A typical scenario (an illustrative situation, not a review from any specific customer)

The small door under the stairs that nobody wants to open

Nid lives in a three-storey townhouse on the eastern side of Bangkok with her three-year-old son and her father, who is in his seventies. During the heavy rain of 24 to 27 September 2026, water from the soi flowed into the house to roughly a hand-span deep. The family moved upstairs and lifted almost everything off the ground floor, except one place nobody thought about: the little storage cupboard under the staircase.

Like countless city homes, that cupboard held paint left over from repainting years ago, half a bottle of thinner, mosquito spray, toilet cleaner, a bag of fertiliser for the pot plants, mops, shoe boxes, a box of old documents and a broken fan. The small wooden door stayed shut because nobody went in often.

A week after the water receded, the ground floor had been washed and wiped clean. Yet every time someone walked past the stairs, a strange smell drifted out: the damp, sour mustiness of laundry that never dried, mixed with the sharp edge of solvent and something like insecticide. When Nid opened the door, she found collapsed boxes, black-green mold spots on the document box, rusted paint-tin lids with paint oozing out, and a thin film of mud and oil on the floor. Her son loves to play right at the foot of the stairs, and her father had started coughing more at night.

According to news reports, around 300 mm of rain fell in about 48 hours and the Bangkok Metropolitan Administration declared all 50 districts a disaster zone on 26 September 2026. The east of the city was hit hardest, and as of 30 September some roads such as Si Burapha and Lat Phrao still had 20 to 40 cm of water. Many families lived with damp for longer than expected, and forgotten spaces like storage rooms and under-stair cupboards became the worst odour spots in the house.

Why storage rooms and under-stair cupboards smell the worst after a flood

People are often surprised that the living room, which took deeper water, airs out quickly while a small cupboard that took less water keeps smelling for weeks. The reason lies in the nature of these spaces.

1. Closed spaces with no airflow

Under-stair cupboards and store rooms rarely have windows or vents, and the door is almost always shut. Once moisture gets in, it has nowhere to go. The air stays warm, still and humid, which is exactly what mold likes, and chemical vapours build up in a small volume of air until they are more concentrated than anywhere else in the house.

2. They hold most of the household's chemicals

Under the stairs is where we hide things we would rather not see: paint and thinner, fuel for the lawnmower, insecticide, termite treatment, strong cleaners, fertiliser, glue and lubricants. Floodwater tips containers over, rusted caps leak and labels wash off, so products mix with water and seep into the floor.

3. The contents absorb both water and odour

Cardboard, paper, fabric, doormats and the timber used for under-stair panelling all soak up water readily. When that water carries chemicals, the odour is absorbed too and released slowly for a long time. Gypsum or plywood lining under some staircases can also hide moisture inside, letting mold grow where nobody can see it.

4. They are the last place anyone cleans

After the water drops, families rush to clean the rooms they use every day. The store room waits. The longer it waits, the further mold spreads and the deeper the odour sinks. Agencies such as the CDC advise cleaning and drying within 24 to 48 hours to limit mold; many store rooms miss that window entirely.

5. The staircase spreads the smell to every floor

In townhouses and shophouses, the stairwell connects every level. Warm air rises up it, carrying the odour from the cupboard below all the way to upstairs bedrooms. That is why so many families complain that the bedroom smells musty even though the water never got there.

Musty versus chemical: two different smells that both need handling

Mustiness comes from living things

The damp-laundry or old-earth smell mostly comes from mold and bacteria growing on wet materials. As they grow, these microbes release certain volatile compounds with a characteristic odour. Urban floodwater also carries drain water, adding sewer and rotting notes.

Chemical smells come from stored products

Sharp, pungent or oily odours come from solvents in paint and thinner, fuel, insecticides and cleaners. These are sources of VOCs that keep evaporating from residues soaked into the floor and contents.

When the two overlap

In a flooded store room the two are usually layered, and our noses cannot tell which is which. Some people only spray disinfectant, which helps partly with mold but does nothing for solvent smell. Others only run a fan, which helps with damp but does not destroy embedded mold spores. Recovery that works tackles both: remove the source, dry the space, then deal with residual odour and microbes in the air and on surfaces.

How mixed musty and chemical odours affect the household

A store room may be a corner nobody occupies, but its air flows into the spaces where everyone lives. Possible effects include:

• From mold: blocked or runny nose, sneezing, itchy eyes, coughing, and asthma flare-ups in people with allergies or asthma.
• From chemical vapours: headaches, dizziness, stinging eyes and nose, nausea and fatigue after time near the source.
• From touching residue: skin irritation, rashes or burning, especially when hands touch chemical-laden residue and are not washed.

Who is most at risk

Young children breathe faster relative to body weight, crawl and sit on the floor, and put their hands in their mouths, so they meet floor residue and low-level air more. Older people and anyone with lung or heart disease or weakened immunity tolerate airborne irritants less well. Pregnant women should avoid clearing out a room with a strong chemical smell and let others do it. People with allergies or asthma are usually the first to feel symptoms.

When to see a doctor

Fever, muscle pain especially in the calves, or red eyes after wading (possible signs of leptospirosis), diarrhoea, breathing difficulty, wheezing, or a spreading rash all mean seeing a doctor promptly and mentioning flood or household-chemical exposure. Odour removal improves the environment; it is not medical treatment.

What a lingering store-room smell costs a family

Health that slowly wears down

A slight cough, a blocked nose every morning or an afternoon headache may not seem serious, but repeated daily for weeks they erode sleep and everyday life. Children may get sick often enough to miss school; older relatives may need more doctor visits.

Money spent replacing and masking

The odour spreads into clothes in a nearby wardrobe, curtains or the sofa. Families buy air fresheners, charcoal bags and sprays that add up, and the smell still stays.

Time and energy, over and over

Wash it, and the smell returns, so the job is repeated every weekend. Some families haul everything out into the sun again and again while still going to work, caring for children and fixing the rest of the house.

Not feeling at ease at home

Home should be where everyone rests. Holding your breath walking past the stairs, or not letting the children play downstairs, takes that sense of safety away, and families end up arguing about who should deal with it and how.

Value when selling or renting

Viewers notice a musty smell the moment the door opens. A musty-chemical mix makes buyers and tenants wonder what damage is hidden, and they may negotiate down or walk away.

Popular fixes that rarely work in a flooded store room

Keeping the door shut

Closing the door cuts the smell for a while, but in a sealed room the damp has no way out, mold grows faster and vapours concentrate. Next time the door opens, it is worse.

Mothballs or odour blocks

Mothballs are themselves a chemical that evaporates. Adding them to a room that already smells of chemicals just adds another vapour to the air. It does not remove the original smell, and it is a poor choice in homes with small children.

Air freshener and perfume

These mask with fragrance, and some contain VOCs, so a room with an air-quality problem simply gains more volatile compounds.

Bleach in a closed room

Using strong bleach in a tight, unventilated space means breathing chlorine fumes directly, and if the floor has residue from other cleaners containing ammonia or acid, irritating gases can form. Follow the label, never mix products, and always ventilate.

Anti-mold paint over the under-stair wall

If the wall is still damp inside and mold roots remain, painting over only hides the problem, and new paint adds paint odour and VOCs to a tiny, poorly ventilated room.

Charcoal or coffee grounds

These absorb a little odour in a drawer or small box but cannot deal with mold inside walls or vapour soaked into a concrete floor.

The targeted fix: WHD ozone removal of musty and chemical residue odours

Once damaged items are out and the room has been washed and dried, the step that properly finishes the job is the ozone chemical residue and odour removal service from World Health Disinfection (WHD). According to the service information, ozone (O3) breaks odour molecules down at the source instead of masking them. It handles paint, thinner, lacquer and epoxy odours, VOCs and musty smells, helps reduce bacteria, viruses and mold, and then reverts to oxygen with no residue.

Why ozone suits cramped spaces with dead corners

An under-stair cupboard has corners hands cannot reach: the low end under the sloping treads, gaps behind shelves, the joint between wall and floor. Ozone is a gas, so it reaches every gap air can reach and works on odour molecules both in the air and on surfaces, without wiping spot by spot.

One zone or the whole floor

The team checks how far the smell has travelled. If it is confined to the store room, the plan may cover that zone only. If it has risen up the stairwell to the bedrooms, a whole floor or several zones may need treating in sequence. In every case the treated area must be empty and sealed.

Results in numbers

According to the service information, WHD measures TVOC with calibrated instruments before and after treatment, so the family can see how the air has changed rather than relying on noses that may have got used to the smell.

How the service runs

1. Questions and survey. The team asks what chemicals were stored, checks leaking containers have been removed and the room is dry enough.
2. Baseline TVOC in the store room, at the foot of the stairs and in rooms receiving the smell.
3. Preparing the home. Everyone, including pets and plants, leaves the treatment zone. Cupboard doors and drawers in the store room are opened so ozone can reach inside; windows and doors of the zone are closed.
4. Treatment with the Master Ozone Generator in the empty, closed space, for a time matched to room size and odour level.
5. Rest and ventilation. According to the service information, the space is ready to use about two hours after treatment.
6. Follow-up TVOC, compared with the baseline, plus advice on looking after the store room from now on.

Safety first: ozone treatment is done only in an empty, closed space. People, pets and plants must be out and may return only after the rest and ventilation period. Ozone follows mud removal, washing and drying. It does not replace cleaning or medical care.

10 reasons to finish a flooded store room with ozone

1. Musty and chemical smells in one pass, as described for the ozone chemical-odour service.
2. Reaches the low, narrow end under the stairs where cloths and sponges cannot go.
3. No new chemicals in a tiny room, unlike mothballs or sprays, because ozone reverts to oxygen.
4. Helps reduce mold and bacteria, the source of mustiness, according to the service information.
5. Stops the smell climbing the stairwell when the treatment covers every zone it reaches.
6. Before-and-after TVOC figures that reassure the whole family.
7. Quick turnaround: about two hours after treatment the space is ready.
8. Suitable for homes with children and older people when the protocol is followed: out during treatment, back after the rest period. Details on the WHD service page.
9. Supports follow-on repairs: repaint the stair wall or fit new shelves, then treat again for fresh paint odour.
10. Peace of mind: walk past the stairs without holding your breath. Ask the team.

Before and after: a store room restored the right way

Example case: a store room behind the kitchen in an eastern Bangkok house

Example case (an illustrative scenario explaining the process, not a specific customer's testimonial)

Ek, an office worker in his forties, owns a detached house in a housing estate in eastern Bangkok. Behind the kitchen is a store room of about two square metres holding a spare gas cylinder, lawnmower fuel, half a bottle of weedkiller, termite treatment, toilet cleaner and a sack of cat food. In late September 2026 it stood in about thirty centimetres of water for several days.

Once the water went down, Ek had the wiring and the gas cylinder checked before using anything. Wearing rubber gloves, boots and a mask, he moved every chemical container outside. Weedkiller and termite-treatment bottles with faded labels were set aside for disposal through the channel his district office specified rather than poured down the drain. The wet cat food was thrown out. He washed the floor with a cleaner used according to its label, wiped the walls and shelves, then left the door open with a fan running for several days.

Even dry, the room sent a musty, oily, insecticide smell into the kitchen whenever the door opened, and the family cat refused to go near it. Ek called WHD. The team measured TVOC in the store room and kitchen, had everyone and the cat leave the house, sealed the kitchen and store-room zone, treated it with a Master Ozone Generator, rested and ventilated the space, then measured again. In Ek's account, the kitchen smell dropped enough for normal cooking. He decided to stop keeping agricultural chemicals in the house and moved the essentials into sealed plastic boxes on a high shelf.

The lesson: ozone works best when the room is empty, dry and the source of the smell has gone, and reorganising storage means not facing the same problem in the next big rain.

Recovery checklist for store rooms and under-stair cupboards

Before you start

• Check wiring and any sockets that were submerged; if there is a gas cylinder, check it for leaks first.
• Open the store-room door and nearby windows to air the space before going in.
• Keep children, pregnant women, older relatives and anyone with asthma away while clearing.
• Wear rubber gloves, boots, a mask and eye protection.

While clearing

• Bring every chemical container out and inspect it; isolate leaking, unlabelled or unknown ones.
• Never pour chemicals down drains or together; ask your district office about hazardous-waste disposal.
• Discard cardboard, papers, fabric and anything moldy or soaked in chemical residue; photograph important documents first.
• If under-stair gypsum or plywood has swollen, have a builder open or replace it so moisture does not stay trapped.
• Wash floors and walls according to product labels, without mixing cleaners.
• To disinfect washed surfaces, consider a medical-grade disinfectant such as Chemgene HLD4H at the label dilution and contact time.
• Dry thoroughly with fans and an open door before putting anything back.

Treatment day

• The store room should be empty or hold only bare shelving, with cupboards and drawers open.
• Everyone, pets and plants leave the treatment zone.
• Return only when the team confirms the rest and ventilation period is over.

Afterwards

• Store chemicals in sealed plastic boxes on shelves above the previous flood line.
• Keep only what you really need; dispose of old paint and chemicals properly.
• Open the store-room door regularly or fit a vent.
• Watch the family for symptoms; see a doctor if anything persists.

FAQ: musty and chemical smells in storage rooms

The store room is tiny. Do I really need a service?

If washing, drying and removing the source make the smell go away, maybe not. But if it keeps coming back or reaches other rooms, especially with young children or older people at home, ozone treatment with TVOC measurement shows clearly whether the problem is solved.

Do we have to leave the whole house?

Everyone must leave the treated area and any space sharing its air. The team decides which zones to seal. In a townhouse where the stairwell links every floor, the usual advice is for everyone, pets and plants to be out of the house during treatment and the rest period.

Should items going back in be treated too?

Washable, fully dry items such as plastic boxes, tools and shelving can stay in the room during treatment on the team's advice. Anything that has absorbed chemical residue or grown mold should be thrown out, or it becomes a new odour source.

Can an air purifier do the job instead?

Air purifiers filter dust, and models with carbon filters reduce some airborne odour; they are useful afterwards. They do not act on odour embedded in surfaces and dead corners the way ozone treatment does.

There is widespread mold inside the under-stair wall. What then?

Deeply moldy or swollen materials should be removed and replaced, then the space cleaned, dried and treated with ozone as the final step. Ozone cannot replace removing damaged materials.

How soon can we go back in?

According to the service information, the space is ready about two hours after treatment. The team confirms timing for your actual space.

Family members have had a cough and tight chest since the flood. What should we do?

See a doctor, especially for asthmatics, young children and older people, and mention the flood and the chemical or mold smell. Improving indoor air reduces triggers; it is not treatment.

Store room or under-stair cupboard still musty and chemical? Let WHD finish the job

Site check, before-and-after TVOC readings, and ozone treatment for every zone the smell has reached.

Ask about ozone odour removal

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

Related services

• Ozone chemical residue and odour removal
• Residential ozone disinfection
• Chemgene HLD4H medical-grade disinfectant

References

• 2026 Bangkok floods (Wikipedia)
• CDC: Floods
• WHO fact sheets