Stachybotrys vs Cladosporium: What Your Lab Report Actually Means

Stachybotrys and Cladosporium are frequently confused because both produce dark-coloured growth and both appear on indoor surfaces. They are, however, biologically distinct organisms with different growth requirements, different health implications, and different remediation standards. The only way to tell them apart reliably is laboratory analysis of a surface or air sample — and understanding what a lab report says about each species will determine whether you need professional remediation or a cleaning and ventilation fix.

What Is Cladosporium and Where Does It Appear?

Cladosporium is the most common indoor mould species found in air and surface sampling worldwide. It is ubiquitous: present outdoors in soil, on plant matter, and in the air, and it readily colonises indoor surfaces where moisture accumulates. In buildings, it appears most commonly on window sills (where condensation collects), bathroom tile and grout, air conditioning vents and duct surfaces, and on wall surfaces in rooms with persistently elevated humidity.

Key characteristics:

Appearance: green, olive green, or dark brown-black. Under a microscope, characteristic branching chains of spores (cladospores) are identifiable.
Texture: powdery or slightly velvety on surfaces
Health significance: it is an allergen — sensitised individuals may experience hay fever-like symptoms, and it can exacerbate asthma. It is not mycotoxigenic under typical indoor conditions, meaning it does not produce the type of mycotoxins associated with Stachybotrys.
Growth requirements: relatively tolerant of lower moisture levels compared to Stachybotrys. Can establish on surfaces with intermittent humidity.
Remediation implication: surface growth on non-porous materials can be addressed with cleaning and ventilation correction. Growth on porous materials (drywall) requires physical removal.

What Is Stachybotrys and What Makes It Different?

Stachybotrys chartarum is the organism colloquially called “toxic black mold.” It is far less common indoors than Cladosporium, and its presence indicates a specific set of conditions that Cladosporium does not require.

Key characteristics:

Appearance: dark greenish-black, slimy and wet-looking. The slimy texture is due to the hydrophilic nature of the spore masses — they are held together by moisture rather than being dry and powdery.
Texture: distinctly wet and slimy when active. Can appear darker and more compact than Cladosporium colonies.
Growth requirements: Stachybotrys requires chronically wet cellulose material — drywall paper is an ideal substrate — for at least 8–12 days of sustained wet conditions before it establishes. It cannot grow on concrete, metal, or plastic. It does not establish from routine bathroom humidity.
Mycotoxin production: Stachybotrys produces trichothecene mycotoxins, specifically satratoxins. These are the compounds responsible for its “toxic” reputation.
Spore behaviour: critically different from Cladosporium. Stachybotrys spores are sticky and wet — they do not aerosolise readily under normal conditions. This means air sampling often fails to detect an active Stachybotrys colony. A room can have significant Stachybotrys growth inside a wall cavity and show a near-normal air sample.
Remediation implication: per IICRC S520, confirmed or suspected Stachybotrys is treated as Condition 3 regardless of apparent size — full containment, physical removal, and independent clearance testing are mandatory.

Why Colour Alone Cannot Identify the Species

Both species can appear dark — near-black — on surfaces. So can Chaetomium (brown-olive, woolly, common after prolonged water intrusion and frequently found co-occurring with Stachybotrys), Trichoderma (bright green, rapidly spreading on wet wood), and Aspergillus niger (black on surfaces, a Pen/Asp species). A contractor, homeowner, or even a physician who says “it’s black, so it must be Stachybotrys” is guessing, not identifying.

Surface tape-lift sampling — a sample of the actual growth collected on a clear adhesive slide and sent to an AIHA-accredited laboratory — is the standard method for species identification from a visible colony. The laboratory analyst examines the spore morphology under a microscope and identifies the genus and often the species.

Air sampling, by contrast, is far less reliable for Stachybotrys specifically because of the sticky-spore property described above. A negative air sample does not rule out Stachybotrys. If you have visible slimy dark growth on drywall after chronic moisture intrusion, surface sampling is essential — an air sample alone is insufficient.

How to Read a Mold Lab Report: Spore Counts and the Outdoor Control

A mould air sample lab report typically contains:

Spore count in spores per cubic metre (spores/m³): the volume of air sampled and the number of spores identified, broken down by genus/species
Raw species breakdown: what was found, at what count
Outdoor control data: a simultaneous sample taken outside the building on the same visit, under the same conditions

The outdoor control is the most important reference point. Indoor air naturally contains mould spores carried in from outdoors. A completely mould-free indoor air sample is neither possible nor the goal. What you are looking for is whether the indoor sample is significantly elevated above the outdoor control, and whether species present indoors at high counts are absent or rare outdoors.

Normal result: indoor total spore count is at or below the outdoor control; the species profile indoors mirrors what was found outdoors (same dominant species, similar proportions).

Abnormal result: indoor species significantly elevated above outdoor; or species present indoors at high counts that are absent or rare in the outdoor control — this indicates an indoor amplification source (active mould growth somewhere in the building).

What Does Elevated Pen/Asp on a Lab Report Mean?

Penicillium and Aspergillus are often grouped on lab reports as “Pen/Asp” because the spores of these two genera are morphologically similar under light microscopy and cannot be reliably distinguished without more advanced analysis. Both are green to blue-green in colour and are common in soil and on plant material outdoors.

Elevated Pen/Asp indoors — significantly above the outdoor control, especially if Pen/Asp is a minor component outdoors but the dominant indoor species — is a reliable indicator of active indoor mould growth, typically in a hidden location: inside wall cavities, under flooring, inside HVAC ductwork, or in ceiling spaces. Some Aspergillus species are mycotoxigenic; Penicillium species can also produce mycotoxins.

Elevated indoor Pen/Asp is not the same as finding Stachybotrys, but it is a result that warrants professional investigation of the moisture source and the location of the growth.

When Is Surface Sampling Worth the Additional Cost?

Air sampling is useful for detecting amplification of airborne species and establishing whether an overall indoor mould burden is elevated. It has significant limitations for Stachybotrys specifically. Surface sampling is worth the additional cost — typically $50–$150 per sample plus lab fees — in the following circumstances:

Visible dark, slimy mould growth is present on drywall or other cellulose material after chronic moisture intrusion
Air sampling returned a normal result but visible growth is present (this combination strongly suggests Stachybotrys, given its sticky spore property)
You need species-level identification to guide remediation decisions or support an insurance claim
A post-remediation verification of a specific surface is needed

For a professional assessment of suspected Stachybotrys, a combination of air sampling with outdoor control and targeted surface tape-lift sampling of the visible growth is the appropriate protocol. Air sampling alone is not sufficient.

Frequently Asked Questions

Can I identify Stachybotrys by its smell?

Stachybotrys and other active mould colonies typically produce a musty, earthy odour from microbial volatile organic compounds (MVOCs). The smell can indicate active growth before visible mould is apparent, but it cannot identify the species. Cladosporium, Chaetomium, and Stachybotrys can all produce musty odours. Professional assessment with sampling is required for species identification.

What if Stachybotrys shows up on my air sample?

If Stachybotrys is detected on an air sample — even at relatively low counts — take it seriously. Given that Stachybotrys spores do not aerosolise readily, their presence in air sampling suggests either significant disturbance of an existing colony or a very large active colony. A professional assessor should be engaged to locate the source through inspection and surface sampling, and remediation should follow IICRC S520 Condition 3 protocol.

My lab report says “Chaetomium” — what does that mean?

Chaetomium is a brown to olive-black woolly mould that is an indicator of chronic water intrusion. It frequently co-occurs with Stachybotrys because both require sustained moisture on cellulose materials. Finding Chaetomium in a sample is a strong indicator of long-term moisture damage and should prompt investigation for co-occurring Stachybotrys, particularly via surface sampling of any visible dark growth.

What is an AIHA-accredited lab and why does it matter?

The American Industrial Hygiene Association (AIHA) runs a laboratory accreditation programme (EMLAP) that certifies mould analysis laboratories for proficiency and quality control. Results from an AIHA-accredited laboratory are considered reliable for professional and legal purposes. Consumer DIY kit labs are typically not AIHA-accredited. A professional assessor will always use an AIHA-accredited laboratory.

If my indoor total is lower than outdoor, does that mean there is no mould problem?

Not necessarily. A lower indoor total than outdoor is a generally reassuring result, but it is not a guarantee that no indoor source exists. A hidden Stachybotrys colony that is not actively releasing spores, or a localised growth in a poorly sampled room, may not elevate the total count. A thorough assessment combines air sampling with visual inspection, moisture readings, and targeted surface sampling where indicated — not air sampling alone.

How often should I test for mould if I have had a previous mould problem?

After successful remediation and confirmed clearance, there is no universal schedule for repeat mould testing. Testing is warranted if: a new moisture event occurs and materials are not dried within 48–72 hours; musty odour returns; visible mould reappears; or occupants begin experiencing respiratory symptoms without other explanation. Annual visual inspection for moisture damage combined with a functioning dehumidification strategy is more cost-effective ongoing prevention than routine air sampling.