The carpet is almost never the main problem in a smoker’s flat. It gets cleaned first because it is the thing a landlord can see and the thing a cleaning firm can price, and by the third invoice everybody involved has spent money on the wrong surface.

I have done a lot of these. Ex-tenant flats in Catford and Harrow, a maisonette in Bow where the previous occupant had smoked indoors for nineteen years, void properties where the agent wants a viewing on the Friday and the smell hits people in the hallway. The pattern repeats so exactly that I now give the same speech on the phone before I quote.

Why does the smell survive a carpet clean that worked?

Because tobacco smoke does not settle in one place. It deposits everywhere air touched, which means every square metre of every surface in the property, and the carpet is a minority shareholder.

Cigarette smoke carries nicotine along with a range of tars and semi-volatile organic compounds. The nicotine is alkaline and water-soluble, so it comes off a wiped surface with fair ease. The tars are resinous, sticky, and they go on slowly releasing the compounds your nose reads as stale smoke for years after the smoker has gone. On a wall those deposits sit in and under the paint film. On a ceiling they sit in the emulsion and, in an older flat, in the artex texture, which has roughly three times the surface area of a flat ceiling and holds onto everything.

So a carpet clean removes what the carpet was holding. It does nothing to the five or six times greater area above and around it, and within a day or two the room smells the way it did.

The surface-area arithmetic nobody does

Take a small lounge, four metres by three and a half, with a standard ceiling height. Floor area is fourteen square metres. Walls come to about twenty-six, ceiling another fourteen, and then you add the door faces, the window reveals, the skirting, the radiator, the inside of the built-in cupboard, the ceiling rose, the coving. The carpet is somewhere around a quarter of the exposed surface in the room, and it is the only quarter anyone has paid to treat. That is money spent on the wrong surface.

Where is the smoke smell living once the carpet is clean?

In descending order of nuisance: the ceiling and walls, soft furnishings and curtains, the underlay and the back of the carpet, the extract fan and its ducting, inside wardrobes and drawers, the loft hatch, and the lampshades.

Two of those deserve attention because people forget them. The underlay is a smoke reservoir in its own right, because smoke moves with air and air moves down through a carpet at the edges and through the gripper gap. Cleaning the pile leaves that untouched. And the bathroom or kitchen extract, in a flat where the smoker used it as a smoking point, will have tar on the impeller and along the duct, warm, and ready to push smell into the room each time it runs.

The loft hatch matters in a top-floor conversion, because warm smoky air spent years finding its way up through it and settling on the joists and the insulation above a bedroom ceiling. I have stood in a bedroom in a converted terrace that smelled clean at chest height and distinctly of old tobacco when I put my head near the hatch, and no amount of work downstairs was going to change that.

The wardrobe test

Open a fitted wardrobe in a smoker’s flat a week after the property has been cleaned and aired. The air inside will be worse than the room, because it has been sealed and nothing has washed through it. That is the quickest check I know for whether a property has been decontaminated or deodorised, and it takes four seconds to perform in a void inspection.

Does ozone remove smoke odour or does it pause it?

Ozone is a strong oxidiser and it does react with odour molecules in the air. That part is real, and a well-run ozone treatment in a sealed void will make a flat smell dramatically better on the Wednesday.

The trouble is what it cannot do. Ozone is a gas with poor penetration into dense, non-porous deposits. It reaches air and it reaches the outermost few microns of a surface. A tar film sitting in and under a paint layer is largely out of its reach, and that film continues releasing compounds afterwards at the rate it always did. What you get is a reset that lasts anywhere from four days to a couple of months depending on how heavy the contamination is, and then the room smells again. Landlords read the return as a failed treatment and book a second one.

Ozone also acts on the building. It attacks rubber and elastomers, perishing washing machine hoses and window seals with repeated exposure, it dulls some paints and fades certain dyes, and it can leave its own sharp metallic note behind. It reacts with terpenes from cleaning products and air fresheners to produce formaldehyde and ultrafine particles, which is an odd thing to introduce into a property you are preparing for a family. The HSE workplace exposure limit for ozone is 0.2 parts per million over a fifteen-minute reference period, and the concentrations used for odour work sit far above that, which is why the flat must be empty, sealed, signed and then ventilated properly before anyone goes in.

Where ozone earns its fee

After the source has been removed. Run it at the end of a decontamination as a finishing step on residual air and on the cavities you could not wash, and it does a good job. Run it instead of the decontamination and you have bought a fortnight. I use it in that first way perhaps a dozen times a year, and I decline to use it in the second way, which loses me the easy version of the job.

What sequence works in a two-bed ex-council flat?

Strip, wash, seal, then soft furnishings, then the carpet last.

Take down curtains and blinds and send them out or replace them. Remove lampshades. Wash every hard surface with an alkaline degreaser, working ceilings first, and expect the water to run brown in a heavy property. Wash inside cupboards, the tops of doors, the backs of radiators, the window reveals. Clean or replace the extract fan and, where you can reach it, the duct.

Work top down and change your water often. The cloth tells you when to change it: once the rinse water has gone the colour of weak tea it stops removing tar and starts spreading it in a thin film, which is how a ceiling ends up looking evenly yellow rather than patchy. Sugar soap suits the job, a strong alkaline degreaser suits it better, and on a heavy ceiling you will go over the same square metre twice before it stops giving anything up.

Then seal. This is the stage that fixes it, and it is a decorator’s job rather than mine. A shellac-based stain-block primer over ceilings and walls locks the residual tar film underneath a barrier that stops it off-gassing, and then two coats of emulsion on top. Water-based trade primers are not a substitute here. I have watched a landlord in Stockwell repaint twice with ordinary contract emulsion and a sealer and then, on the third go with shellac primer, get a flat that held its improvement for the whole tenancy.

The carpet comes last for a reason

Cleaning the floor before the ceilings get washed means recontaminating the pile with everything that rinses off above it. Any technician who turns up to a smoker’s void and starts before the decorator has finished is taking your money in the wrong order, and it happens constantly because the carpet clean is the easiest thing to book.

When should a landlord stop cleaning and replace?

When the carpet has had one proper clean and the property still fails the wardrobe test, the fault lies above head height. When the property has been washed and sealed and the room still smells at floor level, the underlay is the thing left to change, and underlay is cheap.

My blunt version for landlords: pay me once. If the smell survives a single thorough hot water extraction in a decontaminated property, do not pay me again. Lift the carpet, bin the underlay, put down new, and either refit the carpet or replace it depending on what the pile looks like. A second and third carpet clean is money spent on the wrong surface for a second and third time, and the trade knows it and keeps taking the bookings.

The void-period sum

Every week a flat sits empty costs a London landlord a week’s rent, and the temptation is the quick fix that lets a viewing happen on Friday. Ozone gives you that Friday. It also gives you a tenant who complains in week three, and the same works to do six months later with somebody’s furniture in the way and a rent reduction being discussed. Every landlord I have said this to has already worked it out and wanted permission to spend the money. Decontaminating properly takes four to six working days including drying and two coats. In most London postcodes the rent lost across those days is smaller than the cost of doing it twice.

The other argument I lose regularly is about air fresheners and thermal fogging. Fogging deposits a scented oil over the contaminated surfaces, which reads as an improvement for a fortnight and then leaves you with stale tobacco underneath a cherry note. I would rather hand back a deposit than put that in a flat somebody is about to sleep in.

In the Bow maisonette the ceiling washings came off the size of a ten-pence piece on the white cloth, brown and sticky, and the decorator kept one of the cloths on the windowsill to show the landlord why the primer was going on before the second coat of emulsion.