What a spore trap sample is
A calibrated pump pulls a measured volume of air through a small cassette, and spores and particles stick to a slide inside. An accredited lab counts them under a microscope and reports them as spores per cubic meter of air, grouped by mold type.
Spore traps identify mold to the genus or group level, not the exact species, and they count both living and dead spores.
There is no single "normal" number
The EPA states that no federal standards or threshold limits exist for airborne mold spores. Spores are in all outdoor air and in every home. So an indoor number on its own tells you very little. The meaning comes from comparison.
Step 1: Compare indoor to outdoor
A good report includes an outdoor sample taken the same day. In a home without an indoor mold source, indoor counts are usually lower than outdoor counts, and the mix of mold types looks similar. Indoor totals well above outdoor point to a source inside.
Step 2: Compare the types
Look at which mold types dominate. Outdoors, types like Cladosporium and basidiospores (mushroom spores) usually lead. When an indoor sample is dominated by types that are scarce in the outdoor sample, such as Penicillium/Aspergillus, or includes water damage indicators like Stachybotrys or Chaetomium, that suggests growth inside the home even if the total count looks modest.
Step 3: Compare rooms
With more than one indoor sample, compare a suspect area to a cleaner room. A basement with far higher counts than an upstairs bedroom tells you where to look.
Mold types you will see on most reports
Cladosporium
One of the most common outdoor molds. Found indoors too; it matters mainly when indoor counts are well above outdoor.
Basidiospores and ascospores
Mostly from mushrooms and plants outside. High outdoor counts are normal, especially in wet weather.
Penicillium/Aspergillus
Reported together because they look alike under the microscope. Common in homes with indoor growth. An indoor count well above outdoor is one of the most useful signals on a report.
Stachybotrys and Chaetomium
Associated with materials that have stayed wet, like drywall after a leak. Their spores are heavy and do not stay airborne long, so even low indoor counts deserve attention.
What air tests cannot tell you
A spore trap is a snapshot of a few minutes. Spore levels change with air movement, the furnace cycling and activity in the room. A clean result means airborne spores were low during the sample, not that the house has no mold anywhere. That is why air sampling belongs alongside a visual inspection and moisture readings, not in place of them.
When the results point to a problem
If lab results suggest an indoor source, the next step is finding and fixing the moisture feeding it. If lab results suggest remediation is warranted, the scope should be based on where the growth actually is, not on the numbers alone. After cleanup, follow-up air samples compared to outdoor air are the standard way to verify the work.
Frequently asked questions
I have a report from another company. Can you help?
If you want a second opinion on a report you have already received, reach out and tell us what you are seeing.
Are home petri dish tests comparable?
No. They do not measure a known volume of air and have no outdoor comparison, so they nearly always show growth and cannot tell you whether your home is different from the air outside.