Mold & indoor air quality
Every house has mold. The right question is "is what's in my house different from what's outside my house, and is it affecting how I feel." This page walks through what the literature says and how Canary tests against it.
The two-sentence summary
The clinical evidence linking water-damaged indoor environments to respiratory and atopic symptoms in sensitized individuals is strong (WHO 2009 Guidelines for Indoor Air Quality: Dampness and Mould; Institute of Medicine 2004 Damp Indoor Spaces and Health). The actionable evidence linking specific airborne spore counts in a non-water-damaged home to symptoms is weaker. We test in a way that surfaces the first picture, not the second.
What is mold, mechanically
Indoor mold is filamentous fungi growing on building materials. Growth requires four things: a fungal spore (always present in any indoor environment), a nutrient source (most building materials qualify), oxygen, and moisture above ~60% relative humidity at the surface for 24-48 hours. The last item is the only one you can control after the fact, and it's where almost every mold remediation focuses.
Why "all mold is everywhere" is true but unhelpful
The atmosphere has thousands of spores per cubic meter at baseline. So does your living room, almost always. Reporting a single "total spore count" from inside without comparing it to the outside is meaningless. What matters is the indoor:outdoor ratio — if the indoor count is meaningfully higher than the outdoor count on the same day from the same site, that's the signal of indoor amplification (AIHA Field Guide for the Determination of Biological Contaminants in Environmental Samples, 2nd ed.).
Inspector visits include both indoor and outdoor air samples for this reason. A bad lab report leaves out the outdoor sample entirely.
Genus matters more than count
Outdoor air is dominated by Cladosporium and Alternaria in most climates and seasons. Indoor amplification typically shows up as elevation in genera that outdoor air normally doesn't have much of:
- Stachybotrys (the "black mold" you've heard of) — requires sustained wet cellulose; almost always indicates a moisture event left untreated.
- Chaetomium — same dampness signal; often grows on the back of drywall where you can't see it.
- Aspergillus / Penicillium — generic indoor amplification; mostly indicates excess humidity rather than acute water damage.
- Memnoniella, Trichoderma, Acremonium — same water-damage signal as Stachybotrys in most contexts.
A report that shows elevated Cladosporium indoors but no Stachybotrys or Chaetomium means something very different from one that shows the reverse. Our report explains which is which.
Fragments + mycotoxins, briefly
Spores aren't the only thing mold produces. Fungal fragments — broken-off pieces of hyphae and cell walls — are 10-100x more numerous than spores and small enough (often <1 µm) to bypass the upper airway defenses. The clinical implications are still being worked out, but the literature increasingly suggests fragments may explain why some patients respond to remediation even when their total spore counts looked unremarkable (Reponen 2007; Górny et al. 2002).
Mycotoxins (secondary metabolites produced by some mold genera) are a separate and more contested area. We do not currently offer mycotoxin testing in urine or building materials — the analytical methods are not standardized across labs and the clinical interpretation framework is unsettled. We may revisit as the literature matures.
Where mold actually grows in your home
The top six locations in our pilot data, by frequency of elevated indoor sample:
- Behind drywall in the room with a past leak (kitchen, bath, below-grade walls).
- HVAC return plenum and the first 3 feet of duct around it.
- Crawl space — especially with a dirt or partial dirt floor.
- Attic at the eaves, where insulation meets the soffit and condensation forms.
- Under wall-to-wall carpet in a basement or first floor built on slab without a vapor barrier.
- Inside the wall cavity adjacent to a window with failing flashing or sealant.
An IAQ sampling visit is informed by where the building is most likely to have a problem — not by where it looks worst.
How we sample
- Air samples via spore trap (Air-O-Cell or equivalent cassette) at 15 L/min for 5 minutes, in the affected room AND outdoors on the same day. The outdoor sample is the baseline.
- Surface samples via tape lift on any visible suspect growth.
- Bulk samples of removable material (insulation, carpet pad fragment) when warranted.
- Relative humidity readings at 3+ locations including the most-affected room and the room directly above any moisture source.
Lab analysis is by light microscopy + spore identification at an AIHA-LAP accredited environmental microbiology lab. Culture analysis is available where the report would benefit from differentiating viable vs nonviable spores.
What we report
- Total spore count (indoor + outdoor + ratio).
- Genus-by-genus breakdown of both samples.
- Whether Stachybotrys, Chaetomium, or other water-damage indicators were observed.
- Indoor relative humidity readings.
- Inspector's plain-English interpretation: is this indoor amplification? What's the most likely source? What action would close the loop?
What a positive result usually means
- Indoor:outdoor ratio > 1.5 on Stachybotrys or Chaetomium: active water-damage source somewhere. Find it (the inspector usually has an opinion) and address it before remediation — mold returns to a dried-up surface that's wet again.
- Elevated Aspergillus/Penicillium with normal water-damage indicators: typically a humidity-control story. Dehumidification + HVAC adjustment is usually enough.
- Indoor counts comparable to outdoor: no indoor amplification detected. The clinical workup focus may need to shift elsewhere (food, non-allergic mechanisms, structural breathing). A negative result is a real answer.
When to test
- Persistent indoor-only allergy or asthma symptoms.
- History of leak or flood within the last 5 years.
- Musty smell anywhere in the home.
- Visible suspect growth.
- Worsening symptoms in basement, bathroom, or specific room.
- Symptoms in a child that started after moving into a new building.
When NOT to test (yet)
- You already know where the leak is, you haven't fixed it, and the wall is still wet. Fix the source first. The lab result will not change the answer ("there's mold") and may delay remediation.
- The visible damage is large enough that the remediation scope is obvious. Skip diagnostic testing; engage a remediation contractor with a post-remediation verification (PRV) plan.
Selected references
- World Health Organization. WHO Guidelines for Indoor Air Quality: Dampness and Mould. 2009.
- Institute of Medicine. Damp Indoor Spaces and Health. National Academies Press, 2004.
- AIHA. Field Guide for the Determination of Biological Contaminants in Environmental Samples, 2nd ed., 2005.
- Reponen T, et al. Submicrometer particles. Indoor Air 2007.
- Górny RL, et al. Fungal fragments as indoor air biocontaminants. Appl Environ Microbiol 2002.
- Mendell MJ, et al. Respiratory and allergic health effects of dampness, mold, and mold-related agents. Environmental Health Perspectives 2011.