The flat is spotless. Every surface has been scrubbed, the carpets cleaned, the windows polished, and yet the moment you step inside, the smell is still there. Stale cigarette smoke that has soaked into the walls over years of tenancy. The faint, persistent trace of a previous tenant’s dog. A musty note that no amount of airing seems to shift. Some odours live not on surfaces but in the air and deep within soft furnishings, and conventional cleaning, however thorough, simply cannot reach them. This is where ozone treatment earns its place in the professional toolkit. Used correctly, it does something ordinary cleaning cannot: it destroys the molecules responsible for an odour rather than masking them. For end of tenancy work across Camden Town and the wider NW1 area, it is often the difference between a property that merely looks clean and one that genuinely is, ready to hand on without a trace of the tenancy before.
What Ozone Is, and Why It Tackles Odour Differently
The oxygen we breathe is O2, a stable molecule of two oxygen atoms. Ozone is O3, the same element with a third atom attached, and that extra atom makes it both powerful and unstable. Ozone does not want to remain as O3; its third oxygen atom readily breaks away and latches onto whatever it meets. When it meets an odour-causing molecule, such as the compounds left behind by smoke, bacteria, mould or a pet, it oxidises it, altering its chemical structure so that it no longer registers as a smell. Having done its work, the ozone reverts to ordinary, harmless oxygen.
This is the crucial distinction between ozone and an air freshener. A freshener simply adds a stronger scent to cover an unpleasant one, leaving the original molecules intact to reassert themselves once the perfume fades. Ozone removes the source. It is a chemical reaction rather than a disguise, which is exactly why a properly executed ozone treatment can clear an odour that has resisted every other method tried before it.
How the Treatment Actually Works, Step by Step
Professionals do not simply switch on a machine and walk away. The process follows a careful sequence, and skipping any part of it undermines the result.
Thorough cleaning comes first
Ozone is a finishing step, not a substitute for cleaning. It deodorises; it does not wash away dirt, grease or residue. A professional will always deep-clean the property first, removing the physical sources of smell, and ensure that carpets and fabrics are fully dry, since damp surfaces interfere with the treatment. Only once the property is clean and dry does the ozone stage begin. Treating a dirty property with ozone in the hope of shortcutting the cleaning would waste the process entirely, since the residue producing much of the smell would still be sitting there once the gas had dispersed.
Generating and sealing in the ozone
The treatment uses an ozone generator, which produces O3 from the surrounding air, most commonly by passing it through a high-voltage electrical field in a process known as corona discharge. The machine is placed in the empty property, the room or whole flat is sealed as far as possible, and the generator runs on a timer. Depending on the strength of the odour and the size of the space, this might take anything from around thirty minutes to several hours. Stubborn smoke or animal odours often call for a longer cycle, or more than one. Sealing the space matters as much as the generator itself, because ozone that escapes through an open vent or gap is ozone that never reaches the odour, and the concentration the treatment depends on is lost before it can act.
Aeration and safe re-entry
Because ozone is unstable, it begins reverting to oxygen the moment the generator stops, with a half-life of roughly twenty to thirty minutes. After treatment, the property is ventilated, with windows opened and air allowed to circulate, until ozone levels have fallen back to those of ordinary outdoor air. Only then is it safe to re-enter. This aeration period is not optional; it is a core part of the method and a measure of whether the job has been done responsibly.
The Odours It Handles Best
Ozone is particularly effective against odours that are organic in origin and have permeated the air and soft materials of a property rather than sitting on a single surface.
Cigarette and cigar smoke is the classic example. Tobacco residue clings to walls, ceilings, carpets and curtains, and is notoriously difficult to remove by cleaning alone, yet ozone reaches it where wiping cannot. Fire and bonfire smoke responds in much the same way. Pet odours, the smell a cat or dog leaves in a property over months or years, are another strong candidate, as are persistent cooking smells and the general mustiness of a flat that has been closed up or touched by damp. In each case the odour exists as molecules dispersed through the air and absorbed into furnishings, which are precisely the conditions ozone is suited to address.
It is worth understanding why these particular smells are so hard to shift by conventional means. A surface clean tackles what can be seen and touched, but smoke and pet odours migrate into porous materials and the air itself, settling into fibres, plaster and the gaps that no cloth ever reaches. Ozone, being a gas, fills the same spaces the odour occupies and reacts with it there, which is the single reason it succeeds where scrubbing reaches a natural ceiling.
The Limits Professionals Respect
A skilled contractor is as clear about what ozone cannot do as about what it can, and honesty here prevents disappointment later.
Most importantly, ozone treats the symptom, not the cause. If a smell comes from an ongoing problem, such as a water leak, rising damp, or mould growing behind a wall, ozone will reduce the odour for a time, but it will return until the underlying fault is repaired. Ozone is a deodoriser, not a building repair, and no responsible contractor will pretend otherwise. It also cannot reach odours sealed deep within a structure that the gas cannot penetrate. And because ozone is a strong oxidiser, prolonged or excessive exposure can degrade certain materials, particularly rubber and some plastics, so professionals control the dose and the duration with care rather than simply maximising both in the hope of a faster result.
Why Safety Dictates a Professional Approach
The very property that makes ozone effective against organic molecules also makes it hazardous to breathe. Ozone is a respiratory irritant: inhaling it can cause coughing, chest tightness and throat irritation, and it can aggravate conditions such as asthma. In the UK, workplace exposure to ozone is addressed by Health and Safety Executive guidance and falls under the Control of Substances Hazardous to Health Regulations, which is precisely why the gas is treated with such respect by those who use it regularly.
The cardinal rule is that ozone treatment takes place only in an unoccupied space. No people, no pets and no live plants should be present during a cycle, and the area is kept sealed and clearly marked while the generator runs. This is the strongest argument for leaving ozone treatment to professionals rather than attempting it with a hired machine. Judging the correct concentration, exposure time and safe re-entry point is not guesswork, and getting any of it wrong carries a genuine health cost rather than merely a disappointing one.
Where It Fits in the End of Tenancy Picture
End of tenancy is, in many respects, the ideal moment for ozone treatment, precisely because the property is empty. The void between one tenant leaving and the next arriving provides exactly the unoccupied, controllable window the method requires, with no residents or animals to work around and no need to disrupt anyone’s daily life.
This matters in an area like Camden, where much of the rental stock consists of older houses and period conversions, including the canalside flats near Regent’s Canal, where damp and lingering smells tend to be more common than in newer builds. A flat that has housed a heavy smoker or a much-loved pet, or one that has sat closed and musty between tenancies near Kentish Town or Chalk Farm, can carry an odour that would otherwise put off prospective tenants or trigger a dispute over the deposit. Removing it properly, during the empty period before the next tenancy begins, leaves the property genuinely fresh rather than merely scented over for a viewing.
A Precise Tool, Properly Handled
Ozone treatment is neither a gimmick nor a magic wand. It is a specific chemical process that excels at one demanding task, the destruction of stubborn organic odours, and it is governed by clear rules about cleaning beforehand, dosing during, and ventilating afterwards. Understood and applied correctly, it allows a professional to resolve smells that would defeat any conventional clean, while the safety discipline surrounding it ensures that nobody is put at risk in the process. At the end of a tenancy, when a property must be handed on in genuinely fresh condition rather than simply looking the part, ozone treatment is one of the more quietly impressive instruments a cleaning contractor can bring to bear, provided it is used with the knowledge and caution the process demands.