Most mold disputes are not about whether a standard was followed but about which standard was supposed to apply. The references overlap, and reaching for the wrong one — or naming all of them indiscriminately — is its own kind of error. This guide maps each standard to the task it actually governs.
Which mold standard applies to my inspection?
It depends on the question you are answering. Use IICRC S520 to classify mold contamination and scope remediation. Use IICRC S500 to classify the water event that caused it. Use the AIHA Green Book and ACGIH Bioaerosols to choose a sampling strategy and interpret the lab results. Use ASTM D7391 to describe the air-sample analytical method. Use ASHRAE 160 only when the dispute is about moisture-control design. A defensible report cites the two or three that are operative for that job, not the whole list.
The mistake at both extremes is real. Citing one standard for everything ignores that water classification and mold classification are different jobs. Citing all six on a routine inspection reads as padding and invites the question of which one you actually applied.
When do I use IICRC S520 versus IICRC S500?
S520 governs the mold; S500 governs the water that caused it. Reach for IICRC S520 when you are classifying mold contamination into a condition category and scoping remediation — that is the standard for the assessment-to-remediation chain. Reach for IICRC S500 when you are classifying the water event itself: how contaminated it was and how much material it soaked.
S500 splits into two axes that a report needs to keep separate. The S500 water categories (1 through 3) describe contamination — clean, gray, black. The S500 water classes (1 through 4) describe evaporation load — how much is wet and how hard it is to dry. A Category 3 sewage event and a Class 4 hardwood-saturation event are different problems even on the same job. On a water-caused mold inspection, you typically cite S500 for the cause and S520 for the mold consequence.
When do AIHA and ACGIH apply instead of IICRC?
AIHA and ACGIH govern sampling and interpretation, where IICRC governs classification and remediation. When the question shifts from "what condition is this" to "should I sample, and what does the result mean," you are in AIHA Green Book and ACGIH Bioaerosols territory. The Green Book drives sampling strategy and the indoor-versus-outdoor comparison; ACGIH anchors the relative, no-fixed-limit interpretation and the indicator-species commentary.
These two are what you cite when a result needs reading. Neither sets a pass/fail number, and both insist the analysis is comparative — which is exactly why your report frames an air result against a same-day outdoor control rather than against an invented threshold (AIHA, Recognition, Evaluation, and Control of Indoor Mold).
Where do ASTM D7391 and ASHRAE 160 fit?
These two are narrow and specific. Cite ASTM D7391 when you are describing how a spore-trap air sample was analyzed — direct microscopy — because it is the analytical method, attributed to the lab, not an interpretive standard. Cite ASHRAE 160 only on a moisture-design question: a wall assembly that condenses, a vapor-barrier dispute, a hygrothermal-failure opinion. On a routine "is there mold" inspection, neither is the operative interpretive standard.
There is one more reference that comes up by client request rather than by the standards consensus: the ERMI and HERTSMI-2 DNA dust indices. Those are not assessment standards at all, and the post-remediation clearance criteria describe what "done" looks like after remediation. Knowing which document answers which question is what keeps a report from naming standards it never actually used.
How does this map onto a single report?
A typical water-caused mold report touches several at once, each doing one job. The water event gets an S500 category and class. The mold gets an S520 condition category. The air samples get an AIHA/ACGIH comparative interpretation and an ASTM D7391 method note. ASHRAE 160 appears only if the root cause is a design failure. Listed that way, every citation has a reason to be there.
MoldMind assigns the S520 condition category per finding, records the S500 category and class on water intrusions, and structures the lab results for the indoor-versus-outdoor comparison — so each standard lands on the field it governs instead of being name-dropped in a narrative. See the sample report for how the standards-referenced section is built from the underlying data.
Sources
- IICRC, Standards overview — S520 (mold remediation) and S500 (water restoration) scope.
- EPA, Mold resources for professionals — assessment and remediation guidance.
- AIHA, Recognition, Evaluation, and Control of Indoor Mold — comparative sampling interpretation.