A mold inspector came out, collected air samples and surface swabs, and sent the results to a lab. Now you have a report with names like Cladosporium, Aspergillus/Penicillium, and Stachybotrys — and counts like 320 spores/m³ — and you have no idea what any of it means.
Here is how to read it.
This is the most important structural thing to understand about a mold air quality report. The inspector took samples inside your home and also took an outdoor baseline sample. The comparison between the two is what determines whether you have a problem.
Mold spores exist everywhere — in the outdoor air, in every home, in your lungs right now. The question is whether the indoor levels are elevated relative to what’s naturally present outside. If indoor counts are significantly higher than outdoor counts, or if indoor samples contain species not present outdoors, that’s an indicator of an indoor mold source.
Spore counts are expressed as spores per cubic meter of air (spores/m³). There is no universally agreed “safe” threshold — different labs and different industrial hygienists use different benchmarks — but here are the commonly used reference points:
In Houston specifically, outdoor baseline spore counts are often higher than national averages because of the climate — warm, humid, vegetated. A Houston outdoor sample in summer may read 800–1,200 spores/m³ naturally. This is why the indoor-to-outdoor ratio matters more than the absolute number alone.
Cladosporium: The most common outdoor mold worldwide. Found on dead plant material, soil, and decaying wood. Its presence indoors at levels comparable to outdoors is normal and not a concern. Elevated indoor Cladosporium relative to outdoor may indicate a moisture source but it’s the least alarming of the common species.
Aspergillus/Penicillium: Labs often report these together because they’re visually indistinguishable under the microscope. They’re ubiquitous but also the most common indoor mold genera. Elevated Asp/Pen indoors relative to outdoors is a significant finding — these species grow readily on drywall, insulation, and building materials. They’re also the species most associated with respiratory symptoms in sensitive individuals.
Stachybotrys: This is what people call “black mold.” It’s slow-growing, requires consistently very wet conditions (saturated cellulose materials), and is almost never found in significant amounts in outdoor air. Any Stachybotrys in an indoor sample — even a few spores — is significant because it indicates a chronically wet indoor source. It doesn’t grow from ambient humidity; it grows from sustained water intrusion.
Chaetomium: Another water-indicator mold. Found on paper and cellulose-based materials with sustained moisture. Like Stachybotrys, its presence indoors at elevated levels indicates a chronic moisture problem, not just ambient humidity.
For Stachybotrys and Chaetomium, the count matters less than the presence. These species don’t naturally occur at high levels in outdoor air, so finding them indoors at any meaningful level points to an active, wet indoor source. A total spore count of 400 with 50 Stachybotrys is more concerning than a total of 2,000 that’s all Cladosporium.
This is one of the most common and confusing outcomes. The inspector didn’t see mold. The report shows elevated spore counts. The explanation is almost always that the mold is hidden — inside a wall cavity, under flooring, in the HVAC system, in the attic above the living space — and is releasing spores into the air that you’re breathing without being visible.
Elevated air counts with no visible mold do not mean the report is wrong. They mean you need to find the source — typically through additional investigation, moisture mapping, and targeted opening of suspect wall or floor cavities.
After mold remediation is complete, a clearance test uses the same air sampling methodology to confirm that spore counts have returned to levels comparable to outdoor baseline. Clearance testing should be conducted by an independent party — not the remediation company — to avoid a conflict of interest.
If a remediation company tells you clearance testing isn’t necessary, that should give you pause. IICRC S520 standards recommend post-remediation verification before containment is removed and spaces are reoccupied.
The two numbers that matter most on your report: the indoor-to-outdoor ratio (is indoor air significantly higher?) and the species present (are Stachybotrys or Chaetomium in the sample?). Everything else follows from those two questions.
If you have a report you’re trying to interpret or you need remediation based on what the report found, call 247 Restoration Specialists. We’ll walk through the results with you and explain exactly what the findings mean for your home.
Air sampling tells you that elevated spore levels are present in the air — it does not pinpoint where the mold is growing. Surface swabs of visible suspected mold confirm whether a specific visible growth is mold and what species. Finding a hidden mold source requires additional investigation using moisture meters, thermal imaging, and sometimes targeted opening of wall or floor cavities.
The “toxic black mold” label refers to Stachybotrys chartarum, which produces mycotoxins under certain conditions. However, color alone doesn’t identify mold species — many molds appear black, green, or gray. Stachybotrys is significant because it indicates chronic water intrusion, not simply because of its color. Any mold found in significant indoor quantities warrants professional assessment regardless of color.
As soon as practically possible once the source is identified. Active mold continues to grow and release spores until the moisture source is eliminated and contaminated materials are removed. Delays allow the affected area to expand and increase remediation cost. If the report confirms active growth, remediation within days — not weeks — is the appropriate timeline.
