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Reading and interpreting mold test results

How to Read Your Mold Test Results

Reports can look technical, but the key is simple: compare indoor results to outdoor controls and look for water-damage clues.

Understanding Report Sections

Most reports include: sample details (location, date, time), lab method, results table, and an outdoor control sample. Focus on where samples were taken and whether conditions were noted (humidity, HVAC running, recent cleaning). Without context, results are hard to interpret. A quality inspector provides this context in their written interpretation.

Indoor vs Outdoor Comparison

Outdoor air naturally contains mold spores. The goal is to see whether indoor levels and species mix are similar to outdoor conditions. If indoor counts are much higher or the species mix is very different, it suggests an indoor source.

Common Mold Types

Many reports list genera like Cladosporium, Aspergillus/Penicillium, Alternaria, and Chaetomium. Some are common outdoors, while others are more associated with water-damaged materials indoors. The presence of water-damage molds indoors (especially when not present outside) is more important than total counts alone. Finding Stachybotrys (black mold) or Chaetomium typically indicates chronic moisture.

Water-Damage Indicators

Certain genera are more commonly associated with chronic moisture (for example, Stachybotrys and Chaetomium). Finding them indoors suggests a sustained moisture issue and a need to inspect for damp materials. Check common problem areas like basements, crawlspaces, and areas affected by past leaks when looking for the moisture source.

Water-Damage Molds

  • Stachybotrys (black mold) - chronic water damage
  • Chaetomium - paper/drywall with prolonged moisture
  • Ulocladium - wet surfaces and water-damaged areas
  • Fusarium - very wet conditions, flooding

Common Outdoor Molds

  • Cladosporium - most common outdoor mold
  • Alternaria - outdoor, also near windows
  • Basidiospores - mushrooms, wood decay fungi
  • Ascospores - outdoor decomposition

What the Numbers Mean

What the Numbers Mean

Spore counts are usually reported as spores per cubic meter. Higher numbers are not automatically more dangerous. A smaller indoor count can still matter if outdoor counts are very low, or if the species mix is unusual. Think of counts as clues, not diagnoses.

  • Compare indoor vs. outdoor counts for the same species
  • Look for indoor-dominant species not found outside
  • Consider building conditions at sampling time

Limitations of Test Results

Air samples are snapshots and change with weather, activity, and HVAC use. Hidden mold behind walls may not show up. Testing alone cannot identify the moisture source. Both EPA and NIOSH emphasize that inspection and moisture control are the core of mold assessment.

What to Do Next

Based on your results, follow these steps to determine the appropriate action:

  1. 1

    Review the comparison

    Check if indoor levels are higher than outdoor levels or if the species mix is different.

  2. 2

    Identify water-damage indicators

    Look for Stachybotrys, Chaetomium, or other moisture-associated molds that appear indoors but not outdoors.

  3. 3

    Check for moisture sources

    Inspect common problem areas like basements, bathrooms, and areas with past water damage.

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