The fire is out, the damaged room has been stripped, and the house still smells of smoke. It is the most common complaint after a house fire in New Zealand, and it leads to a reasonable question: can someone test for it? It depends, because the smell and the residue are two different things. A laboratory can measure one of them very well and the other not at all. This article explains which is which, what a fire residue test actually tells you, and when it is worth doing.
What the smell actually is
The smell of smoke is mostly gases and volatile compounds released when materials burn: wood, plastics, foams, fabrics and whatever else was in the room. Those compounds soak into porous materials such as carpet, curtains, plasterboard, insulation and timber, and then leak back out for weeks or months, especially on warm days or when the heating comes on. That is why a house can smell worse a week after the fire than on the day, and why a room that looks clean can still smell.
Alongside the gases, a fire produces particles. Those particles settle on every surface the smoke reaches, and they carry odour compounds with them. Residue on surfaces keeps the smell going, which is why cleaning the residue is usually the first step in getting rid of the smell.
What a fire leaves behind: soot, ash and char
Under the microscope, fire residue falls into three types, and each tells a slightly different story:
- Soot is very fine black carbon from incomplete combustion. The particles cluster into chains and grape-like groups, and they travel the furthest: through doorways, up stairwells, into ceiling spaces and into the house next door.
- Ash is the mineral residue left after material has burned. It is grey or white, light or clumpy depending on what burned, and is more common closer to the fire.
- Char is partly burned material, dark brown to black fragments that often still show the structure of the wood, paper or plastic they came from.
A fire residue test counts each of the three separately, next to the ordinary dust on the same sample. The result shows how much of what has settled on a surface, or is floating in the air, came from combustion.
What testing can tell you
- Whether fire residue reached a room. The core question for cleaning and insurance scope: which rooms need cleaning, and which don't.
- How far it travelled. Samples taken from the fire room, the rooms beyond it and a room the fire didn't reach show where the residue stops.
- Whether cleaning worked. A sample taken after restoration shows whether residue is back to normal background before a room is repainted or handed back.
- Whether it's in the air. Air samples on spore-trap cassettes show whether combustion particles are still airborne in a room, compared against an outdoor or unaffected-room sample taken at the same time.
What testing can't tell you
Microscopy counts particles. It does not measure the smell, the gases, volatile organic compounds (VOCs) or polycyclic aromatic hydrocarbons (PAHs). Those need chemical testing, which is a different service. A room can return a normal residue result and still smell, because odour compounds have soaked into materials that look clean. The reverse also happens: residue can sit on surfaces with little smell.
A fire residue test also doesn't identify the fuel that burned, and on its own it isn't a health assessment. What it gives you is an independent measurement of whether combustion residue is present above normal background, room by room, which is usually the fact that the cleaning and insurance decisions turn on.
Every house has some combustion particles
A clean result is not zero. Cooking, candles, toast, wood burners, barbecues and traffic all leave small amounts of soot and char in every home, and a New Zealand house with a log burner will carry more than one without. That is why a single positive count proves little on its own. Surface results are read against published ranges for normal background residue in homes and buildings, and the most useful sampling plans include a comparison room the fire didn't reach.
When testing is worth doing
- The smell hasn't gone after cleaning, and you need to know whether residue is still on surfaces or whether the problem is odour held in materials.
- Before repainting or sealing. Painting over residue traps it and the odour with it. Test first, then seal.
- The insurer or restorer and you disagree about which rooms or contents were affected.
- A rental property is being handed back after a fire and the landlord or tenant needs independent evidence.
- The fire wasn't yours. Smoke from a neighbouring house or unit is covered in Your Neighbour Had a Fire.
Taking samples
Surface samples are clear tape lifts taken from flat surfaces that have not been wiped since the fire: window sills, the tops of door frames and wardrobes, light fittings, skirting boards and the insides of cupboards. Take samples from the affected rooms, the rooms between, and a comparison room. Our tape lift instructions (PDF) show the technique, and samples go in with our ash, soot & char chain-of-custody form (PDF). The fire residue testing page sets out what the report shows and how air sampling fits in.
If the house was built before 2000, fire damage can disturb asbestos-containing materials. Don't move or clean debris before checking WorkSafe's asbestos guidance.