Home Mold Test: What the Result Actually Tells You
The lab report came back as a two-page PDF with a bar chart on it. Indoor sample: 1,400 spores per cubic meter, mostly Cladosporium. Outdoor sample, taken the same morning on the front steps: 3,100 spores per cubic meter, mostly Cladosporium. Nowhere does either page say pass or fail — the $45 bought a number with no scale printed next to it. That's not a defective report. That's every mold test result, including the petri dish on the laundry shelf that grew four fuzzy dots.
Why there's no number to fail
Spores are a permanent part of outdoor air and therefore of indoor air, because indoor air is mostly outdoor air that came through a door. Counts outside swing by orders of magnitude with season and weather — a few hundred spores per cubic meter over frozen ground in January, tens of thousands on a dry, breezy afternoon in late August. A single indoor number can't be graded against a range that moves that much.
That's why EPA never published a limit. Its guidance is explicit that threshold values for airborne mold have not been established, and it goes further: if you can see or smell growth, sampling is unnecessary, because the remediation protocol is identical either way. Find the water, remove the growth, dry the material. A count adds nothing to that list.
Then there's what the sampler catches. A spore-trap cassette runs a pump for 5–10 minutes and photographs one moment in one room; a slammed door or someone crossing a dusty basement moves the count by a factor of several. Two samples an hour apart routinely disagree — normal instrument behavior, not a bad lab.
What each result format actually says
Each format answers a different question, and each has one sentence it is genuinely allowed to say.
| Test format | What comes back | What it can prove | What it can't | Cost |
|---|---|---|---|---|
| Settle plate (open petri dish) | Colonies after 48–96 h | Viable spores were in the air. That's it | Any concentration, any comparison — no volume was measured | $10–20 for 2–3 plates |
| Petri kit + mail-in ID | Genus names per colony | Which genera grew on agar | How much was in the air; agar favors fast growers | $10–20 + $30–40 per plate |
| Spore-trap air sample (pump + cassette) | Spores/m³ by genus, indoor and outdoor | Indoor vs outdoor ratio at that moment | Whether it's inside a wall, or where | $30–50 per sample, pump rental or pro |
| Tape lift or swab of a stain | Structures seen under a microscope | Whether that stain is growth or settled dust | Anything about the air in the room | $30–50 per sample |
| ERMI-style dust DNA panel | 36 species, one index score | A history of what has been in that dust | Current condition — EPA calls it a research tool | $150–300 |
The tape lift is the underrated one: it answers something you can act on — is this dark line on the sill growth or dirt in a scratch — for the price of one air sample. The ERMI is the overrated one, developed as a research instrument, not validated for grading a house, and delivered with no remediation step attached.
Reading the number you already have
Assume the report is in front of you. Four questions, in this order, extract everything it holds.
Was there an outdoor control from the same day? If not, the indoor number is uninterpretable — the most common defect in a DIY result.
How do the totals compare? Indoor at or below outdoor, same genera in roughly the same proportions, is the ordinary picture. Indoor running several times outdoor is the signal worth chasing.
Does the mix match? This matters more than the total. Outdoor air across most of the US is dominated by Cladosporium and, in season, Alternaria. When the indoor sample flips to Aspergillus and Penicillium — genera that thrive on damp drywall, paper facing and dust — and the outdoor sample doesn't show them, that's an indoor source. Stachybotrys is heavy, clings to wet surfaces and rarely shows in air at all, so its absence proves nothing about what's behind the drywall.
What does the number change? Usually nothing. Visible growth under about 10 square feet on a hard, non-porous surface is DIY by EPA's own threshold; larger areas, wall cavities, HVAC and any water damage in a building you don't own belong with a certified professional. Porous material that stayed wet — carpet, ceiling tile, insulation, drywall — gets replaced rather than cleaned, at any count. And the humidity that fed the growth is the real fix: hold 30–50 %RH, treat anything sustained above 60 %RH as the problem the test was pointing at.
Common mistakes
- Skipping the outdoor control. Without one, an indoor count has no scale. Every credible protocol samples outdoors the same day, in the same weather.
- Reading a grown petri dish as a diagnosis. Plates grow in clean houses. With no measured air volume and no control plate, colonies prove only that spores exist indoors.
- Testing before looking for water. A moisture meter and a flashlight settle more cases than a lab report, and cost less.
- Treating a low count as an all-clear. A 10-minute pump sample can miss growth sealed inside a wall cavity. A clean report doesn't overrule a musty smell or a stain.
FAQ
What is a normal mold spore count in a house?
There isn't one. EPA has set no threshold values for airborne mold, so no count is officially normal or abnormal. In practice, labs read an indoor total at or below the same-day outdoor total, with a similar genus mix, as unremarkable — and indoor counts several times outdoor as worth investigating.
My mold test came back positive — what now?
Almost every mold test comes back positive, because spores are everywhere. The action doesn't change with the result: locate the moisture, then treat visible growth under about 10 square feet on hard surfaces as a DIY job and anything larger or hidden as professional work. Our 10-square-foot walkthrough covers the protocol EPA lays out.
Should I test for mold if I can already see it?
EPA says sampling is unnecessary when growth is visible, because knowing the genus doesn't change the remediation. The exception is a dispute — a landlord, an insurer, a buyer — where a documented $30–50 sample is evidence rather than information. The $15 vs $400 decision tree sets out when each is worth it.
Why did my DIY petri dish grow mold when my house seems fine?
Because agar is ideal food and every room has viable spores drifting through it. A plate left open for an hour typically grows something within 48–96 hours regardless of the building's condition. We break down where the petri format misleads in the DIY kit guide.
Related:
General information on interpreting home mold sampling, not medical or professional remediation advice. Growth over about 10 square feet, suspected growth inside walls or HVAC, or any related health concern warrants a certified specialist. Sampling methods, lab pricing and report formats vary by provider.