Why "test before you clean" is a rule and not a preference
Every smoke test measures a surface as it is on the day of sampling. Clean the surface and the measurement describes the cleaning, not the loss. In a structure fire the point is obvious enough that most people respect it; after wildfire smoke it is routinely ignored, because the house looks fine, the neighbours are wiping, and the first crew through the door offers a same-day fog. The result is a home with no record of what came in, an insurer whose adjuster reasonably calls what is left "dust," and a homeowner arguing about a smell. United Policyholders, whose claim guidance is written for wildfire survivors, tells them that when the insurer will not commit to a proper inspection they should hire independent testing themselves, because Certified Industrial Hygienists test for evidence of smoke, soot, char and contaminants in the air and on surfaces — and that an adjuster with no training in air-quality or particulate assessment is offering an opinion, not a finding.
The regulators have started to say the same thing in their own voice. After the January 2025 Los Angeles fires, California's insurance commissioner created a Smoke Claims and Remediation Task Force to recommend uniform inspection, testing and remediation standards for smoke-damaged homes and health and safety thresholds for occupied structures. Its March 2026 report found that smoke from a wildland-urban interface fire — structures burning, not brush — carries greater potential for lead, asbestos and heavy metals than a typical wildland fire, and that the particulate-bound residue does not dissipate until it is physically removed. Neither finding can be acted on without a sample that was taken before the removal.
The sampling design: the part that decides the claim
A wildfire-intrusion test differs from a house-fire test in one structural way: there is no unaffected room to compare against, because the plume reached the whole house and the whole street. The design has to build its own comparison. A defensible plan has four classes of location, and a report without all four is where the dispute starts.
- Affected interior locations — horizontal surfaces in the rooms that face the fire, inside cabinets and closets (which show what got past a closed door), the top of the attic insulation, the underside of the roof deck.
- Interior reference locations — surfaces that were cleaned or replaced shortly before the event, or sealed spaces that the air did not reach; they show what the house looks like without the plume.
- Outdoor reference locations — protected exterior surfaces on the property and, ideally, a location upwind of the event; they establish the regional background every house now carries.
- The HVAC system — the filter that ran during the plume, the return plenum, a supply register; ANSI/IICRC S590 governs the inspection of an HVAC system after a fire and is the framework for saying whether the ducts are a reservoir or a pathway.
The laboratory then reads interior results against both references. A finding is specific to your property when the interior loading exceeds the outdoor background and the particle character — char and ash morphology under the microscope, the metals profile where structures burned — matches the event rather than the fireplace or the barbecue. A CIH-led assessment of the type described here lists exactly those elements: combustion by-product sampling for soot, char and ash, surface contamination testing, heavy-metals analysis, microscopic analysis of the particles, all by accredited laboratories and interpreted by a Certified Industrial Hygienist. Our particle testing and fire VOC testing pages cover the laboratory methods themselves.
What is measured, and what each measurement is for
Surface particulate is the loss. Tape lifts and wipe samples, read by light or electron microscopy, count and characterise the particles per unit area; this is the number that says how much came in and where it settled, and it is the number that goes in the remediation scope. Air sampling for volatile organic compounds — canister or sorbent tube, depending on the compounds of interest — is the odor question: it says whether the residue is still off-gassing at a level that matters for occupancy, and it changes with ventilation, which is why it is not the primary evidence of the loss. The furnace filter is a time-integrated record of what the house breathed during the event, and the single best object to bag and hand to the hygienist. Attic insulation is sampled because it is the reservoir with the largest surface area and no finish to clean; the building-science explainer on why attic residue drives the smell that returns on warm days is worth reading before the hygienist's visit, because it explains why the attic is on the sampling plan even when the living space looks clean.
The EPA's guidance on wildfires and indoor air quality is the occupancy layer over all of this: close up, filter, make a clean room, do not add particles indoors. It is written for the days of the plume. The testing described here is for the weeks after, when the outdoor air has cleared and the question is whether the inside of the house has.
From the report to the protocol to clearance
The report ends in a scope, and the scope is written to ANSI/IICRC S700, the standard for professional fire and smoke damage restoration: assessment, work plan, execution, verification. One honesty note belongs here. The IICRC's own summary says S700 addresses fires other than wildfires, and there is no published S760; a wildfire-intrusion protocol is therefore written by the hygienist from S700's principles rather than read out of a wildfire standard, and a report that cites a standard that does not exist is a report an insurer's reviewer will discount. The crew that executes the protocol should hold the IICRC Fire and Smoke Restoration Technician credential; the protocol tells them which surfaces are HEPA-vacuumed and wet-cleaned, which porous materials come out, whether the ducts are cleaned or replaced, and what is sealed.
Clearance is the same locations, the same methods, after the work — compared with the background samples taken on the first visit. Back to the local baseline is the standard; back to zero is not achievable after a regional event and a report that promises it is not credible. The clearance letter is the document that says the house is done, and it exists only because the first sample was taken before anyone cleaned. If embers or radiant heat reached the roof, that is a separate inspection by a roofer or an engineer, and the picker below opens your state's directory.