For landlords and property owners

The Tenant Moved Out, the Smell Stayed In: How a Bangkok Landlord Ended Three Months of Empty Viewings

Fresh paint and a new "For Rent" sign cannot hide what a previous tenant left in the walls, the curtains and the air. Here is what actually resets a unit between tenants.

Keys Returned, Problems Revealed

Wanchai Thongdee spent thirty years as an accountant, and when he retired he did what many careful savers in Bangkok do: he kept his old two-bedroom townhouse in Lat Phrao as a rental and moved in with his daughter's family in Nonthaburi. The rent was never a fortune, but it paid for his medical check-ups, his grandchildren's school fees now and then, and a trip upcountry once a year.

His last tenant stayed three years. He paid on time, rarely called with complaints and seemed perfectly pleasant. He also smoked indoors, kept two medium-sized dogs, and cooked a lot of deep-fried food in a kitchen with a weak extractor fan. Wanchai only visited twice during the lease, both times briefly, and both times at the front door.

On the day the keys came back, Wanchai stepped inside and stopped. The living room smelled of old smoke, the kind that seems to come out of the walls themselves. The staircase carried a sharp, ammonia-like note near the landing where the dogs had slept. The kitchen tiles were filmed with grease, and the small back bedroom, which had been used as a storeroom with the door shut, smelled of mildew. The curtains had turned a faint yellow.

He hired a cleaner, repainted the living room and replaced the curtains. Then he listed the house online, confident it would be taken within weeks, as it always had been before. It was not. Over the next three months, eleven groups came to view it. Several were polite and never called back. One young couple with a baby walked in, looked at each other and left within two minutes. A property agent eventually told him plainly: "Khun Wanchai, the house is fine. The smell is not."

That sentence stung, because Wanchai had done everything he thought a responsible owner should do. He had spent money, hired people and given up his weekends. What he had not done was deal with the one thing every visitor noticed the moment the door opened. His situation is far more common than most owners admit. In a city where rental homes change hands every year or two, the habits of one household can quietly shape the first impression of the next, and a unit that looks spotless in photographs can still lose tenant after tenant at the front door.

What a Lingering Odor Really Costs a Landlord

Many owners treat a smelly unit as a cosmetic issue, something the next tenant will get used to. In reality it touches almost every part of the rental business.

Money lost with every empty month

Vacancy is the most expensive line on a landlord's hidden balance sheet. Every month a unit sits empty, you still pay the common fees, the utilities for viewings, perhaps a loan installment, and you earn nothing. Three months of vacancy can wipe out a large share of a year's net rental profit. Owners who respond by cutting the rent often lock in a lower income for the entire next lease, all because of a problem that could have been fixed.

Health risks for the next occupants

Old tobacco smoke leaves residue that settles into fabrics, ceilings and walls and slowly releases back into the air. Pet urine that has soaked into grout, wooden floors or mattresses breaks down into ammonia and supports bacteria. Closed rooms in a humid city grow mold on walls, in wardrobes and inside the air conditioner. For a family moving in with a newborn, a toddler, or an elderly parent with weak lungs, these are not just unpleasant smells. They can mean coughing, irritated eyes, blocked noses, allergy flare-ups and poorer sleep from the very first week.

Reputation and the quality of tenants you attract

Good tenants have choices. The responsible professional or the young family who will look after your property is exactly the person who walks away from a unit that smells. The tenant who accepts it anyway may be the one least likely to care for it. Over time, a property can slide into a cycle where it attracts short, troubled tenancies and gets harder to rent each time.

Time, stress and awkward conversations

Landlords end up spending weekends at the property, trying one fix after another, arguing with contractors, or disputing deposit deductions with the outgoing tenant. Wanchai's daughter began to worry about him driving across the city in traffic every Saturday to "air the house out." He began to wonder whether he should simply sell.

Why Standard Move-Out Cleaning Isn't Enough

A normal move-out clean is designed to make a unit look ready. It is not designed to remove odors that have soaked into the structure over months or years. Here is what Wanchai tried, and why each step fell short.

Repainting. Fresh paint seals the surface for a while, but smoke residue in the ceiling, in the gaps around window frames and in the unpainted parts of the house keeps releasing odor. On humid days, many owners notice the old smell creeping back through new paint within weeks.

A regular cleaning crew. Cleaners scrub floors, wipe cabinets and clean bathrooms. They cannot reach the inside of built-in wardrobes' back panels, the underside of stairs, the inside of air-conditioning coils or the fibers of a built-in sofa.

Air fresheners and deodorizers. A viewing that smells heavily of perfume makes experienced renters suspicious. They know what fragrance is usually covering.

Leaving windows open. With the owner living elsewhere, windows cannot be left open for days for security reasons, and a humid breeze can make damp rooms worse, not better.

Replacing everything. New curtains helped, but swapping mattresses, sofas, built-in furniture and wooden floors is costly, slow and, in many cases, unnecessary if the odor source itself can be neutralized.

Resetting the Unit: WHD Residential Ozone Disinfection

The property agent who had been honest about the smell was also the one who suggested a way out. Some of the condominium owners she worked with had used a professional ozone service after difficult tenancies, and she had seen units that were considered "unrentable" leased within weeks afterwards.

The service she recommended was residential ozone disinfection by World Health Disinfection (WHD). Instead of adding something on top of the odor, the treatment uses ozone gas, O3, produced by the Master Ozone Generator. Ozone is an unstable, highly reactive form of oxygen. Because it is a gas, it does not stay on the surfaces a cleaner can reach; it moves through the whole volume of the house, into ceiling corners, wardrobe interiors, the fabric of remaining furniture, the gaps in wooden floors and the air conditioner's inner parts. There it breaks apart odor molecules and damages the structure of bacteria, viruses and mold spores.

Once its job is done, ozone naturally decomposes back into oxygen, O2. For a landlord, that detail matters a great deal. There is no chemical film left for the next tenant to breathe in or touch, no liquid soaking into floors or wiring, and no risk to fittings, lights, water heaters or appliances that come with the lease.

Four stages, about thirty minutes

Each treatment cycle follows the same controlled sequence:

1. Start and set intensity (about 2 minutes). The technician positions the Master Ozone Generator and sets the output to match the size of the space and how strong the odor problem is.
2. Distribute (about 12 minutes). Ozone fills the space evenly, reaching every room, corner and soft surface.
3. Purify (about 12 minutes). Ozone works at the molecular level to destroy odor compounds and pathogens.
4. Pause (about 4 minutes). The system stops so the ozone can begin breaking down before the space is opened up and aired.

Larger properties or multi-storey houses may need the team to plan the rooms in sequence; they will advise on this when they assess the unit.

Safety is non-negotiable

No one stays inside while ozone is running. People, pets and plants must be out of the area. The trained technician controls the machine throughout, and only after the pause does the team open the space and ventilate it before anyone re-enters. Between tenancies this is simple, because the unit is empty anyway, but it is still worth saying clearly: ozone disinfection is a job for professionals, not a gadget to run while you are sitting on the sofa.

Credentials that give landlords confidence

WHD's service page states that its Master Ozone Generator is certified and tested by Thailand's Department of Medical Sciences and by Intertek in the UK, with Intertek finding it ten times more effective than standard methods. The page also reports that the treatment eliminates more than 99.9% of bacteria, destroys viruses including SARS-CoV-2, and removes 99.99% of mold and fungal spores in air-conditioning units, while tackling smells such as cigarette smoke, pet odor, cooking odor, stale air and chemical residue. More than 300 organizations, from five-star hotels such as The Ritz-Carlton, Fraser Suites, Andaz and Dusit International to hospitals, schools, offices and factories, rely on the company's services.

10 Reasons Landlords Add Ozone to Their Turnover Checklist

1. It targets the odors tenants leave behind. Cigarette smoke, pet smells, heavy cooking and general mustiness are exactly the odors the service is designed to break down.
2. It works on what you cannot replace cheaply. Built-in wardrobes, wooden floors, ceilings and fitted furniture are treated where they stand.
3. It shortens the empty period. A unit that smells neutral at the first viewing has a far better chance of being leased quickly.
4. It leaves nothing behind. Ozone reverts to oxygen, so the incoming tenant is not moving into a home full of chemical residue or perfume.
5. It is dry. No soaking, no dripping, no waiting for carpets or mattresses to dry, and no risk to electrical fittings.
6. It disinfects as well as deodorizes. Bacteria and viruses on surfaces and in the air are reduced, giving the next household a genuinely clean start.
7. It tackles mold in air conditioners. Units left switched off for weeks are a classic source of that first-day musty blast; the service page cites 99.99% mold and spore reduction.
8. It supports your asking rent. Instead of discounting to compensate for a smell, you can present the property at its proper value.
9. It comes from a tested machine and an experienced team. The Master Ozone Generator has been tested by Thailand's Department of Medical Sciences and Intertek, and the team serves hotels and hospitals.
10. It can come with a bonus clean. Services from 15,000 baht upward include a free medical-grade disinfection spray using CHEMGENE HLD4H from the UK, a useful extra for bathrooms and high-touch areas.

Before and After: The Lat Phrao Townhouse

What the Owner Says

"I am an accountant, so I did the numbers. Three empty months had cost me far more than the treatment. I was nervous about using a gas I did not understand, but the technician explained every step, asked me to wait outside, and only let me back in after the house had been opened and aired. The difference was not subtle. I walked in and, for the first time in three years, the house smelled like my house again. The family who rented it has a baby, and the mother told me she chose it partly because it 'felt clean.' Now I book an ozone treatment every time a tenant leaves, before I even put the sign up."

Wanchai, Lat Phrao, Bangkok

A Smarter Turnover Plan for Rental Owners

Wanchai's experience changed the way he manages the handover between tenants. If you own one or several rental units, the same sequence can save you weeks of vacancy.

1. Inspect with your nose, not just your eyes. At the move-out inspection, walk every room with the doors and windows closed and the air conditioners switched on for a few minutes. Open wardrobes and cabinets. Note any smoke, pet, damp or cooking smell alongside the usual list of scratches and stains.

2. Fix the physical sources first. Remove rubbish, clear drains, repair leaks, replace anything genuinely damaged and have the air conditioners professionally cleaned. Ozone neutralizes odors and microorganisms; it cannot repair a leaking pipe that keeps a wall damp.

3. Deep clean, then treat. Let the cleaning crew and painters finish, then book the ozone treatment as the final step before photographs and viewings. That way the treatment reaches a unit that is already physically clean, and nothing new is brought in afterwards.

4. Photograph and list while it is fresh. Schedule your first viewings soon after treatment, when the unit is at its best. A neutral-smelling home makes a strong first impression that no photo can convey.

5. Keep it fresh during vacancy. If the unit may stay empty for a while, arrange for someone to air it briefly on dry days and run the air conditioners occasionally. Closed, humid rooms are where mold starts again.

6. Put clear terms in the next lease. Many owners now state in writing whether smoking indoors is allowed, how pets are handled, and what condition the unit should be returned in. Clear expectations make deposit discussions far easier.

7. Make ozone part of your routine budget. Rather than an emergency expense, treat it as a standard turnover item, just like repainting or servicing the air conditioners. It protects your rent, your tenant quality and your time.

Surface Smell or Deep-Set Odor? A Quick Self-Check

Not every returned unit needs the same response. Before you spend money on repainting or new furniture, it helps to understand how deep the problem goes. These simple checks give you a clearer picture.

The closed-door test. Close all windows and doors and leave the unit shut for a full day, ideally a warm one. When you return, notice what hits you in the first ten seconds. A smell that builds up in closed air is coming from materials inside the unit, not from outside.

The fabric test. Press your face close to curtains, a mattress or a sofa cushion and breathe in. Soft furnishings are like sponges for smoke and cooking odors, and they usually tell you more than the walls do.

The air-conditioner test. Switch each unit on after it has been off for a while and stand in front of it. A sour or dusty smell in the first minutes suggests mold or bacteria inside the coil and drain tray.

The cupboard test. Open wardrobes, kitchen cabinets and the space under the sink. Musty air trapped in these spaces is often where mildew is quietly growing.

The second-opinion test. Owners become used to their property's smell surprisingly quickly. Ask a friend or an agent who has never been inside to walk through and tell you honestly what they notice.

If the smell only shows up in one obvious spot, a targeted clean may be enough. If it appears in several of these tests, the odor has spread through the air and into materials, and that is exactly the situation whole-space ozone treatment is designed for.

Get Your Rental Ready for Its Next Tenant

Don't let a previous tenant's habits keep your property empty. WHD's residential ozone disinfection neutralizes smoke, pet and musty odors and disinfects the whole space in about 30 minutes, with no chemical residue left behind.

See service & pricing – click here

Phone 065-556-6294 | LINE @whd268 | Email worldhealthdisinfection@gmail.com

Promotion: book 15,000 baht or more and get a complimentary medical-grade disinfection spray service with CHEMGENE HLD4H (UK).

Landlord FAQ

Can ozone remove years of cigarette smoke?

Cigarette smoke is one of the odors the service specifically targets. For heavily affected units, the team assesses the severity and sets the ozone intensity accordingly. Combining treatment with removal of badly stained items, such as old curtains, gives the best result.

What about pet urine smells?

Ozone breaks down odor molecules from pets in the air and in soft surfaces. Where urine has soaked deep into a material, it is sensible to clean or replace that specific item as well, then treat the whole unit so no trace lingers.

Should I treat before or after painting?

Ideally after. Complete painting, repairs and cleaning first, then book the ozone treatment as the last step before viewings.

Will it damage the appliances included in the lease?

No liquid is used and nothing gets wet, so refrigerators, TVs, water heaters and air conditioners stay dry. The treatment is handled by a technician who controls the machine throughout.

Do I need to be present?

Someone should let the team in and receive the unit afterwards, but no one may stay inside during treatment. People, pets and plants wait outside until the technician has reopened and ventilated the space.

Is it suitable for condominiums as well as houses?

Yes. The service is designed for residential spaces of all kinds, from studio condos to multi-storey houses. Tell the team the size and layout so they can plan the visit.

How soon can the new tenant move in?

Once the pause stage is complete and the unit has been opened and aired, it can be used again. Many landlords schedule treatment a day or two before the first viewing or the move-in date for peace of mind.

Related Pages

• About the ozone disinfection machine
• How ozone removes chemical residue and stubborn smells
• WHD ozone cleaning service

For background on indoor air, moisture and mold, see the U.S. EPA Indoor Air Quality pages and Thailand's Department of Medical Sciences.

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