How to Read a Mold Lab Report

If someone has tested your home for mold, you probably have a lab report that looks like a spreadsheet: a column of Latin names, a column of numbers, and a unit you have never seen before. Many companies hand it over with a single line of interpretation, and that line is often written by the same company that wants to sell you the cleanup.

You can read most of it yourself. Here is what each part means and the few comparisons that actually matter.

The parts of a spore trap report

Sample IDs and locations. Each column is one air sample: a room, a crawl space, a basement, and at least one outdoor sample. If there is no outdoor sample, the report cannot be interpreted properly. That is the first thing to check.

Volume. How many liters of air were pulled through the cassette. Typical residential samples are in the range of 75 to 150 liters. Very small volumes produce less reliable numbers.

Spore types. The lab identifies spores by genus or group under a microscope. Some are reported as combined groups, like Penicillium/Aspergillus, because they look nearly identical at that magnification.

Raw count and count per cubic meter. The raw count is what the analyst actually saw. The per cubic meter figure scales that up to a standard volume so samples can be compared. The per cubic meter number is the one to look at.

Background debris. Usually rated on a scale. Heavy debris can hide spores, so a sample rated heavily loaded may undercount what was actually there.

The comparison that matters most

Outdoor air always has mold spores in it. The question is never whether there are spores indoors. It is whether the indoor air looks like a diluted version of the outdoor air, or whether something inside the house is adding spores of its own.

In a typical healthy house, indoor totals are lower than outdoor totals and the mix of types is similar. The dominant outdoor spores, often Cladosporium and various basidiospores and ascospores in this region, are also the dominant indoor ones, just at lower numbers.

The pattern that points to an indoor source is different: one or two types that are much higher indoors than outdoors, or types that show up indoors and barely at all outside.

The spore types worth knowing

Penicillium/Aspergillus. Common everywhere. An indoor count far above the outdoor count is one of the most frequent signatures of hidden growth, often on drywall, framing or dust in damp areas.

Cladosporium. Usually the most common outdoor spore. Indoors it tends to track outdoor levels, but a strong indoor excess can come from growth on window frames, bathroom surfaces or HVAC components.

Stachybotrys and Chaetomium. These grow on materials that have been wet for a long time, such as drywall and wood after a leak. Their spores do not travel easily through the air, so even small indoor counts are meaningful and suggest a water problem that has been going on a while.

Hyphal fragments. Pieces of the mold body itself rather than spores. Elevated fragments indoors can indicate disturbed or active growth nearby.

What a lab report cannot tell you

An air sample is a snapshot of a few minutes of air. It does not locate the growth, it does not tell you how much material is affected, and a normal result does not prove there is no problem behind a wall. That is why sampling should always be paired with a physical inspection: moisture readings, a thermal camera, and someone actually looking in the crawl space and attic.

It also does not tell you what the fix costs. Be cautious of any report that jumps straight from a spore table to a large remediation quote without describing the affected area, the moisture source and a scope of work.

Getting a second read

If you already have a report, from another inspector or from a kit, we are happy to go through it with you. Sometimes the numbers support exactly what you were told. Sometimes there is no outdoor control, the sample volume was tiny, or the interpretation reaches further than the data. Because we do not do remediation, we have no reason to make a report sound worse than it is.

Our reports

Every report we deliver includes the raw lab data, an outdoor control, the sample locations and volumes, and a plain-English explanation of what the numbers mean for your house. You can see a sample report here. A standard inspection is $425 with one indoor air sample, an outdoor control and lab fees included.

Frequently asked questions

What is a normal mold spore count indoors? There is no single normal number. The meaningful comparison is indoor against the outdoor sample taken the same day. Indoor counts well below outdoor, with a similar mix of types, is the typical healthy pattern.

Is Penicillium/Aspergillus bad? They are common everywhere, indoors and out. What matters is whether they are elevated indoors compared with outside. A large indoor excess of Penicillium/Aspergillus is one of the most common signs of hidden indoor growth.

Is any Stachybotrys a problem? Stachybotrys spores are heavy and sticky and rarely become airborne, so even low counts indoors are worth taking seriously. It grows on material that has been wet for a long time, which means there is a water problem to find.

Can you review a report someone else gave me? Yes. If you already have a lab report from another company or a kit, we can walk through it with you and tell you what it does and does not show.