Fire Residue Testing NZ — Soot, Ash & Char Analysis
Laboratory evidence of whether smoke from a fire reached a room, a unit or a home, and whether the clean-up worked. Surfaces and air, from our Hamilton lab.
Smoke Smell or Black Dust After a Fire?
After a fire, the questions are rarely about the room that burned. They are about everything around it: the hallway, the bedrooms upstairs, the apartment next door, the house downwind of a vegetation fire. Is that grey film on the windowsill from the fire, or ordinary dust? Did smoke get into the ceiling space? Has the restoration company's clean actually removed the residue?
Fire residue testing answers those questions with a measurement. We examine samples under the microscope and count the three kinds of particle a fire leaves behind (soot, ash and char) separately from the everyday dust every building carries. The result tells you whether combustion residue in a space is within normal background or above it, sample by sample and room by room.
Ask about fire residue testingWhat the Test Answers
Fire and Emergency New Zealand attends around 5,000 structure fires a year, and fewer than two in three stay in the room where they started. The rest spread smoke and residue further than anyone can see.
Did it reach this room?
Scope for cleaning and insurance: which rooms, units or contents carry fire residue above normal background, and which don't.
Has the clean worked?
Samples taken after restoration show whether residue is back to normal background levels before a room is handed back.
Fire or ordinary dust?
Cooking, candles, fireplaces and traffic all leave small amounts of combustion particles. The test separates normal background from fire-related residue.
Surfaces and air
Tape lifts show what has settled on surfaces. Air cassettes show what is still airborne in a room.
What We Test: Surfaces and Air
Surface samples (tape lifts)
A clear tape lift pressed onto a surface collects the particles that have settled there: on window sills, skirting boards, the tops of door frames, light fittings and inside cupboards. In the laboratory, soot, ash and char are counted across a measured area of the tape under the microscope and reported as counts per square millimetre, alongside the other debris on the sample. The three are combined into a single Total Fire Residues figure, which is read against published ranges for normal background combustion residue in homes and commercial buildings.
Air samples (spore-trap and impaction cassettes)
Air is drawn through a spore-trap cassette such as the Air-O-Cell with a calibrated pump, and the particles captured are counted in the same way and reported per cubic metre of air. There is no published guideline for combustion particles in indoor air, so an air result is always compared with a reference: an outdoor sample, or a room the fire did not affect, taken at the same time. The report's summary sets out that comparison.
Surface and air samples are separate analyses with separate reports, and each is priced per sample. Contact us for pricing.
What Your Report Shows
Every sample gets its own result card. Ash, soot and char are reported individually, next to the non-combustion debris on the same sample, so the fire-related fraction is never hidden inside a single dust figure.
Alongside the cards, the report records how each sample was examined, the analyst's notes on what was seen, and a written summary of the job. Results are reported within 5 working days of the samples reaching our laboratory.
Dealing with smoke from a fire?
Tell us what happened and what you need the testing to show. We'll recommend where to sample and how many samples the question needs.
Who Uses Fire Residue Testing
- Homeowners and tenants after a fire in the home, or smoke from a fire next door, who want to know whether the house is clean or needs more work
- Apartment owners and body corporates where smoke has travelled through shared corridors, ceiling spaces or ventilation into neighbouring units
- Restoration and cleaning contractors who need independent before-and-after results to scope a job and show it was completed
- Loss adjusters and insurers deciding which rooms and contents a claim should cover
- Landlords and property managers who need independent evidence before re-letting a property after a fire
- Homes near vegetation and wildfires, where smoke and ash have come in through windows and ventilation
Taking Samples: Where and How Many
A single sample rarely answers the question. Plan the sampling around a comparison:
- The affected area. The room or unit closest to the fire, or where residue is suspected.
- The path between. Hallways, stairwells, ceiling spaces and shared ducting, where smoke would have travelled.
- A comparison. A room or building the fire did not reach. For air samples, add an outdoor sample taken at the same time.
Surface samples use the same clear tape lift slides as our mould testing. Follow the tape lift instructions (PDF), choose flat, undisturbed surfaces that have not been wiped since the fire, and send them with our ash, soot & char chain-of-custody form (PDF). Tick Reference against any outdoor or unaffected-room comparison sample. Slides and air cassettes are available from our sampling supplies.
Older buildings: fire damage in a building built before 2000 can disturb asbestos-containing materials. Before debris is moved or cleaned, follow WorkSafe's asbestos guidance for homeowners.
Method and Its Limits
Samples are analysed by optical microscopy to ASTM D6602-13 (modified), the standard practice for sampling and testing possible carbon black and other environmental particulate. Our interpretation draws on the AIHA Technical Guide for Wildfire Impact Assessments and ANSI/IICRC S700, the standard for professional fire and smoke damage restoration.
This is an indicative, semi-quantitative method. It is not covered by our laboratory's IANZ accreditation, and every report states that. It identifies and counts combustion particles by their appearance under the microscope. It does not measure gases, odour, VOCs or PAHs, and it does not identify the fuel that burned. Where a result needs definitive confirmation, for example in a dispute, electron microscopy (SEM or TEM with elemental analysis) is the recognised next step, and we can advise on it.
Fire Residue Testing FAQ
What is fire residue testing?
Fire residue testing is laboratory analysis of the particles a fire leaves behind: soot, ash and char. A surface tape lift or an air cassette is examined under the microscope, each type of combustion particle is counted, and the result is reported per square millimetre of surface or per cubic metre of air, separately from ordinary household dust.
Can you test the air as well as surfaces?
Yes. We analyse surface tape lifts and air samples collected on spore-trap or impaction cassettes such as the Air-O-Cell. Surface results are reported in counts per square millimetre and read against published normal-background ranges. There is no published guideline for combustion particles in indoor air, so air results are compared against an outdoor or unaffected-room sample taken at the same time.
How long do results take?
Results are reported within 5 working days of the samples arriving at our Hamilton laboratory.
Does the test measure the smoke smell?
No. The smell after a fire comes mostly from gases and volatile compounds, which microscopy does not measure. Fire residue testing counts the particles: soot, ash and char. The two often go together, because residue on surfaces keeps releasing odour, but a smell can linger after the particles are gone, and residue can be present with little smell. Our guide to smoke smell after a fire explains the difference.
Is the method accredited?
No. Ash, soot and char analysis by optical microscopy is an indicative, semi-quantitative method performed to ASTM D6602-13 (modified). It is not covered by IANZ accreditation, and each report says so. Where a result needs definitive confirmation, electron microscopy (SEM or TEM) is the recognised next step.
My neighbour had a fire. Can you tell whether smoke residue got into my home?
Yes, that is one of the main uses of the test. Samples are taken from the rooms closest to the fire, from a room further away, and where possible from an unaffected space for comparison. The report shows whether soot, ash and char in your home sit within normal background or above it. Read more in Your neighbour had a fire.
How much does it cost?
Fire residue analysis is priced per sample, and the price is the same for surface and air samples. How many samples a job needs depends on the question, so tell us about the situation and we'll send a quote.
Ask About Fire Residue Testing
Tell us what happened, where the property is, and what you need the testing to show.