A mold report typically contains a table of spore counts broken down by species, an indoor-to-outdoor comparison, and sometimes moisture readings or photos. The key to reading it is knowing which numbers matter, what “normal” looks like, and which mold types deserve your attention. Here’s how to work through each section of a standard report.
Raw Count vs. Spores Per Cubic Meter
Most air sample reports have two main columns for each mold type detected. The “raw count” is simply how many spores the lab technician counted on the sample slide. The “count per cubic meter” (often written as counts/m³) takes that raw number and scales it up based on the volume of air that was sampled, giving you a standardized concentration. This is the number you should focus on, because it allows you to compare one room to another or compare your indoor air to the outdoor sample, even if the sampling times were slightly different.
A commonly referenced benchmark: a “clean” building typically has fewer than 2,000 total spores per cubic meter, with Aspergillus and Penicillium (the most common indoor molds) at 700 or below. Total counts between 5,000 and 10,000 usually indicate active indoor mold growth. These aren’t federal standards, though. The EPA has not set regulatory limits for airborne mold concentrations, so these thresholds come from industry experience rather than law.
The Indoor-to-Outdoor Comparison
This is the single most important part of the report. A good mold inspector collects at least one outdoor air sample alongside the indoor samples. The outdoor sample acts as your baseline, because mold spores are always present in outdoor air, and they drift inside through doors, windows, and HVAC systems.
The basic rule: indoor spore counts should be lower than or roughly equal to outdoor counts, and the species found indoors should mirror what’s found outside. If your outdoor sample shows 500 spores/m³ of Cladosporium and your living room shows 400, that’s normal. But if a mold type appears indoors at concentrations significantly higher than outdoors, or if a species shows up indoors that isn’t present in the outdoor sample at all, that points to an indoor source.
Pay attention to both the total count and the individual species breakdown. A report might show a reasonable total, but one specific mold genus could be disproportionately elevated indoors. That’s still a red flag.
Which Mold Types Matter Most
Your report will list mold by genus, and some names carry more weight than others.
- Aspergillus/Penicillium: Labs often group these together because their spores look nearly identical under a microscope. They’re the most common indoor molds and are present in virtually every home at low levels. Counts above 700/m³, or indoor levels noticeably higher than outdoor, suggest a moisture problem feeding growth.
- Cladosporium: The most common outdoor mold. Finding it indoors at levels near or below the outdoor count is completely normal.
- Stachybotrys: This is what people call “black mold.” It doesn’t travel through the air as easily as other molds because its spores are sticky and heavy, so even a small count on an air sample is significant. The American Industrial Hygiene Association ranks it first among molds producing mycotoxins harmful to human health. Any confirmed presence of Stachybotrys on your report warrants investigation, regardless of the count.
- Chaetomium: Another water-damage indicator. Like Stachybotrys, it typically means sustained, serious moisture intrusion rather than everyday humidity.
If your report lists “background” or “other” spores, these are typically fragments and unidentifiable material. A high number in this category isn’t necessarily alarming on its own, but it adds to the total load your lungs are processing.
Moisture Readings on the Report
Many inspectors include moisture meter readings for walls, floors, or ceilings. These numbers are reported as a percentage of moisture content in the material being tested.
For wood and wood-based materials like framing or subfloor, the thresholds are straightforward. At 19% moisture content or below, wood is too dry to support mold growth. Between 20% and 25%, conditions are marginal, and mold can begin to establish. Above 25% is the optimum range for wood decay and active mold colonization. Drywall doesn’t have the same standardized scale, but most inspectors flag anything above 1% on a pin-type meter or elevated readings on a non-invasive scanner as worth investigating further.
Moisture readings are arguably more actionable than spore counts. A high moisture reading tells you where the problem is, while spore counts tell you there is a problem. If your report shows elevated moisture in a specific wall, that’s the wall to open up or dry out.
ERMI and HERTSMI-2 Scores
Some reports use DNA-based testing instead of (or alongside) traditional air sampling. These tests analyze dust samples for the genetic material of 36 mold species and produce a single score.
An ERMI (Environmental Relative Moldiness Index) score compares 26 water-damage-associated species against 10 common background species. The result falls on a scale from roughly -10 to +30. A negative score means your home has less water-damage mold than the average U.S. home. The higher the positive number, the more water-damage species are present relative to normal background mold. Scores above 5 are generally considered elevated, and scores above 10 are high.
The HERTSMI-2 is a narrower test that focuses on just five of the most problematic species. It’s often used by people with mold sensitivity or chronic inflammatory conditions. The scoring breaks down into three zones:
- Below 11: Generally considered safe, even for sensitive individuals. Roughly a 98% likelihood the environment is safe to remain in.
- 11 to 15: Borderline. Remediation is advised, and the home is not recommended for people with suspected mold illness.
- Above 15: Considered dangerous for sensitive individuals.
These DNA-based tests capture mold that may have settled into carpets and furniture over months, so they reflect long-term conditions rather than a single-day snapshot. That makes them useful for a different question than air sampling: air samples tell you what you’re breathing right now, while ERMI and HERTSMI-2 tell you what’s been accumulating over time.
Checking Lab Credibility
Before you trust any numbers on your report, look for the lab’s accreditation. The gold standard for mold analysis labs is accreditation through AIHA Laboratory Accreditation Programs (specifically their Environmental Microbiology Laboratory Accreditation Program, or EMLAP). This is a third-party, internationally recognized accreditation body that audits labs for quality and accuracy. The lab name and accreditation status should appear on the report header. If you don’t see it, ask your inspector which lab they use and verify accreditation on the AIHA website.
An unaccredited lab may still produce reasonable results, but you have no independent verification that their analysts are proficient or their equipment is calibrated. For something that could influence a remediation decision costing thousands of dollars, accreditation matters.
What the Report Means for Next Steps
If indoor counts are similar to or lower than outdoor counts across all species, and moisture readings are normal, your home is in good shape. No action needed.
If one or two areas show moderately elevated counts (say, indoor totals between 2,000 and 5,000/m³) with no Stachybotrys or Chaetomium, and moisture readings are only slightly elevated, you may be dealing with a small, contained issue. Improving ventilation, fixing a minor leak, or cleaning a visible patch of mold on a hard surface could resolve it.
If the report shows total counts above 5,000/m³, the presence of water-damage indicator species that aren’t in the outdoor sample, or moisture readings above 25% in building materials, you’re looking at a situation that likely needs professional remediation. The EPA’s remediation guide recommends hiring experienced contractors for anything beyond small, visible patches, and suggests verifying that contractors follow established industry guidelines from organizations like the American Conference of Governmental Industrial Hygienists.
One last thing to keep in mind: a single set of air samples is a snapshot of one moment in time. Spore counts fluctuate with humidity, air movement, and activity in the room. If results are borderline or confusing, a second round of testing on a different day can clarify whether you’re seeing a real pattern or a one-time spike.