What Is CFU
CFU stands for Colony Forming Unit. It's a count of living mold spores capable of growing into visible colonies when placed on a nutrient medium in a laboratory. When a mold inspector collects an air or surface sample from your home, the lab culture it on culture plates and counts how many individual mold spores germinate into colonies over 7 to 10 days. Each colony that grows represents one CFU from your original sample.
Why CFU Matters for Mold Testing
CFU counts tell you whether mold levels in your home exceed normal background levels. The EPA doesn't set a single "safe" CFU threshold because mold tolerance varies by person and building type, but baseline comparisons are critical. Inspectors typically collect outdoor air samples and indoor samples from the same property during the same visit. If your indoor CFU count is 25 percent higher than the outdoor baseline, that suggests active mold growth inside your home that warrants investigation.
This distinction matters because you want to know if you're dealing with routine environmental spores versus an actual moisture problem. High CFU counts concentrated in one area, like a basement or bathroom, point to a specific moisture source you need to address. Low counts throughout the home suggest your air quality is acceptable.
How CFU Testing Works
- Sample collection: Inspectors use viable sampling methods, typically air cassettes or swabs, to collect mold particles from specific locations in your home.
- Laboratory culture: Samples are placed on nutrient agar plates in controlled conditions (usually 72 to 96 hours at 22 to 27 degrees Celsius) to encourage mold growth.
- Colony counting: A technician counts visible colonies using a microscope and records results as CFU per cubic meter of air (for air samples) or CFU per square centimeter (for surface samples).
- Species identification: Labs often identify which mold types are present, since Stachybotrys and Aspergillus present greater health risks than Alternaria or Cladosporium.
Interpreting CFU Results
A typical outdoor baseline might show 500 to 2,000 CFU per cubic meter depending on season and location. Indoor counts below the outdoor baseline generally indicate acceptable conditions. Counts significantly higher than outdoor levels, especially from specific problem areas, justify remediation. During remediation work, contractors monitor CFU levels to confirm that moisture control and cleanup are working. Post-remediation, a clearance test confirms CFU counts have returned to normal ranges before occupants return to the treated area.
Keep in mind that CFU counts measure only living, culturable spores. Dead mold spores and non-viable fragments won't register in a CFU count, so this test captures only part of the full mold picture. That's why inspectors also document visible mold growth and investigate moisture sources directly.
CFU and Remediation Protocols
Remediation contractors use CFU baseline data to set targets. EPA remediation guidelines typically call for cleaning affected areas until CFU counts match outdoor baseline plus 10 to 15 percent. For mold affecting areas larger than 10 square feet, professionals use containment barriers and HEPA filtration to prevent cross-contamination during cleanup. Once remediation is complete, a third-party inspector performs clearance testing to verify CFU levels have dropped to acceptable ranges before the space is reoccupied.
Common Questions
- What CFU count means I need remediation? There's no universal number, but if your indoor count is 25 to 50 percent higher than outdoor baseline, or if you have visible mold growth anywhere, professional remediation is warranted. A certified mold inspector can advise based on your specific results and local conditions.
- Why don't all mold spores show up as CFU? CFU testing only counts living spores that can germinate on culture media. Old, damaged, or nonviable spores won't grow colonies. This is why inspectors also look for visual evidence and conduct moisture assessments alongside CFU testing.
- Can I rely on CFU testing alone to clear my home? No. Remediation clearance requires CFU testing, visual inspection confirmation, and moisture control verification. A property isn't truly remediated until moisture sources are eliminated and CFU counts stabilize at acceptable levels for at least 30 days.