Cladosporium is one of the most common molds in the world, indoors and out, and one of the most frequently identified in homes. It typically appears as olive-green, brown, or black growth with a suede-like or powdery texture, and it shows up on damp surfaces from bathroom grout and window sills to walls, fabrics, and HVAC surfaces. If you've seen dark speckled growth on a cool, damp surface, there's a fair chance it was Cladosporium.
This page is educational, not medical advice. It covers what Cladosporium is, why it's so ubiquitous, the honest health framing, and how to deal with it — with the same recurring theme that the removal approach doesn't depend on identifying the species by name.
Health questions belong with a doctor
This page is educational, not medical advice. Reactions to mold vary widely from person to person; if you or someone in your home has symptoms you're worried about, talk to a doctor. Summit's crews handle the building side — finding, containing, and fixing mold and its moisture source.
What is Cladosporium?
Cladosporium is a large genus of molds found almost everywhere — it's one of the most abundant molds in outdoor air, and it drifts indoors freely through windows, doors, and ventilation. Indoors, it colonizes damp surfaces and materials: window frames and sills, bathroom and kitchen surfaces, damp drywall and wood, textiles, and the moist surfaces inside HVAC systems. It can grow at cooler temperatures than some molds, which helps explain its prevalence in refrigerated and cool, damp spots.
Its typical look — olive-green to brown or black, often in speckles or patches with a slightly fuzzy or powdery surface — is common enough that it's frequently what people are seeing when they find “black mold” on a window frame or bathroom surface. But as always, color and appearance don't confirm a species.
Why is it so common in homes?
Two reasons: it's everywhere in outdoor air to begin with, so spores are constantly entering, and it's tolerant — able to grow on many materials and at cooler temperatures than pickier molds. Give it a reliably damp surface, and it colonizes readily.
In the Pacific Northwest, cool damp conditions for much of the year play to exactly those strengths. Condensation-prone windows, humid bathrooms, and damp crawl spaces and basements give Cladosporium the moisture it needs, which is why it's such a routine find in regional homes.
What's the honest health picture?
Cladosporium is a well-recognized allergen — it's one of the more common airborne mold allergens, and for sensitized people it can trigger the familiar allergy and asthma symptoms: sneezing, congestion, itchy eyes, coughing, and for those with asthma, worsened breathing symptoms. People with allergies, asthma, or weakened immune systems tend to be more sensitive, consistent with the general framing for indoor mold.
For most healthy people, ordinary exposure is an allergen-and-irritant matter rather than a serious infection risk. As with any mold, health symptoms are a question for a doctor, not something to diagnose from the fact that Cladosporium is present.
How do I remove and prevent Cladosporium?
Same playbook as any indoor mold, which is the practical reason species ID rarely changes what you do: address moisture, remove growth, and keep humidity in check.
Removal and prevention:
- Reduce indoor humidity and improve ventilation — especially in bathrooms and around condensation-prone windows.
- Clean small amounts of surface growth on non-porous materials; remove affected porous materials that hold growth.
- Keep HVAC surfaces and drip pans clean and dry, since Cladosporium colonizes them and air distributes spores.
- Fix the underlying moisture source so growth doesn't simply return.
- For extensive growth (beyond ~10 sq ft per EPA guidance) or HVAC-wide contamination, use professional remediation.
Sources referenced on this page
Questions we hear about this
Is Cladosporium dangerous?
It's primarily an allergen — one of the more common airborne mold allergens — so sensitized people, especially those with allergies or asthma, may react to it, while serious infection is mainly a concern for people with weakened immune systems. Health symptoms are a doctor's question; the growth itself should be removed and its moisture fixed.
Is the black mold on my window Cladosporium?
It very well could be, since Cladosporium loves cool, condensation-damp window frames — but you can't confirm a species by sight. Fortunately it doesn't change the fix: reduce condensation and humidity, clean or replace affected material, and the growth stops recurring.
Can Cladosporium grow in my fridge or cool rooms?
Yes — it tolerates cooler temperatures better than many molds, so it can appear on damp surfaces in refrigerators, cool basements, and around AC systems. Keeping those surfaces clean and dry, and controlling humidity, keeps it in check.
Why does Cladosporium show up on my window sills every winter?
Window frames and sills are the coldest interior surfaces in winter, so humid indoor air condenses there first, and Cladosporium — which happens to tolerate cool, damp conditions well — colonizes the reliably wet sill. It's such a common pairing that dark window-sill growth is very often this mold. Reducing indoor humidity, improving airflow to the glass, and wiping up morning condensation removes the dampness it depends on.
Does finding Cladosporium mean my air is unhealthy?
Cladosporium is present in almost all indoor air at some level, so its mere presence isn't alarming — what matters is whether indoor levels are elevated and whether there's visible growth, both of which point to a moisture problem to fix. For anyone with mold allergies or asthma, reducing that exposure can help symptoms, and specific health concerns are best taken to a doctor.
Next step
You understand the problem — now weigh your options.
Side-by-side comparisons show where DIY genuinely works, what the methods differ on, and how restoration companies actually vary.