You’ve had professional air testing done and received a report with numbers, spores per cubic metre of air, percentages, species names you don’t recognise, and a conclusion you’re not sure how to interpret. This guide explains what those numbers actually mean, what the relevant benchmarks are, and what action they indicate.
Understanding air sampling results is genuinely useful, it tells you whether there’s a mould problem, how serious it is, where it’s coming from, and whether remediation has worked.
How Air Sampling Works
During professional air sampling, a calibrated pump draws a known volume of air through a collection cassette (typically an Allergenco-D or RCS cassette, or a spore trap). The cassette is then analysed by a NATA-accredited laboratory, where a trained analyst counts and identifies the mould spores captured.
Results are expressed as spores per cubic metre of air (spores/m³), sometimes also expressed as counts per sample volume.
A standard sampling protocol includes:
- At least one indoor sample from the area of concern
- One outdoor control sample (taken at the same time in the same conditions)
- Sometimes, an additional indoor sample from an unaffected reference room
The outdoor control sample is critical, it establishes what mould spore counts look like in the ambient environment at the time of testing. In the Northern Rivers, outdoor spore counts are high due to the subtropical rainforest environment. What matters is the relationship between indoor and outdoor counts, not the absolute indoor number.
The Core Interpretation Principle
Indoor mould spore counts should be equal to or lower than outdoor counts.
An indoor count that is significantly higher than the outdoor control indicates that there is an additional mould source inside the building, an active colony somewhere in the structure that is contributing spores to the indoor air above what would naturally enter from outside.
This is the foundation of air sampling interpretation. It’s why a result of “5,000 spores/m³ indoors” cannot be evaluated without knowing what the outdoor count was on the same day. If outdoor was 8,000, 5,000 indoors is excellent. If outdoor was 1,500, 5,000 indoors is a significant finding.
Northern Rivers Outdoor Baseline Counts
In the Northern Rivers subtropical environment, outdoor mould spore counts are naturally higher than in temperate Australian cities. Typical ranges:
| Season | Typical Outdoor Spore Count | Notes |
|---|---|---|
| Wet season (Nov, Mar) | 5,000-25,000 spores/m³ | Very high, warm, moist conditions favour outdoor mould |
| Transition (Apr, May, Sep, Oct) | 3,000-10,000 spores/m³ | Moderate baseline |
| Dry season (Jun, Aug) | 1,500-5,000 spores/m³ | Lower, but still elevated vs temperate regions |
These figures vary considerably by microclimate, hinterland rainforest locations (near Mullumbimby, Federal, Byron Bay hinterland) will run higher; more open inland areas (Casino, Coraki) will run lower.
What Do the Numbers Mean? Interpretation Framework
| Indoor:Outdoor Ratio | Interpretation | Recommended Action |
|---|---|---|
| < 1.0 (indoor lower than outdoor) | Indoor air is better than outdoor, excellent | No action required |
| 1.0-1.3 | Indoor approximately equal to outdoor, acceptable | Monitor; no remediation needed |
| 1.3-2.0 | Mildly elevated indoors, low-level concern | Improve ventilation; identify potential sources; retest |
| 2.0-3.0 | Moderately elevated indoors | Professional inspection to identify source; targeted treatment likely warranted |
| > 3.0 | Significantly elevated indoors | Active indoor mould source identified, professional remediation indicated |
| > 5.0 | Severely elevated indoors | Significant active mould problem requiring urgent professional remediation |
Species Matter: Not All Moulds Are Equal
The spore count number tells you how much mould is in the air. The species identification tells you what type of mould and provides information about where it’s coming from and what health risk it may pose.
| Species Group | What It Indicates | Health Concern |
|---|---|---|
| Cladosporium | Common environmental mould; found outdoors and on many building surfaces | Low, Moderate at high concentrations |
| Aspergillus/Penicillium | Ubiquitous; elevated counts suggest indoor source | Moderate, some species produce mycotoxins |
| Stachybotrys chartarum | Black mould; associated with wet cellulose materials | High, produces trichothecene mycotoxins |
| Chaetomium | Associated with water-damaged materials | Moderate, High |
| Fusarium | Associated with wet building materials, soil | Moderate |
| Alternaria | Common outdoor species; elevated indoors suggests moisture issue | Moderate |
Key point: Finding Stachybotrys (black mould) indoors is a significant finding even at low counts, because it is rare in ambient outdoor air and almost always indicates an active colony on wet cellulosic material (plasterboard, paper, wood) inside the building. Even a count of 50-100 Stachybotrys spores/m³ indoors should be taken seriously.
The Aspergillus/Penicillium group is reported together in most analyses because the spores are morphologically similar. Counts that are disproportionately high relative to outdoor baseline are a common finding in Northern Rivers homes with ongoing moisture issues.
Interpreting Your Specific Report
What to Look For in a Professional Report
A well-presented air sampling report should include:
- Sample locations and collection times
- Volume of air sampled per cassette
- Count per cubic metre for each sample
- Species breakdown for the dominant organisms
- Indoor:outdoor ratio calculation
- Professional interpretation of findings
- Recommendations for action
If your report only provides raw counts without an outdoor reference and interpretation, it is of limited practical value.
Elevated Counts in Specific Rooms
If your report shows elevated counts in one room but not another, this narrows the source location significantly. The room with elevated counts most likely has a local mould source, inside the wall cavity, above the ceiling, or under the floor. This targeting makes professional inspection and remediation more efficient and less expensive.
Pre- and Post-Remediation Comparison
Air testing before and after remediation is the industry standard for confirming that remediation has been effective. Post-remediation counts should return to indoor:outdoor ratios below 1.3. This provides documented assurance and is increasingly required by insurance companies and property managers.
When Air Sampling Is and Isn’t the Right Tool
| Situation | Air Sampling Appropriate? | Better Alternative |
|---|---|---|
| Visible mould, small area | Not needed | Visual assessment + surface treatment |
| Suspected hidden mould (smell without visible sign) | Yes, confirms indoor source | Thermal imaging can also help |
| Dispute between landlord and tenant | Yes, provides objective evidence | |
| Post-remediation clearance | Yes, industry standard for clearance | |
| Property purchase due diligence | Yes, if suspected mould | Also moisture metering and visual |
| Asthma/allergy worsening indoors | Yes, quantifies exposure | |
| Routine monitoring (no symptoms, no visible mould) | Probably not needed | Annual visual inspection sufficient |
Northern Rivers Considerations for Air Testing
Test in the wet season if possible. An air test taken during the dry season (June, August) will record lower ambient counts and may understate a problem that becomes significant in the wet season. If you can only test once, the wet season result is more informative.
Outdoor counts vary hour to hour. Air testing should ideally be done at a consistent time of day (typically mid-morning) with both indoor and outdoor samples taken at the same time. Results taken at different times on different days are harder to interpret.
NATA accreditation matters. Ensure the laboratory analysing your samples holds NATA accreditation for biological air sampling. NATA-accredited labs use standardised methods and quality controls that ensure reproducible, defensible results.
Frequently Asked Questions
My report says 3,000 spores/m³ indoors. Is that bad? It depends entirely on what the outdoor count was at the same time. If outdoor was 8,000, a result of 3,000 indoors is excellent. If outdoor was 800, it’s a significant finding. Always ask for the indoor:outdoor ratio in your report.
What is an “acceptable” indoor mould count in a Northern Rivers home? Given the high natural outdoor counts in the Northern Rivers, what constitutes “acceptable” indoors is higher in absolute terms than it would be in a drier city. The ratio (indoor vs outdoor) is the key metric. An indoor:outdoor ratio below 1.3 is generally considered acceptable; above 2.0 indicates an indoor source requiring attention.
My test showed Stachybotrys but only a few spores. Should I be worried? Yes, Stachybotrys is a significant finding at any concentration indoors because it indicates the presence of a colony on wet cellulosic material somewhere in the building. It doesn’t mean emergency action is needed immediately, but it does mean a professional inspection to find and treat the source is warranted.
Is one air test enough to assess my whole house? A single sample from the living area gives useful general information, but it won’t pinpoint problem areas within the building. Multiple samples, one per room, one outdoor control, provide a much more detailed picture that can direct targeted remediation.
How much does air sampling cost in the Northern Rivers? A standard two-sample set (one indoor, one outdoor) with NATA lab analysis and a professional interpretation report costs $300 to $600. Additional samples are $150 to $250 each. Post-remediation clearance testing is typically the same cost as initial testing.
Want professional air testing for your Northern Rivers property? Northern Rivers Mould Removal can connect you with an independent local contractor who uses NATA-accredited air sampling and provides full interpretation reports. Request air quality testing.