A story from the field: the small door nobody wanted to open

Khun Wittaya is forty-five. He lives with his wife, their primary-school daughter and his seventy-year-old mother in a two-storey detached house in a housing estate in Bang Na. They bought the house more than ten years ago, and the feature he liked most when they first viewed it was storage. There was a cupboard room under the staircase, a storeroom at the back of the garage, and a rear room on the ground floor that sat about three steps lower than the rest of the house. The family used that sunken room as a mix of storeroom and hobby space: camping gear, his old guitar amplifier, his wife's sewing table, boxes of school books, and the artificial Christmas tree that came out once a year.

When the 2026 floods reached Bang Na, water seeped into the house to a depth of about forty centimetres and stayed for almost ten days. The sunken room was a different story. Because its floor was lower, the water there stood nearly a metre deep, and it was the last place in the house to drain. When the street finally dried out, the family did what most families do. They cleaned the living room and kitchen first, because that is where life happens. The under-stair cupboard and the storerooms were shut. "We'll deal with them later," everyone agreed.

Two weeks later, a musty smell started drifting out every time someone walked past the stairs. His mother complained that her nose stung. His daughter began sneezing at night. When Khun Wittaya finally opened the little door under the stairs, he found cardboard boxes that had collapsed into a single soggy block, grey-green mold spreading across the inside wall, and a sleeping bag and tent covered in black spots. In the sunken room, a muddy tide line ran almost to waist height, and the smell was so heavy he had to prop the door open for several minutes before he could walk in.

His story is not unusual. In almost every flooded detached house we visit, the strongest and most persistent odor does not come from the rooms people clean first. It comes from the rooms people forget.

Why storage spaces become the worst odor source in the house

Under-stair cupboards, garages and sunken rooms share three characteristics. They have little airflow, they get no sunlight, and they are packed with things. When floodwater enters, it soaks into cardboard, fabric, foam and timber, and then it is trapped there in a dark, still space. That combination of moisture, darkness, organic material and stagnant air is exactly what mold and bacteria need.

The under-stair cupboard is usually closed in with a timber frame or gypsum board. Behind that board there is a hollow cavity, and water wicks upward into it through the board itself. Even after the visible surface looks dry, the cavity can stay damp for weeks. Garages hold a mixture of materials: hand tools, paint tins, thinner, engine oil, insecticide, garden fertilizer, sports equipment and bicycles. Sunken rooms are the worst of all, because their floor is below the rest of the house. Water drains from them last and sometimes does not drain at all without pumping. Even after the street outside is dry, there may still be standing water in that room for days.

The deeper problem is timing. These rooms are almost always tackled last. In the meantime mold keeps growing, and its spores drift through door gaps and ventilation openings into the living room and bedrooms. The family notices a stale smell everywhere, but nobody can tell where it is coming from. They clean the kitchen drain again, they wash the curtains, they buy air fresheners, and the smell keeps returning, because the source is still sitting behind a closed door.

There is also a psychological side. Storage rooms are full of decisions people do not want to make: whether to throw away a child's first school bag, an old photo album, a parent's documents. It is easier to keep the door shut. Unfortunately, every week of delay makes more of those items impossible to save.

The risks hiding behind the storeroom door

Health

Floodwater that has flowed across roads and through drains can be contaminated with sewage and germs. E. coli is associated with diarrhea and stomach illness. Leptospira bacteria, carried in the urine of rats, cause leptospirosis, which people in Thailand know as rat urine fever. The risk rises when someone with a cut on their hands or feet handles mud without protection. Mold spores from wet boxes and damp walls can trigger allergies, sneezing, a blocked nose and asthma, especially in children and older adults. Those are exactly the symptoms Khun Wittaya's daughter and mother were showing.

Chemicals in the garage

Paint tins, thinner, oil and insecticide that were under water may have leaked or lost their lids. The result is a chemical smell layered on top of the musty smell. Do not try to fix this by mixing cleaning products. In particular, never mix bleach with toilet cleaners, acids or ammonia-based products, because the combination can release dangerous gases. Separate damaged chemical containers, keep them away from children, and dispose of them according to your local authority's guidance.

Money and time

Storage rooms are full of things with sentimental value: old photographs, house documents, children's toys, family camping equipment. Left too long, mold spreads beyond the point where those items can be cleaned. The longer you wait, the more you have to throw away, and the more the musty smell soaks into the structure of the house itself, which makes it harder and more expensive to deal with later.

Electricity

Garages and storerooms often contain low sockets, consumer units, water pumps or chest freezers. Before switching anything back on, ask a qualified electrician to inspect the system. Floodwater and electricity are a dangerous combination, and no amount of disinfection replaces a proper electrical check.

Why the usual fixes do not make the smell go away

• Pouring bleach on the floor: Bleach can kill some germs on the surface it touches, but it does not reach the cavity behind the under-stair wall, the joints in the timber or the air in the room. Its sharp fumes also irritate the nose and lungs, which is the last thing an allergic child needs.
• Running a fan: Fans do help surfaces dry. But in a room packed with boxes, air never reaches inside the boxes or behind the shelving. Worse, a fan can blow mold spores out of the storeroom and into the rest of the house.
• Air fresheners and scent sachets: These only mask the smell. When the fragrance fades, the musty odor is still there, sometimes mixed with the perfume into something even less pleasant.
• Painting over mold stains: If the wall is still damp and the mold has not been treated, the paint will bubble and peel, and the mold will grow back underneath the new coat.
• Closing the door and waiting: A closed, unventilated room becomes more humid, not less. Keeping the door shut is the most reliable way to let mold settle in.

Step one is sorting: keep, clean or discard

Before any disinfection or ozone treatment, take as much as possible out of the room. Wet items left inside are the odor source that will always bring the smell back. Put on rubber gloves, boots and a mask before you start. Work with the door and any windows open. Then sort everything into three groups.

• Discard: Collapsed cardboard boxes, foam, cushions, small rugs, pillows, sleeping bags that soaked for days and now carry black mold, dry food and medicines that touched the water.
• Clean and keep: Plastic storage boxes, metal tools, hard sports equipment, plastic buckets and similar items. Wash them with soapy water, let them dry fully, then wipe or spray them with CHEMGENE HLD4H spray.
• Sentimental items: Photos, documents, title deeds and certificates. Set them aside, dry them in a shaded spot with good airflow, photograph or scan them, and consult a document-recovery specialist for anything truly irreplaceable.

Plywood shelving that has swollen, sagged or shows mold inside the wood should usually be replaced. If the gypsum board enclosing the under-stair cupboard soaked up water, ask a builder whether it needs to be opened up so the frame inside can dry. Opening a damp cavity is often the single most important step in stopping a smell that otherwise lingers for months.

A practical tip from our crews: sort outside the house, not inside. Lay a tarpaulin on the driveway and make the keep, clean and discard piles there. That way you are not carrying wet, moldy items back through the living room, and spores stay outdoors.

The solution: disinfection fogging plus ozone treatment by World Health Disinfection

Once the room is empty and the mud has been washed out, the next job is dealing with what you cannot see: germs on surfaces, mold spores in the air, and odor molecules soaked into walls and timber. WHD uses two steps that work together.

1. Disinfection fogging service with CHEMGENE HLD4H. Using ULV (ultra-low volume) fogging, very fine droplets drift into the corners under each stair tread, behind shelving, into wall joints and into the corners of the garage that a cloth or brush cannot reach. The disinfectant treats the surfaces where floodwater bacteria remain.

2. Ozone treatment service. Ozone is a gas that spreads through an enclosed room. It helps destroy mold spores floating in the air and breaks down the odor molecules absorbed into walls and wood. It is especially well suited to small, poorly ventilated rooms such as under-stair cupboards, where nothing else circulates.

How the service works

1. Survey: We inspect the under-stair cupboard, the garage and the sunken room, measure moisture, and locate the odor sources and mold patches.
2. Clearing and mud-removal advice: We tell you which items should come out, which materials should be removed by a builder, and which points need an electrician's check.
3. Disinfection fogging: CHEMGENE HLD4H is applied by ULV fogging across floors, walls, ceilings and the timber frame under the stairs.
4. Ozone shock treatment: The room is sealed, and the ozone generator is set according to room size and odor level.
5. Ventilation: Doors and windows are opened and air is extracted until the space is safe to re-enter.
6. Re-check: We assess odor and moisture again and advise on how to reorganize storage so the problem does not come back.

Timing matters. Fogging can happen soon after the mud is gone, while ozone treatment works best once the room has started to dry and there is no standing water. In a sunken room that drains slowly, our crew may recommend fogging first, then a short drying period with fans and dehumidifiers, then the ozone shock.

Safety during ozone treatment

• No people, pets or plants may be inside the treated area during ozone treatment. The team seals off the area and tells you exactly how long it will take.
• In homes with older adults and children, plan for them to stay outside the area, or away from the house, during treatment and ventilation.
• Sensitive electronics, collectibles, artworks and rubber items should be removed or covered. The team controls the ozone dose and duration to suit the room and its contents.
• After treatment, the space must be ventilated until the ozone smell has faded and the team confirms it is safe to go back in.
• Cars in the garage should be moved outside before work begins.

Ozone is effective precisely because it is reactive. That is also why it must be handled by a trained team, with the room empty and properly ventilated afterwards. Please do not try to sit in a room while an ozone generator is running.

Reorganizing after recovery so the smell never returns

Once the room is clean and dry, the way you store things again decides whether the smell comes back. Swap cardboard boxes for lidded plastic containers. Raise shelving at least a hand span off the floor. Leave a gap between shelves and walls so air can move. If it is possible, add a vent or a small extractor fan to the under-stair cupboard, and place moisture absorbers that you replace on a schedule.

In the garage, create a dedicated shelf for chemicals and never store paint or oil on the floor. In the sunken room, ask a builder about a drainage channel or a sump with a pump so water leaves faster if the street floods again. Write down how high the water came this year; that number becomes the basis for next year's plan.

Open storage doors once a week on a dry day. If you start to notice a musty smell again, check for damp spots straight away, before they grow. Small, early checks are far cheaper than another full recovery.

For families who keep documents at home, consider a waterproof document box kept on the upper floor. It sounds simple, but it is the single change most of our clients say they wish they had made before the water arrived.

What a realistic recovery week looks like

Families often ask how long all of this takes. Every house is different, but for a home like Khun Wittaya's the sequence usually runs over about a week. On the first weekend, the family sorts the storage rooms on the driveway, bags up what must be discarded and washes the hard items. Early in the week, a builder opens the damp gypsum board under the stairs and removes swollen shelving, while an electrician checks the garage sockets and the pump in the sunken room.

Once the rooms are empty, our team fogs the under-stair cupboard, the garage and the sunken room with CHEMGENE HLD4H. Fans and dehumidifiers then run for a few days to bring moisture down. When the surfaces are drying and there is no standing water, we return for the ozone shock treatment while the family spends a few hours out of the house, followed by thorough ventilation and a re-check.

By the following weekend, the family can start putting cleaned items back in lidded plastic boxes on raised shelves. The important point is the order: remove, clean, disinfect, dry, ozone-treat, ventilate, then reorganize. Skipping a step, or doing them in the wrong order, is the most common reason a musty smell returns a month later.

10 reasons Bang Na households choose WHD for post-flood storage rooms

1. We know that a house's musty smell usually starts in the room nobody opens, so we inspect storage spaces first.
2. We use CHEMGENE HLD4H disinfectant, a product used in professional disinfection work.
3. ULV disinfection fogging reaches the corners under stairs and behind shelving.
4. Ozone treatment reduces airborne mold spores and odors absorbed into walls.
5. Our ozone generators are adjusted to room size.
6. We give step-by-step advice on what to keep, clean or discard.
7. We take care with garage chemicals and never mix hazardous products.
8. Clear safety procedures for homes with children and older adults.
9. Follow-on dust-mite removal service for mattresses in bedrooms where spores may have spread.
10. Easy to reach by phone and LINE; send photos of the room for an initial assessment.

Before and after storage-room recovery

Customer voice (illustrative scenario)

"At first I thought the smell was coming from the kitchen drain. I cleaned it again and again and nothing changed. When the team surveyed the house, they found the real source was the under-stair cupboard and the sunken room we had closed off. After clearing them out, fogging and ozone treatment, the house could breathe again. What I appreciated most was that they also explained how to store things differently."

— Khun Wittaya, detached house, Bang Na (illustrative scenario from field work)

Frequently asked questions

Q: Do I need to empty the storeroom completely first?
A: Remove as much as you can, especially anything wet and anything made of paper or fabric, because those are the main odor sources. Hard items that have been washed can be disinfected separately.

Q: The room is still damp. Can it be ozone-treated right away?
A: It is better to disinfect first and speed up drying. Ozone treatment works best once the room has started to dry and there is no standing water. Our team will advise on the right timing.

Q: My garage has paint tins and oil. Can it still be ozone-treated?
A: Damaged chemical containers should be removed first, and cars should be taken out of the garage. The team will assess the space and choose a safe approach.

Q: Does everyone have to leave the house during ozone treatment?
A: Everyone must leave the treated area and any connected spaces, along with pets and plants, until ventilation is finished and the team confirms it is safe.

Q: The gypsum wall under my stairs has mold. Does it need to come out?
A: If the board soaked up water, sagged or has mold growing into it, a builder should replace it. Fogging and ozone treatment can then treat the frame inside once the wall is open.

Q: Which areas do you cover?
A: Bang Na, Prawet, Phra Khanong, Suan Luang, Bang Phli and nearby areas. Ask us on LINE @whd268.

Open your storeroom door again without holding your breath

Book a survey of your storage rooms, garage and ground floor in Bang Na and nearby areas.

Book Ozone Treatment — Click Here

See Disinfection Service — Click Here

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

Related links

• Residential ozone disinfection
• Ozone treatment for odors and chemical residue
• Master Ozone Generator
• All articles
• External sources: WHO: Floods and health and Thailand Department of Disease Control