Mold Library
Spore-trap analysis identifies fungi by the shape of their spores. Here are some of the most common types we report — and what they mean for your indoor air.
The most common mold worldwide — indoors and outdoors
Appearance: Olive-green to brown or black colonies. Under the microscope, lemon-shaped (ellipsoidal) spores form branching, tree-like chains.
Where it's found: Living and dead plant material, soil, and indoors on damp window sills, fabrics, HVAC surfaces and bathrooms.
Why it matters: A very common airborne allergen that can trigger hay-fever-type symptoms and asthma. It rarely infects healthy people. Indoor counts well above outdoor often point to a moisture source.
Reported together — their spores look nearly identical on a spore trap
Appearance: Tiny round spores (3–5 µm) produced in chains. Colonies range from blue-green to yellow to black depending on the species.
Where it's found: House dust, water-damaged building materials, soil and stored food. They grow on many materials with only modest moisture.
Why it matters: Common allergens. Some species produce mycotoxins, and Aspergillus fumigatus can cause respiratory infection in people with weakened immune systems or lung disease. High indoor counts warrant investigation.
The notorious “toxic black mold” — it needs sustained moisture
Appearance: Greenish-black, often slimy colonies. Dark, ellipsoidal spores cluster at the tip of the conidiophore.
Where it's found: Chronically wet, cellulose-rich materials — drywall, ceiling tiles, cardboard and wood that has stayed damp for days or longer.
Why it matters: It produces mycotoxins and is tied to water damage. Its heavy spores are not easily airborne unless disturbed, so surface sampling is often more informative than air sampling. Its presence signals a significant moisture problem.
Large, distinctive spores — a major outdoor allergen
Appearance: Large brown, club- or drumstick-shaped spores with both cross-walls and lengthwise walls (muriform), often in short chains.
Where it's found: Soil and plants outdoors; indoors in damp spots — showers, window frames and around leaks.
Why it matters: One of the most important airborne allergens, strongly linked to asthma and allergic rhinitis. Indoor growth usually indicates dampness.
Environmental spores from outdoor fungi
Appearance: Small, varied spores released by mushrooms (basidiospores) and sac fungi (ascospores).
Where it's found: Predominantly outdoors; they enter buildings through normal air exchange and spike after rain.
Why it matters: Mostly a background outdoor signal rather than a sign of indoor growth. They are useful as a reference — which is why we always pair your indoor sample with a same-day outdoor one.
Screening identifies and counts spores by morphology; it is not a medical diagnosis. Health effects vary by individual, species and exposure.