Mold Testing: How to Read a Spore Report
Mold testing is sold as a way to find out whether there is a problem. A laboratory report answers a different question: how the air indoors differs from the air outside, and which genera are present in it.
The EPA’s position is that in most cases, where visible growth is present, sampling is unnecessary. What follows is about the situation where a sample has been taken anyway and a report is in hand.
Why Mold Testing Has Nothing to Compare Against
The EPA states that since no federal limits have been set for mold or mold spores, sampling cannot be used to check a building’s compliance with federal standards. Threshold limit values for airborne concentrations have not been set.
The CDC puts it the same way: there are no set standards for what is and is not an acceptable quantity of different kinds of mold in a home.
New York City’s Department of Health, in its 2008 guidelines, states that no standards or clear and widely accepted guidelines exist against which results can be compared.
Health Canada goes further on interpretation: in the absence of exposure limits, results of air tests for fungi cannot be used to assess risk to occupants, and its approach compares the diversity of taxa rather than any multiple.
Four public health bodies, one position. That is the starting point for reading any report, and the same principle runs through our mold section.
The Ratio That Does Not Exist
Numbers circulate in commercial practice: an indoor count two to three times the outdoor count is called elevated, five times or more significant. A “3:1 ACGIH ratio” also gets cited.
None of these multiples is supported by a primary source. Neither ACGIH in Bioaerosols (1999), nor the AIHA Green Book, nor New York City’s health department, nor Health Canada publishes a numerical threshold. The multiples appear only in material from commercial laboratories and inspectors.
What ACGIH did state, in 1989, is qualitative: a situation is unusual when bioaerosol levels are at least an order of magnitude higher than in symptom-free control spaces, or when the mix of organisms differs between the control and the space in question. An order of magnitude is roughly tenfold, and the comparison is between rooms, not a formalised indoor-to-outdoor figure.
How Genera Are Read: Water Activity
This is where a report becomes usable.
Genera differ in the water activity they require, which is the equilibrium relative humidity of the substrate divided by 100. Some manage on slight moisture; others need sustained wetting. The classification by Grant and colleagues (1989) sorts them into three groups.
Primary colonisers, the xerophiles, grow below roughly 0.80. Wallemia, Penicillium, Aspergillus, Eurotium.
Secondary colonisers occupy roughly 0.80 to 0.90. Cladosporium, Alternaria, Phoma, and part of Aspergillus.
Tertiary colonisers, the hydrophiles, need above 0.90. Stachybotrys, Chaetomium, Ulocladium, Trichoderma, Acremonium, Fusarium, Aureobasidium.
The idea of a “water damage indicator” follows from this. Xerophiles grow almost anywhere and are ubiquitous outdoors. Hydrophiles need cellulose that has stayed wet, are rare in outdoor air, and so carry meaning indoors on their own.
| Genus | Minimum water activity |
|---|---|
| Aspergillus restrictus | 0.71–0.76 |
| Aspergillus versicolor, Penicillium spp. | 0.79–0.80 |
| Aspergillus fumigatus | 0.85–0.86 |
| Cladosporium | 0.85–0.88 |
| Alternaria alternata | 0.85–0.88 |
| Curvularia | 0.855 |
| Fusarium | 0.87–0.91 |
| Aureobasidium pullulans | 0.89–0.90 |
| Acremonium | 0.90 |
| Ulocladium | above 0.90 |
| Chaetomium globosum | above 0.90, optimum 0.94 |
| Stachybotrys chartarum | 0.94, optimum above 0.98 |
Reported minimums from the mycology literature, not standards. Grant et al. 1989; INSPQ fact sheets. No published minimum exists for Trichoderma.
These are reported minimums from the mycology literature, not standards. No published minimum exists for Trichoderma, where only an optimum of 0.98 to 0.995 is available. The measure does not apply to basidiospores or ascospores.
Genera That Count Regardless of the Outdoor Control
Codina and colleagues (2008) state that the presence of Chaetomium, Stachybotrys, and Ulocladium in indoor air samples should be considered relevant when interpreting laboratory reports, regardless of whether those types appear in outdoor samples.
The same authors give their basis directly: fungal ecology, familiarity with spore types, and personal experience, rather than potential health effects. That qualification belongs alongside the claim.
Industry sources extend the same treatment to Trichoderma, Acremonium, and Fusarium.
Two Genera an Air Test May Miss
Stachybotrys and Chaetomium aerosolise poorly and call for contact sampling rather than air sampling.
The practical consequence is worth stating plainly: a negative air sample for these genera does not mean they are absent from the house.
The Florida Background
Reference outdoor levels for Florida, measured at ground level in spring and early summer:
| Category | Low | Moderate | High | Very high |
|---|---|---|---|---|
| Total | under 4,813 | 4,813–8,164 | 8,165–108,509 | above 108,509 |
| Ascospores | under 1,117 | 1,117–2,206 | 2,207–12,975 | above 12,975 |
| Basidiospores | under 1,064 | 1,064–2,506 | 2,507–23,861 | above 23,861 |
| Cladosporium | under 744 | 744–1,635 | 1,636–19,278 | above 19,278 |
| Penicillium and Aspergillus | under 650 | 650–1,076 | 1,077–14,887 | above 14,887 |
Spores per cubic metre. Ground-level sampling, spring and early summer, Florida. Codina, Cohen and Mele.
The authors note scattered distributions and the need for year-round study.
What normal Florida homes look like. In a peer-reviewed study of Tampa-area homes without obvious moisture problems, sampled in summer during the rainy season, the median total indoor count was 335 spores per cubic metre, against 2,355 outdoors. The Aspergillus and Penicillium group appeared in 52 of 53 indoor samples and made up a median 52 percent of all indoor spores. The median indoor-to-outdoor ratio was 0.78.
In a normal house, in other words, there are fewer spores indoors than out. That is the reference point that does not exist as a standard.
Year-round background. The subtropics bring no winter pause. Outdoor levels stay high all year, unlike northern states under snow cover, which raises the denominator and calls for caution when reading the Aspergillus and Penicillium group.
What invalidates an outdoor control: active rain during collection, strong wind, proximity to compost, leaf piles or freshly cut grass, construction work, and sampling under tree canopy.
For three to four days after rain, ascospores and basidiospores rise. The dry spores of Cladosporium and Alternaria lift during warm daytime hours in low humidity and wind.
Sample Types and What Each One Misses
Air sampling by spore trap. A calibrated pump draws a known volume: typically 75 litres over 5 minutes at 15 litres per minute, which is 0.075 cubic metres. Results come in spores per cubic metre.
Microscopic counting cannot, by design, distinguish species within a genus or living spores from dead ones, and against a heavy debris background the small spores of Aspergillus and Penicillium can be masked.
The “Aspergillus/Penicillium-like” category exists because those spores are morphologically indistinguishable under light microscopy. Identification to species requires culture or DNA analysis.
Culture-based sampling. Only viable propagules grow. The choice of medium strongly shapes the spectrum, fast-growing genera suppress slower ones, and results depend on medium, temperature, and an incubation period of 7 to 14 days. Units are colony-forming units per cubic metre.
ERMI and MSQPCR. The EPA Office of Inspector General report 13-P-0356, dated August 23, 2013, found these tools had not been peer reviewed or validated for public use, and licensees are barred from claiming EPA approval.
Home petri dish kits. These come back positive almost every time, because spores are everywhere. They neither quantify nor locate a source.
Laboratory, Accreditation, and Chain of Custody
Analysis should follow AIHA and ACGIH methods. The recommended accreditation is AIHA-LAP EMLAP, or an ISO/IEC 17025 equivalent.
The AIHA EMPAT programme tests only an analyst’s ability to identify taxa from digital images. It is not equivalent to a full quality system, and participation in it is not EMLAP accreditation.
Chain of custody is the documented handover of a sample from collection to laboratory. It becomes critical when results are used in an insurance dispute or in court, which is also where the cost of a mold inspection stops being the main question.
No published inter-laboratory comparison of commercial spore trap reports in the United States, with a numerical estimate of variation, could be located.
Hurricanes
No peer-reviewed studies of indoor spore composition exist for Ian, Milton, or Irma.
The closest studied analogue is Katrina. Solomon and colleagues, writing in Environmental Health Perspectives in 2006, estimated 24-hour concentrations of 21,000 to 102,000 spores per cubic metre outdoors and 11,000 to 645,000 indoors. Mean outdoor concentration in flooded areas was roughly double that in non-flooded areas, at 66,167 against 33,179.
What It Costs
The ranges below are compiled by aggregators of user-submitted quotes and are not measurements. Fuller figures are in our breakdown of mold remediation cost in Florida.
| Service | Range, $ |
|---|---|
| Visual inspection, no samples | 150–250 |
| Visual inspection plus 2–3 air samples | 300–450 |
| Single air sample | 75–150 |
| Laboratory analysis of a sample | 35–75 |
| Surface sample | 50–125 |
| Clearance test after remediation | 250–600 |
Ranges compiled by aggregators of user-submitted quotes, 2026. Not measurements.
Where the work goes beyond testing, the threshold questions are covered in DIY vs professional mold removal.
Frequently Asked Questions
Is there a safe level of mold spores?
No federal standard exists. The EPA states that threshold limit values for airborne concentrations of mold or mold spores have not been set, and the CDC states there are no set standards for an acceptable quantity.
Is a 3:1 indoor-to-outdoor ratio a real standard?
No. Neither ACGIH, nor the AIHA, nor New York City’s health department, nor Health Canada publishes a numerical multiple. Such ratios appear only in material from commercial laboratories.
What is a normal indoor spore count in Florida?
In a study of Florida homes without obvious moisture problems, the median indoor count was 335 spores per cubic metre against 2,355 outdoors, with a median ratio of 0.78. That is one study, not a standard.
Why are Aspergillus and Penicillium reported together?
Their spores are morphologically indistinguishable under light microscopy. Identification to species requires culture or DNA analysis.
Can an air test miss Stachybotrys?
Yes. Stachybotrys and Chaetomium aerosolise poorly and call for contact sampling. A negative air result does not mean they are absent.
Are home mold test kits reliable?
Home petri dish kits come back positive almost every time, because spores are everywhere, and they neither quantify nor locate a source.
Sources
- U.S. Environmental Protection Agency: Mold Testing or Sampling and Mold Course, Chapter 7
- U.S. EPA Office of Inspector General: Report 13-P-0356, August 23, 2013
- Centers for Disease Control and Prevention: About Mold
- Grant, Hunter, Flannigan and Bravery: The moisture requirements of moulds isolated from domestic dwellings, International Biodeterioration 25, 1989
- Codina et al.: Journal of Investigational Allergology and Clinical Immunology, 2008
- Solomon et al.: Environmental Health Perspectives 114(9), 2006
- ACGIH: Bioaerosols, Assessment and Control, 1999
- New York City Department of Health and Mental Hygiene: Guidelines on Assessment and Remediation of Fungi in Indoor Environments, 2008
- Health Canada: Residential Indoor Air Quality Guideline, Moulds
- Institut national de santé publique du Québec: mould fact sheets
- AIHA: FAQs About Spore Trap Air Sampling for Mold