If your mould air counts are only slightly higher than outdoors, fix the moisture source first. If radon sits above the 200 Bq/m³ guideline, plan mitigation on the recommended timeline. For CO₂, VOCs or particulates, improve ventilation and retest before assuming a bigger problem exists.
TL;DR:
- Indoor mold counts should be compared to outdoor samples, as only significantly higher indoor levels with water-damage species indicate a problem.
- Radon levels above 200 Bq/m³ require mitigation within two years, with long-term testing during heating season providing the most reliable result.
- VOC and particulate levels fluctuate with activity; sustained elevations suggest a persistent source that needs addressing, such as poor ventilation or outdoor pollution.
- Visual inspection and moisture source detection remain the primary steps, with air sampling used mainly for specific questions or documentation.
- Relying on a single air test for definitive health risk assessment is unreliable due to the high variability in indoor air quality measurements.
Table of Contents
- What indoor air tests measure and the common units you’ll see on reports
- When to use mould air sampling versus a visual or moisture inspection
- Interpreting mould air sampling results: counts, species, and comparisons
- Interpreting VOC, particulate, CO₂ and radon results
- Testing limitations and how to avoid misreading a single result
- What to do after you get your results, step by step
- What a professional report adds that a DIY kit can’t
- Testing is an investigation, not a verdict
- Book a professional assessment or get a long-term monitor
- FAQ
- Sources
What indoor air tests measure and the common units you’ll see on reports
A single report can cover several different pollutants, each measured a different way. Spore trap air sampling pulls a volume of air through a sticky slide or cassette and counts fungal spores under a microscope. Viable culture sampling grows mould on a nutrient plate and reports colonies that actually grew, which is why those results come back in colony-forming units. Surface swabs and tape lifts check a specific stain or surface rather than the air, and PCR or DNA-based identification narrows down species when that detail matters.
Gas and particle readings work differently. VOC panels and particulate counters give direct electronic readings, often in real time, while radon detectors range from cheap short-term kits to lab-grade long-term monitors.
The units tell you what kind of measurement you’re looking at:
- CFU/m³ or spores/m³: mould concentration in air, from culture or spore-trap methods.
- Bq/m³: radon activity, the unit behind Health Canada’s residential guideline.
- ppm: carbon dioxide and carbon monoxide concentration.
- µg/m³: fine and coarse particulate matter (PM2.5 and PM10).
- ppb or µg/m³: volatile organic compounds, reported individually or as a total.
The lab method shapes how you should read the number. A culture result only counts spores that were alive and able to grow under lab conditions, so it tends to undercount compared to a spore trap, which counts everything present, dead or alive, but can’t always identify species as precisely. Neither method is wrong, they just answer slightly different questions, and a report that mixes both without saying so is harder to interpret than one that states its method plainly.
When to use mould air sampling versus a visual or moisture inspection
Health Canada’s guidance on moisture and mould does not recommend routine air testing in homes. Visual inspection and identifying the moisture source are the primary steps, and air sampling is secondary, useful mainly when it answers a specific question rather than as a general health check.
- Start with a visual and moisture inspection whenever you can see or smell a problem, since this identifies the source faster than air sampling does.
- Order air sampling when there’s an investigative question an inspection alone can’t answer, such as whether a specific room is driving symptoms or whether an HVAC system is recirculating contamination.
- Use air sampling for documentation purposes in real estate transactions or legal disputes, where a dated, lab-backed result carries weight that a visual assessment doesn’t.
- Reserve surface tests, tape lifts or more invasive inspection (opening a wall cavity, for example) for cases where visible growth needs species confirmation or where hidden moisture is suspected but not yet located.
- Skip air sampling altogether when the moisture source is obvious and the fix is straightforward, since the result rarely changes what you need to do.
This sequencing matters because guidance for indoor air quality professionals notes that mould is always present in outdoor and indoor air at some level, and that air sampling results are highly variable from hour to hour. Treating a single air sample as a definitive verdict, rather than one data point in an investigation, is where most homeowners go wrong.
Interpreting mould air sampling results: counts, species, and comparisons
A typical mould lab report lists total spore or colony counts, breaks them down by genus or species, and sometimes shows percent composition for each. The number that matters most, in most cases, is not the total count but how it compares to a paired outdoor sample.
- Indoor roughly equal to or below outdoor: a normal background pattern, not a red flag on its own.
- Indoor moderately higher than outdoor: worth a closer look at ventilation and any damp areas, but not automatically a remediation trigger.
- Indoor dominated by a single genus, especially one associated with water damage, at levels well above outdoor: a strong signal of an indoor moisture source.
- Species identification: useful for understanding exposure in specific cases but, per Health Canada’s professional guidance, it rarely changes the remediation approach, which focuses on removing the moisture source and the affected material regardless of which mould is growing.
The red flags that warrant a professional follow-up are consistent: a dominant indoor genus tied to water intrusion, visible growth paired with elevated air counts, or occupant symptoms that track with time spent in a specific room. Our guide to reading mold analysis results walks through a sample lab table line by line if you want to check your own report against a worked example.
Pro Tip: Always ask for a paired outdoor sample with your indoor mould results. A number without that comparison tells you almost nothing.
Interpreting VOC, particulate, CO₂ and radon results
Each of these pollutants has its own logic for deciding when a number means “act now” versus “keep an eye on it.”
- VOCs: a total VOC (TVOC) reading tells you something is off-gassing, but it can’t tell you what. Speciated results that flag a specific compound, especially one tied to a known source like new flooring or recent painting, are the ones that justify source removal or a ventilation fix. Our practical guide to VOCs breaks down which compounds tend to show up and why.
- PM2.5 and PM10: short-term spikes usually trace to cooking, candles, or outdoor smoke infiltrating through open windows, and resolve once the activity stops. Sustained elevated readings point to a persistent source, like a leaky wood stove or heavy outdoor pollution, and call for filtration or a sealed combustion appliance.
- CO₂: this is a stand-in for how well a space is ventilated, not a pollutant in itself at indoor levels. Readings climbing well above outdoor background during occupied hours suggest stale air building up faster than fresh air is replacing it.
Health Canada ties CO₂ above roughly 1,000 ppm to more complaints of headaches, fatigue and reduced performance among occupants, making it a practical threshold for checking whether a room needs more fresh air. Opening a window, running an exhaust fan, or adjusting an HVAC system’s fresh-air intake usually brings the number back down within a day.
Radon works on a longer timescale. The Health Canada guideline sets 200 Bq/m³ as the level above which remedial action is recommended, with action suggested within two years for results between 200 and 600 Bq/m³, and within one year above that. Because radon fluctuates daily and seasonally, Health Canada recommends a long-term test of at least three months, ideally during the heating season, before making a mitigation decision based on a single number.
Testing limitations and how to avoid misreading a single result
Indoor air is not stable. Occupancy, cooking, cleaning products, HVAC cycling and even outdoor wildfire smoke can shift a reading within hours, which is exactly why guidance for indoor air quality professionals describes air sampling results as highly variable and of limited use for health risk assessment on their own.
- Watch for sample placement errors: a monitor near an open window, a kitchen, or a bathroom fan will skew a result in ways that have nothing to do with the rest of the home.
- Confirm the lab is accredited and uses recognized methods, since results from an uncertified lab or an inconsistent protocol aren’t comparable to guideline values.
- Check sample timing: a mould sample taken right after cleaning, or a radon test run for two days instead of three months, measures a moment, not a pattern.
- Don’t rely on TVOC alone to decide on action, since it hides which specific compound, if any, is worth addressing.
- Prefer paired indoor and outdoor sampling and, for radon, a long-term or continuous monitor over a short-term snapshot whenever the decision has real cost attached.
Pro Tip: Write down what was happening in the home during testing, cooking, renovation, new furniture, windows open or closed. That context often explains an odd number faster than a retest does.
What to do after you get your results, step by step
Once you have a result in hand, the order of operations matters more than the exact number.
- Mould: find and fix the moisture source first, whether that’s a leak, poor grading, or a bathroom fan that’s not venting outside; remediation of the affected material should happen alongside or immediately after.
- Radon: if your long-term average is above 200 Bq/m³, schedule mitigation within the timeframe tied to your level, then confirm the fix with a follow-up long-term radon test.
- VOCs, particulates and CO₂: increase ventilation, address the specific source if one is identified, and verify with a short follow-up monitoring period rather than assuming the fix worked.
- Any elevated result that doesn’t resolve with source control: bring in a professional for a site inspection and a documented remediation plan.
| Pollutant | Trigger for action | Verification step |
|---|---|---|
| Radon | Long-term average above 200 Bq/m³ | Long-term retest after mitigation |
| Mould | Indoor counts well above outdoor, dominant water-damage genus | Visual clearance plus moisture check |
| CO₂ | Sustained levels well above 1,000 ppm during occupancy | Recheck after improving fresh-air exchange |
| VOCs | A specific speciated compound tied to a known source | Retest after source removal or added ventilation |
Professional testing timelines vary by pollutant: a mould inspection and sampling visit typically wraps in a single appointment, while a proper radon measurement takes a minimum of three months to produce a reliable annual average. Improving ventilation design is often the lowest-cost fix for CO₂ and VOC issues, since it addresses the root cause rather than just filtering the symptom.
What a professional report adds that a DIY kit can’t
A professional assessment starts with a site inspection and moisture diagnostics before any sampling happens, which is the step most DIY kits skip entirely. That sequence, paired indoor and outdoor sampling, accredited lab analysis, and a prioritized remediation plan, turns a raw number into a decision you can actually act on.
- We specialize in testing and mitigation for radon, mold, and asbestos to protect family health.
- We emphasize certified expertise and responsive, personal service so clients have confidence in their results.
- Resources like the air quality home inspection guide and the mold analysis results guide walk through interpretation in more detail.
- The Indoor Air Quality Testing Kit gives homeowners a DIY option for monitoring between professional visits.
Testing is an investigation, not a verdict
The mistake I see most often is treating a single air sample as a final answer. A good test starts with a question: is this room making someone sick, do I need documentation for a sale or a dispute, or did my mitigation actually work? Each of those questions points to a different sampling strategy, and none of them is answered well by a reflexive, test-everything approach.
Pattern matters more than any one number. A radon reading taken over three months during heating season tells you something a two-day kit can’t. A mould sample paired with an outdoor control tells you something a single indoor number never will. And the only way to know a fix worked is to retest after the work is done, not to assume it from the invoice.
— Mark
Book a professional assessment or get a long-term monitor
A short-term kit tells you what the air looked like for a few days. Simon Air Quality’s approach starts with a site visit and moisture diagnostics, then builds a sampling plan around the specific question you’re trying to answer, backed by accredited lab analysis and a remediation plan ranked by priority, not a generic checklist.
- Radon measurement and mitigation services, including long-term testing and system installation.
- Mold testing and inspection with lab-backed sampling and a clear remediation path.
- Long-term radon test kits and indoor air quality monitors for ongoing verification.
When you book, expect a site visit, a sampling plan tailored to your situation, accredited lab turnaround, and a written set of next steps ranked by urgency. If you’d rather start with a monitor before committing to a full assessment, the long-term radon test kit is a practical first step. For everything else, see our full range of services and book an assessment that fits your situation.
This article is general information, not a substitute for advice from a qualified doctor. Consult a qualified healthcare professional about your own circumstances before acting on anything here.
FAQ
What is a bad mold score?
There’s no single universal cutoff, since mold spore counts are judged against a paired outdoor sample rather than a fixed number. A result where indoor counts are far above outdoor, especially dominated by one genus tied to water damage, is the pattern that signals a problem worth remediating.
How accurate is air testing for mold?
Mold air sampling results are highly variable hour to hour, which limits their accuracy as a stand-alone measure, according to Health Canada’s guidance for indoor air quality professionals. That’s why visual inspection and moisture investigation are treated as the primary diagnostic tools, with air sampling reserved for specific investigative questions.
How to tell if the air in your house is making you sick?
Look for symptoms that track with specific rooms or times, such as headaches that ease after you leave the house, paired with visible signs like musty odors, condensation, or water stains. Testing for carbon monoxide and radon is worth doing regardless of symptoms, since neither can be detected by smell or sight.
How to interpret your mold air sampling results?
Compare your indoor spore or colony counts to a paired outdoor sample taken at the same time, since a result close to outdoor levels is normal background. A result where indoor counts are substantially higher, especially with one genus dominating, points to an indoor moisture source that needs to be found and fixed.
