with asthma who are allergic to mold.” Irritation of eyes, skin, nose, throat, and lungs can occur in people who are not allergic.
The 2004 Institute of Medicine review found sufficient evidence linking indoor mold with upper respiratory symptoms, cough, and wheeze in otherwise healthy people, and with asthma symptoms in people who already have asthma.
That is the honest health frame for Cladosporium in a house. It is a well-documented allergen. It is not a reason to tell a healthy family they have been poisoned. It is a reason to take indoor growth seriously if anyone in the home has asthma, a mold allergy, or a weakened immune system. If you have health concerns, EPA says consult a health professional before starting cleanup.
Do not call it “non-toxic” as a way to leave it in the wall. Indoor growth means moisture. Moisture means material damage and a damp-building exposure, whatever the genus.
When it is a remediation job
EPA: “If the moldy area is less than about 10 square feet (less than roughly a 3 ft. by 3 ft. patch), in most cases, you can handle the job yourself.” Window tracks, tile, glass, and metal usually qualify if you can see the whole patch, the humidity source is under control, and you use an N-95, gloves, and goggles. Detergent and water, then dry the surface. See DIY or professional.
It becomes a contractor job when any of these are true.
- The area is larger than about 10 square feet, or several rooms add up past that. EPA’s remediation guide recommends limited containment between 10 and 100 square feet.
- The growth is on porous material. Drywall, ceiling tile, carpet, insulation, and many fabrics cannot be cleaned to a reliable standard. EPA: absorbent or porous materials “may have to be thrown away if they become moldy.”
- The HVAC system is involved. Registers, the air handler, lined ducts, or a drain pan that stays wet. Do not keep running the system as a “test.”
- You cannot see the edges. EPA lists hidden mold on the back of drywall, under carpet, above ceiling tiles, and behind wallpaper. A musty closet with clean-looking paint still needs moisture readings and, often, a cutout.
- Sewage or floodwater caused the dampness. That is a professional cleanup regardless of genus. See flood mold.
- Someone in the home has asthma, a mold allergy, or a weakened immune system. Size is no longer the only test.
Bleach on a window track is theater. EPA does not recommend chlorine bleach as a routine mold cleanup practice. On porous drywall it does not reach the paper facing. Read does bleach kill mold.
What it costs
Contractors do not price Cladosporium differently from other indoor molds. They price containment, demolition of unsalvageable material, HEPA work, and whether the air handler is in scope.
Typical mold remediation cost in 2026 is $1,200 to $3,750 for most homes, about $2,300 to $2,400 on average, or $10 to $25 per square foot of affected area. A small bathroom can be $500 to $1,000. HVAC or a contaminated air handler is often $3,000 to $10,000. Duct-only cleaning, when it is actually warranted, is cheaper than full system remediation.
For a number based on your rooms, use the mold remediation cost calculator. An independent mold inspection at $300 to $600 is the cheapest way to stop contractors from guessing at square footage. Clearance testing after the work is how you confirm the job.
Final Thoughts from Brad
If I had a dollar for every “toxic black mold” photo that came back Cladosporium from a window sash, I could replace a lot of bath fans. The Latin name is common. The moisture is still the job.
Wipe what is on glass and metal. Throw out what is in the fabric if it will not come clean. Open the air handler if the vents keep dusting dark. And if anyone in the house is wheezing at night and fine at work, treat the indoor growth as an allergen source, not a Facebook diagnosis.
Frequently Asked Questions
Is Cladosporium toxic mold?
No. It is one of the most common indoor and outdoor molds and is mainly an allergen and irritant. CDC’s household mold guidance is about damp buildings, allergy, and asthma, not a special toxin attached to this genus. Indoor growth still needs to be cleaned and the moisture fixed.
Why is Cladosporium on my windows?
Windows run cold. Indoor humidity condenses on the glass, the gasket, and the track. That thin film of moisture plus household dust is enough. Cleaning the track without lowering humidity or insulating the opening is why it returns.
Can Cladosporium grow in my air conditioner?
Yes. Coils, drain pans, blower wheels, and lined ducts are common sites. EPA says not to run an HVAC system you know or suspect is contaminated. Wet or moldy internal duct insulation should be replaced, not sprayed.
Do I need to test to confirm it is Cladosporium?
Not just to name it. CDC does not recommend routine household mold sampling for species ID. Test when you need documentation, when growth may be hidden, when the HVAC is in question, or when you are clearing remediation.
When should I call a remediator for Cladosporium?
When the area is larger than about 10 square feet, when it is in drywall or other porous material, when the HVAC is involved, when you cannot see the full extent, or when someone in the home has asthma or a weakened immune system.
Sources
- About Mold and Moisture (CDC) — damp indoor environments may cause health effects or none at all; allergy and asthma reactions; infections mainly in immunocompromised people and people with chronic lung disease
- Health Problems | Mold (CDC/NIOSH) — allergic responses, irritation in non-allergic people, association between damp indoor spaces and asthma
- Can mold cause health problems? (EPA) — molds produce allergens and irritants; asthma attacks in people with asthma who are allergic to mold
- A Brief Guide to Mold, Moisture and Your Home (EPA) — 10 square foot DIY threshold, moisture control, discarding porous materials, HVAC caution
- Mold Remediation in Schools and Commercial Buildings Guide: Chapter 3 (EPA) — hidden mold, containment by size, do not run a contaminated HVAC system
- Mold allergy (Mayo Clinic) — Cladosporium listed among common allergenic indoor and outdoor molds
Explore Related Topics:
- Common Types of Mold Found In Your Home
- Is Black Mold Dangerous? Signs and What to Do (2026)
- White Mold: Harmless Dust or a Real Problem? (2026)
- Green Mold in the House: How Worried Should You Be?
- Pink Mold in the Shower: Why It Keeps Coming Back
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