Why Surface Cleaning Never Removes Vomit Odour
A flat off Kilburn High Road, three cleans in eighteen months, two of them mine. The first was a proper job on a bedroom carpet after a stomach bug, and the carpet came up clean – no mark, no smell, nothing at the handover. The tenant rang in July. Warm week, windows shut all day, and the room smelled the way the room had smelled originally. I went back, cleaned it again, same result, same phone call the following August.
The third visit I lifted the carpet at the skirting and put my hand on the underlay. It was still down there. It had been down there the whole time, and everything I had done in that room had been performed a centimetre and a half above the actual problem.
What is in vomit, and which part of it smells?
Very little of the smell comes from the food.
Stomach contents arrive at a pH of somewhere between 1.5 and 3.5 – hydrochloric acid, pepsin, partly broken-down protein, fats and starches, sometimes bile if the episode went on. The acid does its own damage, and on a nylon carpet it will shift dye and leave a colour change that survives every odour treatment you can name. That is a separate fault and worth mentioning to whoever cleans it.
The odour itself is mostly short-chain fatty acids. Butyric acid is the dominant one, which is the same compound that gives Parmesan and old sick their shared character, and the human nose detects it at concentrations measured in parts per billion. Isovaleric and propionic acid sit alongside it. Then there are amines from protein breakdown, and sulphur compounds – hydrogen sulphide, methanethiol – which are detectable at even lower thresholds than the acids.
The practical consequence is that you can remove 99 per cent of the material and still have a room that smells. The nose is calibrated to the last one per cent, and that one per cent is still down there.
Why it gets worse over three weeks
Vomit is a bacterial culture medium with a bacterial population already in it. Once the acid is diluted by whatever you flushed it with, the pH climbs into a range those organisms like, and they carry on working on the protein and fat left in the fibre and the backing. Fermentation produces more butyric acid, not less.
So a spot that smelled unpleasant on the night can smell worse a fortnight later, which people find hard to believe and read as their own imagination. It is not imagination. The odour source is being manufactured on site.
Why doesn’t surface cleaning reach it?
Because of what happened in the first ninety seconds, before anyone got there with a cloth.
Vomit is largely liquid, and liquid in a carpet travels down. It passes the face fibre in a couple of seconds, wets the primary backing, soaks the latex layer, and goes into the underlay – and if the volume was significant, it goes through the underlay and onto the floor beneath. On a Woolwich job last year I lifted a fitted carpet and found the outline of the spill printed on the floorboards, perfectly legible, a full year after the event and two cleanings later.
Every cleaning method that works from the top faces the same physical limit. A wand at the surface pulls with real force, but that force falls away very quickly with depth, and it is pulling through a latex barrier layer specifically designed to resist the passage of liquid. You get the pile clean. You get some of the primary backing clean. Below the latex you are achieving close to nothing, and the fatty acids and the bacteria producing them are principally below the latex.
Then there is the geometry nobody thinks about. Liquid spreads sideways as it descends, so the contaminated area underneath is always larger than the mark on top – reckon on half as wide again, sometimes double. Cleaning the visible spot, even brilliantly, leaves a ring of untouched contamination around it that is doing all the smelling.
The twenty per cent you can see
What sits on the surface is the solids, and the solids are the least of it by odour weight. People spend an hour on the solids because that is the horrible part, and the horrible part is the easy part.
Get the bulk off with a spatula or a plastic scraper, working from the outside inwards, without pressure – pressing solids into the pile grinds them into the yarn twist and gives you a second problem. A metal dough scraper from a kitchen shop is the best tool for it. Then get white towel down and stand on it, because bodyweight through a towel pulls more liquid out of a carpet in ten seconds than a domestic wet vac will manage in five minutes. That is the highest-value thing an ordinary person can do, and almost nobody does it.
After that, whatever you spray on the top is competing with a problem it cannot reach. Deodorising sprays and fragranced foams do worse than nothing – fragrance sits alongside odour rather than cancelling it, and what you end up with is butyric acid and mandarin, which is a smell that belongs in no house in Greater London.
Why does it always come back in warm weather?
Two mechanisms, both moisture-driven.
Odour compounds have to be airborne for you to smell them, and volatility rises sharply with temperature. A residue that is sitting quietly in a backing at 14 degrees in February becomes actively volatile at 24 degrees in a closed south-facing bedroom in Streatham. Same quantity of residue, several times the release rate.
The second is bacterial. Dormant organisms in a dried residue rehydrate when humidity climbs and resume producing exactly what they were producing before. This is why the smell comes back after a rainy spell as reliably as after a hot one, and why a room shut up for a fortnight smells worse on the owners’ return than it did when they left.
The summer phone call
Every contractor I know gets a run of these in the first properly warm week of the year, and a fair share are jobs somebody else cleaned in November and signed off as finished. The clean was fine. The diagnosis was wrong. It is still down there, and the weather has done nothing more than turn the volume up on something that was never removed.
What does removing it properly involve?
You have to get the treatment to where the contamination is, and there are two ways: inject through the carpet, or lift it.
Sub-surface injection uses a needle or a flood tool to put treatment through the face into the backing and underlay, then a heavy extraction from the top, repeated. It works on smaller contaminations and it uses a lot of solution – three or four litres for something the size of a dinner plate is normal. Anyone treating a vomit odour with half a litre of anything is treating the appearance of it.
Lifting is better and I do it wherever the fitting allows. Fold the carpet back off the gripper, cut out and bin the contaminated underlay – underlay is £6 to £9 a square metre and cleaning it makes no sense at any price – treat the carpet backing directly from underneath, treat the floor, then reline and refit. A competent fitter will do the refit in twenty minutes.
Enzymes, and why they need time
Enzyme treatments are the right chemistry here: proteases for the protein, lipases for the fats, amylases for the starch. They are catalysts, and they work at a rate rather than instantly, so the thing that kills most enzyme treatments is impatience. They need moisture throughout the dwell – an enzyme in a dried carpet has stopped – warmth, and hours rather than minutes. Four to eight is realistic. Cover the treated area with plastic sheet to hold humidity, weight the edges, come back the next morning.
They also need the right pH range and get destroyed by high alkalinity, so an enzyme applied straight after a strong alkaline prespray is dead on arrival. And on wool, keep everything mild – wool tolerates a lot of acid and very little alkali, and a wool carpet that survives the vomit can still be ruined by the cure.
When is the carpet not worth saving?
When the contamination reached the floor and the floor is timber.
Floorboards absorb, and a Victorian pine board in an Enfield Town bedroom will hold odour indefinitely because you cannot extract from wood. The treatment there is to clean, dry properly, then seal the affected boards with a shellac-based primer, which locks the residue in. That is a decorating job and it is unavoidable if the smell is going to end.
Concrete is easier in one respect and worse in another – it does not soak in as deeply, but a screed with a crack takes the liquid down the crack and out of reach.
The honest arithmetic
A large episode on a mid-range carpet in an Ilford flat, contaminating two square metres with underlay and boards involved, will cost more in labour, materials and repeat visits than the carpet is worth. Say so at the quote stage. I would rather lose the job than take three payments to fail three times, which is what the Kilburn flat became before I lifted the thing.
Small and fresh is the opposite case. If a child was sick on a synthetic carpet over a foam underlay last night and you dealt with it in the first minute, cut a square out of the underlay, put an offcut back, treat the backing with an enzyme and give it a day. Total cost about eight pounds.
Which leads me to the practice I would happily see the back of: the ozone generator, left running in a sealed room for four hours to “kill” an odour. Ozone is a lung irritant, it degrades rubber and elastics throughout the room, and it does nothing whatever to the fatty acid residue in your underlay. It masks by damaging your ability to smell it for a while. Plenty of London operators still own one.
The Kilburn carpet came out and went in a skip. The boards took two coats.…



