A typical scenario (an illustrative situation based on what is commonly seen, not a review from any specific customer)

The roller shutter is open again, but the musty smell has climbed all the way to Mum's bedroom

Tom is the second generation to run a grocery store from a four-storey shophouse in an older neighbourhood on Bangkok's eastern side. His father bought the unit decades ago. The ground floor is the shop, with a mezzanine for stock and a small kitchen at the back that was extended out towards the rear alley. The second floor holds the living room and the bedroom of Tom's mother, who is in her seventies. Tom, his wife and their two daughters sleep on the third floor, and the fourth floor is a shrine room and storage. One narrow wooden staircase rises straight up from behind the counter and connects every level.

In late September 2026 the rain did not stop for days. Water crept up the street, spilled over the kerb and slid under the steel roller shutter. More water pushed up through the floor drain in the back kitchen, bringing grease from the grease trap, bits of rubbish and the unmistakable smell of the sewer. Cartons of water, rice sacks and boxes left on the floor were soaked.

When the water went down, everyone shovelled mud, hosed the floor, scrubbed the shelving and hauled ruined boxes out to the street. The shop reopened and regulars came back. Yet every evening, when Tom pulled the shutter down, a damp, stale smell drifted up the stairwell to the second floor. His mother began coughing at night. His youngest daughter sneezed every morning. And under the staircase, black dots reappeared on the old plaster wall only a week after it had been scrubbed clean.

Tom's problem is not unusual. Countless shophouses and commercial buildings in Bangkok's older districts share the same pattern of life: business on the ground floor, home above. When the ground floor floods, germs and moisture do not stay behind the counter. They have a direct route up to the bedrooms of every generation in the household. This article explains why, and where zone-by-zone ozone disinfection fits into recovery.

The late-September 2026 Bangkok floods and the shophouse districts

According to news reports, continuous heavy rain of roughly 300 mm fell on Bangkok in about 48 hours between 24 and 27 September 2026. On 26 September the Bangkok Metropolitan Administration declared all 50 districts a disaster zone. Eastern Bangkok was hit hardest, with areas such as Khlong Chan and Bueng Kum mentioned in coverage. Governor Chadchart said the canals were full and that drainage would take two to three days after the rain stopped.

As of 30 September, reports said some roads, including Si Burapha and Lat Phrao, still had 20 to 40 cm of standing water. Thai media compared the event with the 2011 floods, and one Thai business report cited a Chamber of Commerce estimate of around 12,300 million baht in damage. Lives were also lost, and our thoughts are with the families affected.

One point must be clear from the start: ozone treatment does not prevent flooding and it is not medical treatment. It is a tool that helps reduce the risk from germs and mold in the air and on surfaces during recovery, and only after mud has been removed and the building has been washed and dried.

Why a shop-below, home-above building carries more risk than people expect

Most shophouses in Bangkok's older districts follow the same design: a narrow frontage, a deep floor plan, windows only at the front and back, and party walls shared with the neighbours on both sides. That layout gives far less cross-ventilation than a detached house, especially in the middle of the building where air barely moves. After a flood, humidity lingers much longer, which is exactly what mold needs.

The stairwell works like a chimney

The staircase in a shophouse is usually an open shaft from the ground floor to the top. Warm, humid air rises naturally, so the stairwell carries moisture, musty odours and mold spores from the shop up to the living room and bedrooms. At night, with the shutter closed, that air has only one way to go: up. This is why many families find that the person sleeping on the second floor develops allergy symptoms even though the water never reached that level.

Mezzanines and stock corners with still air

The mezzanine above the shop has a low ceiling, still air, and is often packed with cardboard cartons, fabric sacks and stock that was rushed up there as the water rose. Cardboard and textiles are ideal food for mold, and when they are stacked tightly in a stagnant corner, mold can start growing within days and release spores back down into the shop or up the stairs.

Back kitchens, grease traps and rear alleys

Shophouses that sell food, or simply have a kitchen behind the shop, carry an extra layer of risk. Water backing up through floor drains is usually mixed with wastewater, grease and food waste. The rear alleys behind older shophouse rows are also routes for rats and cockroaches, so their urine and droppings can be carried in with the floodwater. That can include the bacteria behind leptospirosis.

Old plaster that wicks water above the flood line

Older shophouses are often finished with porous cement plaster, and many have no damp-proof course at the base of the walls. When water stands for days, the wall draws moisture upward by capillary action, well above the actual water level. Such walls dry slowly and grow black spots weeks later.

The germs and mold to know about, and who is most at risk

Urban floodwater is not clean water. It flows across roads, through drains and manholes and over rubbish. The WHO, the US CDC and Thailand's Department of Disease Control all warn that floodwater can be contaminated with sewage, and they advise cleaning and drying buildings as quickly as possible, ideally within 24 to 48 hours, to limit mold growth.

Sewage and faecal bacteria

When wastewater backs up into a kitchen or ground-floor bathroom, faecal bacteria can be left on floors, walls, table legs, shelving and kitchen equipment. Missed spots can cause diarrhoea and vomiting. Shops that sell food or drinks need to be particularly careful, because the risk extends beyond the family to every customer who walks in.

Leptospirosis

Shophouse neighbourhoods with back alleys and old drains often have resident rats. Leptospira bacteria from rat urine enter the body through wounds, scratches, or the eyes and mouth. Anyone who waded through water or walked in mud while cleaning the shop and then develops a high fever, headache, muscle pain (especially in the calves) or red eyes should see a doctor promptly and mention flood exposure. Do not wait for it to pass.

Mold in walls, cardboard and furniture

Common indoor molds after flooding grow readily on cellulose materials such as paper, wood, fabric and gypsum ceiling boards. Spores become airborne, and breathing them in can trigger allergies, a blocked nose, sneezing, coughing, eye irritation and asthma attacks. In people with weakened immune systems, heavy exposure can raise the risk of more serious infection.

Who in a shophouse is most vulnerable

• Older relatives, who often sleep on the second floor to avoid climbing too far and are therefore closest to the shop and the stairwell.
• Young children, who crawl and play on floors and put their hands in their mouths.
• Pregnant women, who should avoid exposure to germs and to strong chemicals during the clean-up.
• People with chronic illness, such as diabetes, heart disease, lung disease or weakened immunity.
• Customers, particularly at food shops, pharmacies and shops frequented by children or older people.

Anyone with a fever, calf pain after wading, persistent diarrhoea, breathing difficulty or other worrying symptoms must see a doctor. Ozone treatment looks after the environment; it does not replace medical care.

How the damage shows up, in the shop and in the home

Health: The family upstairs breathes air rising from the ground floor every night. Lingering coughs, stuffy noses, itchy skin and poor sleep tend to creep in gradually, so many households never connect them to damp and mold left by the flood.

Money: Most shophouse businesses are family operations living on daily takings. Every extra day with the shutter down is income lost immediately. Stock kept in damp corners can go moldy and must be thrown away, and if mold is allowed to spread inside walls or under timber floors, the eventual repair bill can be many times what early action would have cost.

Reputation: Regulars in old neighbourhoods have known the shop for years, but a musty smell at the door or mold on a shelf erodes trust instantly, and one photo on social media can damage a business built over decades.

The usual fixes, and why they fall short

Pouring bleach on the floor and wiping the walls: Correctly diluted bleach can disinfect smooth surfaces, but it cannot reach the air in the stairwell, the mezzanine, the underside of timber floors or the pores of old plaster. Bleach must never be mixed with other cleaning products because dangerous gases can form.

Running fans all day: Fans genuinely speed up drying, but in a shophouse with limited airflow they can simply circulate damp air and spores around the building, or push more of them up the stairwell, unless they are positioned to exhaust air outdoors.

Painting over mold: If the wall is still damp and mold remains in the plaster, the paint blisters, peels in sheets, and the black stains bleed through again before long.

Cleaning only the ground floor: This is the most common mistake in shophouses. People assume that because only the ground floor flooded, only the ground floor needs attention, when damp air and spores have already settled into bedrooms, curtains, mattresses and clothes upstairs.

WHD ozone disinfection for shophouses and commercial buildings

World Health Disinfection (WHD) provides residential and building ozone disinfection, which suits buildings that combine several floors and several uses under one roof, like a shophouse. According to the service information, ozone (O3) is a strongly oxidising gas that breaks down the cell structures of viruses, bacteria and mold, as well as odour molecules, and then reverts to oxygen (O2) without leaving a chemical residue. According to test results certified for the company, from Thailand's Department of Medical Sciences and Intertek (UK), it eliminates viruses, bacteria and mold at 99.99%.

The advantage of a gas is that it spreads through an enclosed space and reaches the corners under the stairs, the mezzanine, the gaps between shelving units, the space behind display fridges and other places that cloths and brushes cannot, and shophouses have plenty of those.

How the work is done in a shophouse: one zone, one floor at a time

1. Site survey: The team walks every level, from the shopfront, mezzanine, back kitchen and bathroom to the stairwell and upstairs bedrooms, noting tide marks, wall moisture and visible mold so the building can be divided into zones.
2. Readiness check: Mud must be removed and the space washed and dried. Badly damaged, sewage-soaked materials, such as flooded mattresses or moldy cartons, should already be out of the building.
3. Preparing the space: People, pets and plants leave the zone completely. The roller shutter, windows and openings are closed, and fans and air conditioners are switched off so the gas stays in place for the required time.
4. Ozone release: An ozone generator distributes the gas through the zone. A typical cycle is about 30 minutes per zone, according to the service information, covering preparation, filling and contact time.
5. Rest and ventilation: After the cycle the space must rest and be ventilated until the ozone has reverted to oxygen. Only then may people re-enter, as advised by the team.
6. Floor-by-floor sequencing: In shophouses the team often plans the order so the family still has somewhere to be during the day, for example the shop and mezzanine first and the bedrooms later, or the whole building on a day the family can stay elsewhere.

Safety rule, every time: Ozone treatment is carried out in an empty, closed space. People, pets and plants must leave the zone and may return only after the rest and ventilation period. In a shophouse where one staircase links every floor, nobody should remain on any floor that shares air with the zone being treated.

10 reasons to ozone-disinfect a shophouse before life fully returns to normal

1. It follows the stairwell, the building's main air route. The gas travels the same path damp air used to take, so it treats the route germs travelled.
2. It reaches low mezzanines and stock corners where it is hard for anyone to scrub.
3. No chemical residue on shelving, because ozone reverts to oxygen according to the service information, which suits shops selling food and daily goods once shelves have been washed as normal.
4. It treats upstairs bedrooms that never flooded but breathed the shop's air, the area most often overlooked. See the residential ozone service details.
5. Short cycles of around 30 minutes per zone, according to the service information, help keep closure time to a minimum.
6. Backed by testing from the Department of Medical Sciences and Intertek (UK), according to test results certified for the company.
7. Odour drops along with germs, because ozone breaks odour molecules at the source instead of masking them, and customers notice the difference at the door.
8. Designed for mixed-use buildings. The team can separate shop, kitchen and living zones to match how the building is actually used. Book a survey of your shophouse.
9. Something concrete to tell customers and family: the shop has been cleaned and disinfected in a proper sequence.
10. Better value when the whole building is treated. According to the service information, ozone jobs of 15,000 baht or more come with a free medical-grade disinfectant spray for wiping high-touch points in the shop afterwards.

Before and after ozone treatment in a shophouse

Note: results depend on the building's condition, remaining moisture and whether the source of damp has been fixed. If walls are still wet or water is still leaking in, mold can return.

Example case: a pharmacy in a three-storey shophouse near Lat Phrao

Example case (an illustrative scenario to explain the process, not a verified customer review)

Nok is a pharmacist who runs a pharmacy in a three-storey shophouse. The shop is on the ground floor with a medicine store and bathroom behind it; the second floor is the living room and her mother's bedroom; Nok and her husband sleep on the third floor. The road outside held water for several days, in line with reports that Lat Phrao Road still had standing water. The water was shallow but stayed long enough to soak the base of the walls and cabinets.

After the water receded, Nok closed for two days. She discarded any medicine boxes that had touched floodwater, washed the floor and cabinets, and ran an extractor fan blowing outdoors until everything was noticeably drier. Even so, the medicine store at the back still smelled musty, and her mother said the second-floor bedroom felt damp at night.

After the survey, the team divided the building into three zones: the shop with the medicine store, the stairwell with the second floor, and the third floor. The family stayed with relatives for a day, and medicines were stored in sealed containers as advised. Each zone was treated in turn, rested and ventilated according to procedure, and the family came home in the evening.

Nok says the clearest difference was the musty smell in the medicine store and on the stairs, which dropped considerably, and her mother sleeps better. She kept using the disinfectant spray from the promotion on the counter and high-touch points, and booked a builder to repair the plaster where tide marks remained. She still tells any customer with a fever or calf pain after wading to see a doctor.

Shophouse recovery checklist: before and after ozone

Safety first

• Cut power to the ground floor before entering any area with standing water, and have an electrician check sockets, switches, the consumer unit and the roller-shutter motor before use.
• Wear boots, rubber gloves and a mask when shovelling mud and moving wet goods.
• Wash hands with soap after every work session and dress any scratch immediately.

Clean and dry

• Remove all mud, especially from the shutter track, under shelving and around the kitchen drain.
• Wash floors and walls with clean water and detergent, scrubbing stained areas.
• Remove cardboard, mattresses, rugs and other porous items that were soaked with sewage or have gone moldy.
• Open the front and back for cross-flow, aim fans to blow air outdoors, and use a dehumidifier if available.
• Check upstairs bedrooms, wash curtains and bedding, and air mattresses that smell musty.

On treatment day

• Arrange in advance where family members, older relatives, children and pets will stay.
• Take plants out of the building and store food, medicine and anything you want protected in sealed containers.
• Switch off air conditioners and fans as the team advises.
• Re-enter only after the team confirms the rest and ventilation period is complete.

After treatment

• Keep windows open for a while, then wipe food-contact surfaces as usual.
• Watch previously damp walls for several weeks; returning stains mean moisture or a leak still needs fixing.
• Repair plaster and repaint only once walls are completely dry.
• Watch for symptoms in the household and see a doctor for fever, calf pain, diarrhoea or breathing difficulty.

Frequently asked questions about ozone disinfection for shophouses after a flood

1. Only the ground floor flooded. Do the upper floors need treatment?

It depends on the actual conditions, but in a shophouse with an open staircase, damp air and spores have usually already risen. The team will survey and recommend which floors to include, especially bedrooms used by older relatives and children.

2. How long will the shop have to close?

A typical cycle is about 30 minutes per zone according to the service information, plus rest and ventilation. The number of zones depends on the building, so most shops schedule treatment on a closing day. The team will estimate timing before starting.

3. Can ozone replace cleaning?

No. Ozone comes after mud removal, washing and drying. Remaining mud and dirt shield germs, and remaining moisture lets mold come back.

4. Can an older relative stay upstairs while the shop is treated?

Not in a shophouse where the staircase connects the floors, because the gas can rise up the stairwell. Treatment must be done with no people, pets or plants present, and re-entry happens only after the rest and ventilation period.

5. Someone at home has a fever and calf pain after wading. What should we do?

See a doctor right away and mention the flood exposure, as these can be signs of leptospirosis. Ozone treatment cares for the environment; it is not medical treatment.

Get the shop trading again, and let the bedrooms upstairs breathe easy

Talk to the WHD team about a survey of your shophouse or commercial building. We will plan zones by floor and use, and schedule ozone treatment to disrupt your business as little as possible.

Book a Shophouse Ozone Survey

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

Related links

• Residential and building ozone disinfection
• Ozone removal of chemical residue and odours after repainting or repairs
• Chemgene medical-grade disinfectant
• Contact us

References

• 2026 Bangkok floods (Wikipedia)
• Al Jazeera: Disaster zone declared across Bangkok
• CDC: Floods
• WHO fact sheets