Old frying oil, mostly. Turmeric and cumin sometimes, fish sometimes, and underneath a fair number of them a base note of cigarettes that nobody mentioned during the viewing. It hits you at the top of the stairs before you have the door open, it fades after ten minutes because your nose stops reporting it, and it comes back the moment a friend walks in and pauses half a second too long in the hallway.
Two cleans have been paid for. The flat looks immaculate. The smell is still there, and the reason is that almost nothing that produces it lives on the surfaces a clean touches.
What is the smell made of?
Cooking odour is not a vapour hanging in a room. If it were, opening a window for a day would end it, which is what every letting agent in London tells tenants to do and it never works.
What is in the air during cooking is an aerosol: microscopic droplets of oil carrying dissolved volatile compounds, thrown off a hot pan and moved around the flat by convection. Those droplets land. They land on everything, including surfaces four rooms away, and they land in a layer so thin that nobody sees it accumulate. Over three years of daily cooking in a small flat, that layer becomes a continuous film on every wall, ceiling and door in the property.
The volatile part of it evaporates slowly out of that film for years. That is the smell. The film is the reservoir, the room is the delivery system, and cleaning the visible surfaces removes maybe a fifth of the total.
Why cooking oil behaves differently from other odours
Oil oxidises and polymerises. Left on a warm surface with air across it, unsaturated cooking oil cross-links into something closer to a soft varnish, which is the same process that seasons a cast iron pan and the same one that makes a neglected extractor hood sticky rather than greasy.
That matters because a polymerised film does not dissolve in the way fresh grease does. Warm soapy water takes fresh splatter off a wall. It slides straight over a three-year-old film and leaves it there, slightly shinier than before. A cleaner working at normal pace on a normal domestic rate will do exactly that, twice, and the customer will watch a spotless cloth come off the wall and conclude the wall is clean.
Tobacco tar behaves the same way and worse, because it is stickier and it carries nicotine, which is water-soluble and keeps releasing whenever the air gets humid. On a warm day in a flat above a shop on Osbaldeston Road in Stamford Hill you could smell a smoker who had moved out in February.
Where is the smell held?
Six places, in rough order of how much they contribute.
Walls and ceilings hold the most, by sheer area. A one-bed flat has something like ninety square metres of painted surface and all of it has been collecting aerosol. The ceiling directly above the hob is the worst square metre in the property and it is the one nobody cleans, because it is above head height and looks fine from below. Matt emulsion is porous and takes the film into the paint rather than onto it, which is why washing it does very little.
Kitchen unit interiors are next. The inside of a wall cupboard directly beside a hob gets a share of every meal cooked, and chipboard carcasses with a thin melamine face absorb through the exposed edges and the drilled holes. Open one on a warm day a month after the flat was cleaned and the previous tenant is still in there.
Extract ducting third, and it deserves its own paragraph below.
Soft furnishings and carpet fourth. Underlay is the sleeper here: the carpet gets cleaned, the underlay is never touched, and it has been absorbing for years through a permeable backing. A rental in Wellmeadow Road in Catford had two carpet cleans in six weeks and smelled worse after the second, because hot water extraction rewetted an underlay that then took nine days to dry.
Fifth, the loft hatch and any service voids, which hold air that has been moving up out of the kitchen for years.
Sixth, and last, the surfaces a cleaner gets booked to clean.
The extractor duct
Almost nobody looks in it. A recirculating hood in a flat with no external duct has a metal grease filter and a carbon cartridge, and both should be treated as consumables. The carbon in a flat where somebody fried daily is finished within months, and after that the hood pushes odour-laden air back into the room at speed.
A ducted hood is a different problem. The flexible aluminium duct running through a kitchen ceiling void to a soffit vent is corrugated, which means several hundred small ridges each holding a ring of polymerised grease. Switch that fan on and the airflow drags odour off the whole length of it. A flat on Gaskarth Road in Balham smelled of fish only when the extractor ran, which took two visits to work out and about eleven minutes to fix once the duct came off.
Why did two cleans not fix it?
Wrong scope, wrong chemistry, and in a lot of cases the wrong instruction from whoever booked it.
Scope first. A standard end-of-tenancy clean covers surfaces, appliances, floors and sanitaryware. It does not include washing down ninety square metres of wall and ceiling, and it should not, because that is half a day on its own and it is not what the price covers. Anyone who books a tenancy clean and expects a smell to leave with it has bought the wrong service, and the honest thing is to say so before taking the booking rather than after the second one.
Chemistry second. Grease films need an alkaline degreaser with real dwell time, applied warm, worked with a pad, and rinsed with clean water so the emulsified grease leaves the building rather than being spread. Most general-purpose sprays are pH-neutral surfactants designed to be safe on everything, and safe on everything means weak on grease.
Third, and this is where people waste the most money: masking. A flat sprayed with fragrance smells of fragrance plus old cooking, which is a distinctly worse smell than old cooking alone, and everyone recognises it as a cover-up. The plug-in in the hallway of a viewing flat is doing the property no favours at all.
What ozone and fogging are worth
Less than the invoices suggest. Ozone generators do break down odour compounds, and in a fire-damage or heavy tobacco situation a properly run treatment by someone with the right kit has a place. In a normal flat with a cooking smell, it is being sold as a shortcut around the degreasing work and it does not do that, because ozone does not remove a grease film. It alters the top layer of it. Six weeks later, on a warm day, the smell is back.
Ozone also needs the property empty of people, pets and plants during treatment and ventilated afterwards, and plenty of the operators offering it locally treat that as optional. That alone is reason enough to be sceptical of the cheap end of the market.
What shifts it?
Sequence matters more than product, and doing these in the wrong order means doing them twice.
Strip the source first. Filters out and replaced, hood degreased inside, ducting removed and cleaned or renewed, cooker pulled out, kickboards off. Then wash down hard surfaces with an alkaline degreaser: walls, ceilings, doors, frames, the inside of every cupboard, rinsing as you go. Then deal with textiles, which for a rental usually means the curtains go to a cleaner or in a bin, and the carpet gets lifted so the underlay can be assessed rather than cleaned.
Then ventilate properly, which means cross-ventilation with the heating on for several days, not a window on the latch. Warm air holds more and moves more.
Only after all that does sealing make sense. A stain-blocking primer, shellac-based or a modern water-based equivalent, locks in whatever is left in the plaster, and two coats of emulsion over the top finish it. Do that before the degreasing and you have sealed grease under paint, which will bleed through and smell whenever the room warms up.
The thing that loses me the work
Most flats in this state need a painter more than they need a third clean.
Once the film has gone into matt emulsion in a kitchen, the fastest and cheapest permanent answer is one day of degreasing followed by a stain block and a repaint. The clean is a necessary step and it is not the expensive part. A landlord in Lincoln Road in Enfield spent four hundred pounds across three cleaning visits over two months before painting the kitchen and hallway for two hundred and thirty, and the smell went the same week.
When should you stop buying cleans?
After the second one, if the smell is unchanged.
Two proper attempts is enough to establish that the reservoir is somewhere a cleaner cannot reach. At that point the money goes to a decorator, a carpet fitter for underlay, or a plumber and electrician for a hood and duct that should have been replaced when the last tenant left.
If you are renting, put it to the agent in writing before you move in rather than after, and be specific: the ceiling above the hob, the duct, the underlay, the cupboard interiors. General complaints about a smell get a general answer about ventilation. Name the four things and you get a decorator.
What to ask for before you sign
Ask when the kitchen was last painted and whether it was stain-blocked. Ask whether the extractor is ducted or recirculating, and if recirculating, when the carbon filter was last changed. Ask whether the carpet underlay is original.
Three questions, and the answers tell you whether the smell is a two-hundred-pound problem or a nine-hundred-pound one.
The tenant in Acton took the curtains down on the first afternoon and put them in the communal bin. She said the flat improved by about a third before anyone touched a wall.