Understanding Mold Testing Results: What Your Lab Report Means

The Lab Report That Looks Like a Medical School Exam

Aspergillus/Penicillium: 1,847 sp/m³. Cladosporium: 426 sp/m³. Stachybotrys chartarum: 53 sp/m³.

You're staring at these numbers wondering if you should be terrified. Your Google search isn't helping — one website says 500 is fine, another says any Stachybotrys means evacuate immediately.

Here's the reality: interpreting mold lab results requires context that most online resources don't provide. Let me give you that context.

The Most Important Number on Your Report (That Most People Miss)

Before looking at any indoor reading, find this first: your outdoor control sample.

Here's why this matters: mold spores are everywhere. Outside. Inside. In every home on earth. The question isn't "are there mold spores?" — the answer is always yes. The question is "are indoor levels elevated compared to normal outdoor levels?"

If your outdoor sample shows 800 sp/m³ of Aspergillus/Penicillium and your indoor sample shows 600, you don't have an indoor mold problem. You have normal infiltration from outside.

If your outdoor sample shows 200 and your indoor shows 1,800? That's a different story.

Common Mold Species and What They Mean

Let me translate the Latin names you're seeing:

Aspergillus/Penicillium

These are grouped together because their spores look nearly identical under a microscope. They're also the most common molds you'll find in virtually any environment.

What it means: Small amounts are completely normal. Elevated indoor levels (significantly higher than outdoor) suggest there's a moisture source growing these molds somewhere inside. These are the molds that grow on damp drywall, in AC systems, and anywhere humidity gets too high.

Health implications: Most subspecies are allergenic. A few are more concerning for immunocompromised individuals. For most healthy people, moderate elevations cause allergy-type symptoms — sneezing, congestion, irritation.

Cladosporium

This is an outdoor mold that commonly makes its way inside. You'll often see it on window sills and bathroom surfaces.

What it means: High outdoor levels often mean high indoor levels — this doesn't automatically indicate a problem. It's when indoor Cladosporium is dramatically higher than outdoor that you look for an indoor source.

Health implications: Allergenic, especially for people with hay fever. Generally not considered high-risk.

Stachybotrys (The "Black Mold" Everyone Fears)

This is the one that gets media attention. Let me tell you what the headlines don't mention:

What it means: Stachybotrys needs a lot of water to grow — sustained, heavy moisture on cellulose materials like drywall or wood. Finding it means there's been (or is) significant water damage somewhere. It's not found outdoors in measurable amounts, so any indoor presence suggests an indoor source.

What it doesn't mean: Your house isn't automatically uninhabitable. Stachybotrys is concerning and should be remediated, but it's not the instant health disaster that some websites claim. I've seen homes with Stachybotrys that were fixed properly and returned to perfectly normal conditions.

The honest answer: Stachybotrys produces mycotoxins that can cause health issues with prolonged exposure. It should be taken seriously and professionally remediated. But "seriously" doesn't mean "panic."

Chaetomium

Another "water indicator" mold that, like Stachybotrys, needs sustained moisture to grow on paper and drywall.

What it means: Similar to Stachybotrys — indicates significant water damage. Often found alongside it.

Other Species You Might See

Alternaria, Fusarium, Ulocladium, Epicoccum... there are thousands of mold species. Most reports will list whatever the lab identified. The key questions remain the same: Is it higher inside than outside? Is it a water-indicator species?

Understanding Spore Counts: The Numbers Game

Let me give you some general reference points, with a huge caveat: there are no federal standards for "safe" mold levels. These are industry guidelines, not legal thresholds.

But here's what matters more than these numbers: the comparison to your outdoor sample and which species are elevated.

Finding 2,000 sp/m³ of common outdoor molds when outdoor levels are 1,500 is different from finding 200 sp/m³ of Stachybotrys when outdoor levels are zero.

What "Raw Count" vs "Count/M³" Means

Your lab report might show two numbers: a raw count and a calculated count per cubic meter.

Raw Count: The actual number of spores the lab technician counted on the sample slide.

Count/M³ (or sp/m³): A calculated estimate of spores per cubic meter of air, based on the sample volume collected.

The Count/M³ is the number used for comparisons and assessments. Raw counts are useful for quality control but less meaningful for interpretation.

The Limitations of Air Sampling (Honest Talk)

Air sampling has real limitations you should understand:

• Point-in-time snapshot: We're capturing what's in the air at one moment. Mold spore levels fluctuate throughout the day.
• Some molds don't release easily: Stachybotrys, for example, has sticky spores that don't become airborne as readily. Low air counts don't always mean low contamination.
• Hidden growth: Mold behind walls may not show up in living space air samples until the problem is severe.

This is why professional mold inspections include visual assessment and moisture mapping — not just air sampling. The lab results are one piece of the puzzle.

When Results Are "Borderline"

Sometimes results aren't clearly good or bad. Indoor levels are slightly elevated. There's a species that could be concerning in small amounts.

Here's how I approach it: I give you the data and my professional assessment, then we talk about your specific situation.

• Does anyone in the home have respiratory issues or immune compromise?
• Is there visible growth or musty odor, even if air counts are moderate?
• Was there a clear water event that could explain the source?

Borderline results often say: "investigate further" rather than "remediate immediately" or "nothing to worry about."

What Your Lab Report Can't Tell You

Lab results show what's in the air or on samples. They can't tell you:

• Why the mold is there (the moisture source)
• How long it's been growing
• Whether it's making you sick (that's a medical question)
• Whether your specific health symptoms are related to mold exposure

If you're concerned about health effects, talk to your doctor — not just your mold inspector. I can tell you what's in your environment. I can't practice medicine.

Questions to Ask Your Inspector

When reviewing results with your inspector, ask:

1. How do indoor levels compare to the outdoor control?
2. Are any species present that shouldn't be indoors at all?
3. Do these results align with what you saw during the visual inspection?
4. What's the likely moisture source for any elevated species?
5. What's your recommendation based on these findings?

A good inspector will explain results in plain English and tie the lab data back to what they observed in your home.