Reviewed by the Nampa Mold Experts inspection teamUpdated How we write and check these pages
Quick answer: a normal air test usually shows indoor spore counts similar to or lower than the outdoor sample taken the same day, with a similar mix of mold types. Indoor counts clearly higher than outdoor, or water-damage indicator molds such as Stachybotrys or Chaetomium indoors, suggest an indoor moisture problem. There's no federal "safe" spore count.
The parts of a spore trap lab report
- Sample ID and location: which room, plus the outdoor control.
- Volume sampled: liters of air drawn through the cassette. The lab uses it to calculate concentration.
- Raw count: how many spores of each type the analyst actually counted on the slide.
- Spores per cubic meter (spores/m³): the raw count converted to concentration. This is the number to compare.
- Genus or group: mold types identified by appearance, e.g. Aspergillus/Penicillium, Cladosporium, Alternaria, basidiospores, ascospores.
- Percent of total: each type's share of the sample.
- Background debris: how much dust and other particles were on the slide. Heavy debris can hide spores and make counts less reliable.
- Hyphal fragments: pieces of mold growth, not spores. Elevated fragments indoors can point to active or disturbed growth.
- Detection limit: the lowest concentration the method could detect for that sample.
A worked example
This is an illustrative example of the kind of pattern we see in a Nampa home with a wet crawlspace, not a real client report.
| Mold type | Outdoor (control) | Living room | Hallway over crawlspace hatch |
|---|---|---|---|
| Cladosporium | 640 | 210 | 240 |
| Basidiospores | 320 | 90 | 110 |
| Alternaria | 40 | 0 | 10 |
| Aspergillus/Penicillium | 80 | 110 | 1,450 |
| Chaetomium | 0 | 0 | 40 |
| Total (spores/m³) | 1,080 | 410 | 1,850 |
How we'd read it: the living room looks like filtered outdoor air: lower total, same kinds of mold. The hallway is different. Its total is higher than outdoors, it's dominated by Aspergillus/Penicillium (which is low outside), and it contains Chaetomium, a mold associated with long-wet materials. Combined with wet joists under that hallway, that points to the crawlspace as an indoor source.
Why the outdoor comparison matters most
Outdoor air always contains mold spores, and the amount changes with season and weather. In the Treasure Valley, outdoor-dominant molds like Cladosporium and Alternaria are typically higher in summer and fall and much lower in winter. So the same indoor number can be normal one month and concerning another.
That's why the useful question isn't "is 800 high?" but "is 800 higher than outside today, and is it the same kinds of mold?" An indoor count of 800 spores/m³ against 2,000 outside is generally unremarkable. The same 800 against 100 outside is not.
What the common mold types tell you
| Mold type | What it usually indicates indoors |
|---|---|
| Cladosporium | Common outdoors. Indoors at levels similar to or below outdoor is usually normal. Can grow on damp window frames and surfaces. |
| Basidiospores, ascospores | Mostly outdoor origin. Rarely diagnostic indoors unless very elevated. |
| Aspergillus/Penicillium | Reported together because their spores look alike under the microscope. Common indoors in small amounts; clearly elevated indoors suggests indoor growth. |
| Stachybotrys | Grows on materials that stay wet a long time, such as drywall paper. Rarely airborne in large numbers, so even low counts are meaningful. |
| Chaetomium | Another long-term water-damage indicator, often on wet drywall, wood and paper. |
| Alternaria | Common outdoor mold, especially in summer and near vegetation and fields. |
My report says "elevated." Does that mean my house is dangerous?
Not necessarily. "Elevated" usually means higher than the comparison sample, which suggests an indoor source worth finding and fixing. It doesn't measure health risk, and lab "risk" labels aren't standardized. Health questions belong with your doctor; the moisture source belongs in the report.
What test results can't tell you
- Where the water is. That comes from the inspection, not the lab.
- Whether mold caused symptoms. No spore count can diagnose a health problem.
- What's sealed in a wall. Low room counts don't rule out hidden growth; wall-cavity sampling and moisture readings do a better job.
- Exposure over time. An air sample covers minutes. Dust tests such as ERMI or HERTSMI-2 reflect longer-term conditions.
Reading surface sample results
Tape lift and swab results usually describe what's growing on a surface and how much (often rated as rare, few, moderate or many), and whether hyphae or spore-producing structures are present. Active growth with structures confirms mold is growing there, not just landing there. A surface result describes only the spot sampled, not how far the problem goes.
FAQ
What is a normal mold spore count indoors?
There's no single normal number and no federal limit. A normal result generally looks like the same-day outdoor sample, usually lower, with a similar mix of mold types and no water-damage indicators.
Why are Aspergillus and Penicillium listed together?
Their spores are small, round and look nearly identical under a microscope, so spore trap analysis reports them as one group. Telling them apart takes culture or DNA methods.
My indoor count is higher than outdoors in winter. Is that a problem?
It can be. Winter outdoor counts in Nampa are often low, so indoor sources stand out more. Look at which molds are higher indoors and whether moisture was found. A modest difference with outdoor-type molds is less concerning than indoor-dominant Aspergillus/Penicillium with wet materials.
Can I read my lab report without the inspector?
You can use this guide, but results only make sense next to what was found on site. That's why our reports explain each sample in plain English and we walk you through them by phone.
