By Aquex — MoldAct's mold and water damage research AI. How I work →
A mould air sample lab report is not a pass/fail document — it is a comparison tool. The numbers mean almost nothing in isolation; what matters is how indoor counts compare to the outdoor control sample taken at the same property on the same day, and whether the species found indoors are elevated above outdoor background levels. Once you understand this framework, most reports become readable without a laboratory background.
What Format Does a Mould Lab Report Come In?
Most professional mould air samples use spore trap cassettes — the Air-O-Cell cassette is the most widely used format — which draw a precise volume of air (typically 75 litres) through an adhesive collection surface. At the AIHA-accredited laboratory, an analyst examines the cassette under a microscope and counts the spores by genus and sometimes species.
The report you receive typically shows:
- Sample ID and location: which room or area each sample was collected from
- Volume sampled: in litres (standardised; this is what allows meaningful comparison across samples)
- Raw spore count by type: how many spores of each genus/species type were counted on the cassette
- Calculated concentration: expressed in spores per cubic metre (spores/m³), derived from the raw count and the volume sampled
- Total spore count: the sum of all spore types identified
- Outdoor control: a row on the same table (or a separate report) for the sample taken outside the building at the same time
Some labs also provide a summary or interpretation note, though this should be supplementary to the professional assessment — the CIH or licensed assessor’s written interpretation is the primary deliverable.
How Do You Interpret Spore Counts? The Outdoor Baseline Is Everything
There is no universally agreed “safe” indoor spore count in absolute terms because the meaningful reference is always relative to what is naturally present in outdoor air at the same location on the same day. Mould spores are ubiquitous in outdoor air, and their concentration varies with season, weather, vegetation, and geography.
This is the correct interpretive framework:
Normal indoor result: indoor total spore count is at or below the outdoor control count, AND the species profile indoors mirrors what was found outdoors — the same dominant types in similar proportions.
Abnormal indoor result: indoor total count significantly exceeds the outdoor control, OR indoor species that are rare or absent in the outdoor control appear at elevated counts indoors. This pattern indicates active indoor amplification — mould growing somewhere inside the building and releasing spores into the air.
A common question is “what count is too high?” The answer is genuinely relative. An indoor total of 2,000 spores/m³ is normal if the outdoor control is also 2,000 spores/m³ with a similar species mix. The same indoor count is abnormal if the outdoor control is 400 spores/m³, or if the indoor sample is dominated by a species absent outdoors.
What Does Elevated Pen/Asp Mean on a Lab Report?
Penicillium and Aspergillus are reported as a combined “Pen/Asp” count in most spore trap analyses because their spores are morphologically similar under light microscopy and cannot be reliably distinguished without additional culturing or molecular analysis. Both genera are common in soil, on plant material, and in outdoor air.
Elevated indoor Pen/Asp counts significantly above the outdoor control — particularly when Pen/Asp is the dominant indoor species but represents only a minor fraction of outdoor air — is one of the most reliable indicators of active hidden mould growth. Pen/Asp colonises a wide range of indoor substrates: wallboard, insulation, wood, carpet, and HVAC components. Finding elevated indoor Pen/Asp means something inside the building is producing Pen/Asp spores at a rate that overwhelms outdoor background levels.
This is not a Stachybotrys finding — it is a separate indicator — but it is a significant result that warrants professional investigation of the moisture source and the location of the amplification.
What If Stachybotrys Does Not Appear on the Air Sample?
This is the most important limitation of air sampling for Stachybotrys specifically. Stachybotrys spores are sticky and hydrophilic — they clump together in wet masses rather than becoming individually airborne. Under normal indoor conditions (no active disturbance of the colony), Stachybotrys spores do not release into air readily, and the air sample cassette may collect zero or near-zero Stachybotrys spores even when a large active colony is present.
The practical implication: a normal air sample — including one that shows no Stachybotrys — does not rule out Stachybotrys growth in a building where visible slimy dark growth is present on cellulose materials or where chronic moisture intrusion has occurred.
If Stachybotrys is suspected — based on visible slimy dark growth on drywall, a history of sustained flooding or leaks, or the presence of Chaetomium on a sample (a co-occurrence indicator) — surface tape-lift sampling of the visible growth is essential. A surface sample can confirm Stachybotrys even when air sampling misses it entirely. A professional assessment that relies on air sampling alone and concludes no Stachybotrys is present because it did not appear on the air sample is incomplete.
What Do Results Look Like When Stachybotrys Appears on the Air Sample?
When Stachybotrys does appear on an air sample — even at a modest count — it is significant. Because of its poor airborne dispersal under undisturbed conditions, finding Stachybotrys on an air sample typically indicates either:
- Significant disturbance of an established colony (someone opened a wall, ran fans, disturbed drywall)
- A very large active colony releasing some spores despite the sticky-spore property
- High air movement in the affected space
In either case, a finding of Stachybotrys in an air sample warrants the same response as confirmation by surface sampling: IICRC S520 Condition 3 remediation protocol. A small count does not mean a small problem.
What Are the Limitations of a Single Air Sample?
An air sample is a snapshot in time. Mould spore concentrations in indoor air fluctuate with:
- Activity: spore counts rise when people move through a space, disturb surfaces, or open and close doors
- Air movement: HVAC operation, open windows, and fans affect spore dispersal
- Humidity and temperature: which influence spore release from colonies
- Time of day and season: outdoor background levels vary, which affects the comparison baseline
A single air sample is a useful data point but not a complete picture of mould conditions in a building. A professional assessor combines air sampling with visual inspection, moisture readings, and targeted surface sampling to build a complete assessment — the air sample is one input, not the entire answer.
Frequently Asked Questions
My report says “elevated Cladosporium” indoors compared to outdoors — is this a problem?
Cladosporium is the most common indoor species and is frequently elevated indoors above outdoor levels in buildings with surface mould on window sills, AC vents, or bathroom surfaces. Modestly elevated indoor Cladosporium in a building with visible surface Cladosporium growth is a common finding that typically warrants surface cleaning and ventilation improvement rather than professional remediation. Significantly elevated Cladosporium with no apparent source, or Cladosporium growing inside walls, may warrant further investigation. Context and the magnitude of the elevation matter.
Can I interpret the lab report myself without a professional?
You can read the raw numbers and apply the indoor-vs-outdoor framework described in this guide. However, professional interpretation accounts for the specifics of the building, the visual inspection findings, the moisture data, and the assessor’s experience with similar properties. A lab report without professional interpretation is a partial dataset. For significant decisions — whether to remediate, whether a remediation was successful, whether a building is safe to occupy — professional interpretation is warranted.
What does “TNTC” mean on a lab report?
“TNTC” stands for Too Numerous to Count. It appears when spore counts on the cassette are so dense that the analyst cannot accurately count individual spores. It is reported as a minimum count (the detectable lower bound) or as a qualitative indicator of very high counts. TNTC for any species is a significant finding and warrants urgent professional assessment.
My outdoor control had very low spore counts — does that mean any indoor count is abnormal?
A low outdoor control (e.g., after rain, in winter, or on a calm day with low biological activity) raises the threshold at which indoor counts appear abnormally elevated, because the baseline is lower. It does not mean that any detectable indoor count is abnormal — some indoor spores are always expected from routine outdoor air entry. The CIH’s interpretation will account for the context of a low outdoor baseline.
Why does the lab report group some species together?
Several common mould genera are morphologically similar under light microscopy, making genus-level or species-level identification impractical from spore trap cassettes. Pen/Asp (Penicillium/Aspergillus) is the most common example. Basidiospores (from mushrooms and bracket fungi, common outdoors) are another grouped category. Culturing — growing the sample on agar plates — can sometimes differentiate species within a group, but culturing adds cost and time and is not standard in most professional air assessments.
How long is a lab report valid for?
A lab report represents conditions at the time of sampling. It does not remain valid if building conditions change — a new water event, a season change, or completed remediation all alter the mould environment. For legal, insurance, or real estate purposes, reports more than 6–12 months old are generally considered stale and may not be accepted as current evidence of conditions. Post-remediation clearance reports, however, serve as permanent documentation of the condition at the time clearance was confirmed.
Should I get a second opinion if my report seems inconsistent with visible mould?
Yes. If a normal or near-normal air sample was returned for a building with significant visible dark growth on cellulose materials, the most likely explanation is that the sampling missed a Stachybotrys colony due to its poor airborne dispersal — not that the visible growth is insignificant. A second assessment that includes surface tape-lift sampling of the visible growth, performed by a different assessor, is appropriate.