Why the Smell Comes Back (And How Odor Is Actually Removed)

Smell is molecules. Something in the building is releasing volatile compounds into the air, and it will keep doing so until that something is removed, neutralised or sealed.

Three common sources after a water loss. Microbial — mold and bacteria producing volatile organic compounds, the classic musty smell. Contamination — residue from sewage or grey water absorbed into porous materials. Combustion residue — soot and smoke particles, if fire was involved.

All three share a property: they are held in or on materials, often inside assemblies nobody can see. Wall cavities, subfloor, insulation, the underside of flooring, ductwork, the back of cabinetry.

Which is why the return-on-a-warm-day pattern is so diagnostic. Heat increases the rate at which those compounds off-gas. If your smell tracks temperature or humidity, the source is still in the building and it's responding to conditions.

Fragrance products — sprays, foggers, scented deodorisers — add a molecule rather than removing one. For a day or two the added scent dominates. Then it dissipates, at which point the original compound is still being produced at exactly the rate it was before.

Worse, some products deposit a residue that itself becomes a smell over time, so you end up with the original odor plus a stale perfume note.

There's a legitimate narrow use for masking — making a space tolerable to work in for a few hours. As a treatment it isn't one, and a company that finishes a sewage job by fogging a scent through the house has not completed the work.

The same applies to a sealer applied over an untreated surface. Sealing has a real role, covered below, but sealing over active contamination just delays it.

Find and remove the source. This is 80% of it. Porous materials holding contamination come out — that's why category 3 losses specify removal rather than treatment. Wet insulation comes out. Affected subfloor gets exposed and assessed. If a smell has no identified source, nothing that follows will hold.

Clean everything remaining. HEPA vacuuming to capture particulate, then damp wiping with an appropriate cleaning agent. Soot and residue in particular need the method matched to the residue type — using a wet method on dry soot drives it into the surface permanently.

Air filtration throughout. HEPA air scrubbers running during and after removal capture airborne particulate, which is a genuine part of the odor load rather than an afterthought.

Treat the remaining molecules. This is where hydroxyl or ozone comes in, and it works because the source is already gone — it's finishing, not fixing.

Seal where necessary. On framing or subfloor where residue has penetrated and cannot be fully removed, a shellac-based or specialist odor-blocking sealer locks the remainder in. This is legitimate at the end of the sequence and illegitimate as a substitute for it.

Both are oxidative treatments that break odor molecules apart rather than covering them, and the difference matters practically.

Ozone is a powerful oxidiser and very effective. It is also hazardous to breathe, so a space must be unoccupied during treatment and aired thoroughly afterward. It degrades rubber, some plastics and certain fabrics and finishes with repeated or prolonged exposure. It's a good tool for a vacated property or a contained area.

Hydroxyl generation produces hydroxyl radicals using UV light, mimicking a process that happens naturally in the atmosphere. It's slower and gentler than ozone, and critically it's safe to run in occupied spaces. That's why we default to it — most of our clients are still living in the house, and telling a family to vacate for three days to run ozone is a real cost.

Neither is a substitute for source removal. A hydroxyl generator running in a room where contaminated drywall is still in the wall will reduce the smell while it runs and achieve nothing permanent.

If the HVAC system ran at any point during or after the loss, it distributed whatever was in the air throughout the building and deposited some of it inside the ducts.

That makes the duct system a reservoir. Every time the system cycles afterward, it re-releases residue into every room it serves — which is precisely the pattern of "the smell comes back when the AC runs."

Surface cleaning the rooms while ignoring the ducts is one of the most common reasons an odor job fails. Duct cleaning should be part of the scope any time the system operated during a contaminated or smoke event, and it's essential after wildfire smoke intrusion.

Also worth checking: the attic insulation, if the air handler is up there. Insulation cannot be cleaned, and where residue has settled into it the answer is replacement.

Reasonable questions to ask, and reasonable things to expect: