What happened in Bangkok in late September 2026

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

Reports noted three deaths, and we extend our condolences to the families affected. As of 30 September, some roads such as Si Burapha and Lat Phrao still had 20 to 40 cm of water. Thai media compared the event with the 2011 floods, and one Thai business report cited a Chamber of Commerce estimate of about 12,300 million baht in damage.

Behind those figures is a group that rarely makes the headlines: workers who live in low-cost monthly rental rooms and row houses. Factory staff, construction workers, delivery riders, cleaners and migrant workers often rent ground-floor rooms near canals or in lanes that drain slowly. When the water comes in, everything many of them own is in that one room. When it goes out, they have to return to work immediately, with no time off to repair anything. This guide is written for landlords who need to restore an entire building and for tenants who want to know what they can do to make their room safe again.

A typical scenario (an illustrative situation based on common post-flood experiences, not a review from a specific customer)

Twenty rooms, one shared bathroom block, and mattresses that never dry

Muay owns a two-storey building of twenty rental rooms in a lane in eastern Bangkok, not far from factories and construction sites. The ten ground-floor rooms are small. Most tenants sleep on mattresses laid directly on the floor, with a fan, a rice cooker, a small fridge and a clothes rail inside. Motorbikes park along the front corridor, and at the back of the building is a shared bathroom block and washing area.

On the night the rain would not stop, canal water behind the lane flowed into the ground floor until it was almost halfway up the shins. Some tenants carried belongings upstairs to neighbours. Others were on night shift and could not come back. When they returned, the mattresses, pillows, blankets and clothes left on the floor were soaked with muddy water.

Days later the water was gone, but the problems were not. Daeng, who works the night shift at a factory, told Muay he could not sleep in his room because the musty smell was so strong. Black spots had appeared along the lower walls. The tenant next door had a small child who had started coughing and developed a rash on the legs. The shared bathroom smelled of backed-up drains all the time. Some tenants talked about moving out; others had no choice, because they could not afford a deposit on a new room.

Muay knew that if she did nothing the building would deteriorate quickly, tenants would get sick and empty rooms would be impossible to let. But she was not sure where to begin, what tenants should handle themselves, what was the landlord's responsibility, and whether professional disinfection made sense for budget rooms.

Why budget rental rooms are the hardest to recover after a flood

City floodwater is not clean. It mixes with canal water, drain overflow and sometimes sewage. Agencies such as the US CDC and Thailand's Department of Disease Control warn that it can carry faecal bacteria, diarrhoea-causing viruses and leptospirosis bacteria. These germs remain in mud and absorbent materials after the water recedes. The WHO and CDC recommend cleaning and drying buildings within 24 to 48 hours to limit mold, but for workers' rental rooms, several conditions make that advice very hard to follow.

Small rooms, many people, little ventilation

Many budget rooms have a small window or only a vent near the ceiling, and some house two to four people. Drying clothes indoors, cooking rice and the breathing of several people in a closed space keep humidity high even in normal times. Add walls and floors still damp from the flood, and mold grows very quickly.

Floor mattresses are the number one risk

Many tenants sleep on mattresses placed directly on the floor. Once submerged, foam or coir mattresses almost never dry from the inside and become a mold source right next to the sleeper's face all night. Many people sun them for a day and use them again because they cannot afford a new one, an understandable but dangerous choice.

Shared bathrooms and washing areas

A bathroom shared by dozens of people is where flood germs spread most easily. If drains backed up, floors, lower walls, water scoops and taps can all be contaminated. Washing areas often have shared basins and hangers too.

Tenants go straight back to work

Day labourers and shift workers cannot take long breaks. Some only come back to sleep, so cleaning happens in whatever time is left after work, and rarely reaches every corner.

Who is most at risk in a rental building

• Young children, who crawl and play on the floor and put their hands in their mouths.
• Pregnant women, whose immune systems change during pregnancy.
• Older relatives living with their children in the room.
• People with chronic conditions such as asthma, allergies, diabetes or weakened immunity.
• Anyone who waded through floodwater to work or to move belongings. A high fever, headache, calf pain, red eyes, diarrhoea or an infected wound means seeing a doctor promptly, as these can signal leptospirosis or other infections. Workers covered by social security can use their entitled hospital or clinic.

Losses that landlords and tenants share

Health. Tenants who fall ill from mold or a stomach infection often keep working, because a day off means a day without pay. Minor symptoms can become serious, and in a shared room, one sick person can easily pass it on.

Tenants' money. Many lost mattresses, clothes, appliances and important documents at once. Replacing everything while income is interrupted is a heavy burden.

Landlords' money. Musty, stained rooms are hard to let. Existing tenants may ask for lower rent or leave, and if damp is allowed to damage walls, door frames and floors, later repairs cost many times more than handling things properly now.

Time. Painting over mold or laying new vinyl on damp floors usually means doing the job again within months.

Families. For tenants living with children or elderly parents, the rental room is the family's entire home. When it is not safe, everyone in it is affected at the same time, and parents who work long shifts have little time to watch for early warning signs in a coughing child.

The building's reputation. Workers' rentals rely on word of mouth among co-workers and online groups. A building known as "smelly, people get sick there" will struggle to find tenants for a long time.

Why common low-cost fixes do not end the problem

Sunning the mattress for a day and using it again. The outside may look dry, but the core stays damp and mold keeps growing quietly under the sheet.

Splashing strong bleach everywhere. Properly diluted bleach sanitises cleaned hard surfaces, but overly strong solutions in a small, poorly ventilated room sting the eyes and nose, and mixing bleach with some bathroom cleaners can release dangerous gas. It also does nothing for airborne germs or fabrics.

Mosquito coils or air freshener. They mask smells briefly but add chemicals to a small room where several people sleep, and do not reduce germs.

Painting over mold to re-let the room. The most common shortcut in rental buildings because it is cheap and fast. If the wall is still damp inside, stains return within weeks, and fresh paint adds chemical odour to a poorly ventilated room, which may need separate ozone chemical-residue removal.

Leaving each tenant to sort out their own room. It sounds fair, but if one room is cleaned well and the next is not, and the corridor and shared bathroom are still contaminated, mold and smells keep coming back. Rental buildings have to be restored as a whole.

How ozone disinfection works for rental rooms and row houses

Ozone (O₃) is a strongly oxidising gas. Released at a controlled concentration in a closed room, it spreads through the air, under beds, behind cupboards, into wall corners and through bathrooms. It breaks down the cell structures of viruses, bacteria and mold, along with the molecules behind musty odours, then reverts to oxygen (O₂). No chemical residue is left in small rooms where tenants eat, sleep and live.

According to the test results certified for the company, World Health Disinfection's service has been tested by Thailand's Department of Medical Sciences and Intertek UK, eliminating viruses, bacteria and mold by 99.99%. See the certificates page for details.

Three rules landlords and tenants must agree on

One: rooms must be empty during treatment. No people, pets or plants in any room or zone being treated, and nobody re-enters until technicians confirm the rest and ventilation period is over. In buildings where night-shift tenants sleep during the day, times must be agreed in advance and every room checked before starting.

Two: remove mud and dry first. Ozone does not replace mud removal, floor washing, laundry or discarding contaminated mattresses. A mattress that was submerged and cannot be fully dried should be thrown away, not treated and reused.

Three: ozone is not medical treatment. Anyone with symptoms after wading through floodwater must see a doctor rather than wait for a clean room to make them better.

The whole-building process

1. Building survey. Count rooms, shared bathrooms, washing areas, corridors and storage, and identify the worst-hit rooms.
2. Schedule with the landlord. Choose a day when most tenants are out, such as a day off, and group rooms by block or floor.
3. Room preparation. Tenants pack food, medicines and valuables as advised, open cupboards and drawers, close windows and leave.
4. Ozone release in sealed rooms, typically about 30 minutes per zone depending on size and conditions. Neighbouring small rooms can sometimes be treated as one zone.
5. Rest and ventilation, letting ozone revert to oxygen before doors and windows are opened. Tenants return when technicians say it is safe.
6. Handover with advice on ventilation, drying laundry and caring for the shared bathroom.

For landlords treating a whole building, ozone jobs of 15,000 baht or more come with a free medical-grade disinfectant spray, useful for ongoing wiping of taps, handles and stair rails in common areas. Learn more about the residential and building ozone disinfection service.

10 reasons landlords choose whole-building ozone after a flood

1. Rooms and common areas in one round, so shared bathrooms and corridors do not become places where germs return.
2. No chemicals left in small, poorly ventilated rooms, because ozone reverts to oxygen.
3. Reaches corners tenants have no time to clean, behind cupboards, under shelves and up at the ceiling. See how on the ozone disinfection service page.
4. Removes the musty smell that keeps rooms empty, at its source rather than masking it.
5. About 30 minutes per zone, with rooms grouped so tenants are out only briefly.
6. Helps reduce ongoing illness in densely occupied buildings.
7. Protects long-term building value when combined with fixing the source of damp.
8. Backed by testing, according to results certified for the company by the Department of Medical Sciences and Intertek UK.
9. Builds trust with current and future tenants that the owner cares about their health.
10. Good value at building scale, plus a free disinfectant spray when the job qualifies. Ask through the WHD ozone service.

Before and after a whole-building recovery

Example case: the Sunday the whole building worked together

This example case is a hypothetical scenario used to explain the process. It is not a review or quote from a real customer.

Muay started by posting a notice at the entrance and messaging the tenants' chat group with a whole-building recovery plan. She asked multilingual tenants to help translate it for neighbours less comfortable reading Thai, so every room understood what would happen and when.

Week one. She hired an electrician to inspect the ground-floor wiring and warned tenants not to use extension cords that had been underwater. A truck collected damaged mattresses and furniture. She cut ground-floor rent for that month so tenants could afford new mattresses. Tenants washed their own floors after work, while Muay paid workers to clear drains and clean the shared bathroom and corridors.

Week two. Rooms with damp walls kept windows open, with fans blowing outwards during the day. A builder found that a corner room was leaking from a broken gutter, so that was repaired first.

Sunday. With most tenants off work, the ozone team arrived late morning. Tenants packed food and medicines into boxes, opened cupboards and gathered at a rice shop at the end of the lane. Technicians checked every room to make sure nobody was asleep inside, including the night-shift tenants, who had agreed to rest at relatives' homes. Rooms were treated group by group, followed by the shared bathroom and corridors. After rest and ventilation, tenants moved back in during the afternoon.

In a case like this, what changes is not only the smell but tenants' sense that their landlord has not left them alone with the problem. For the family whose child had a rash and cough, Muay suggested seeing a doctor at a nearby clinic, because a clean room does not replace treatment.

Rental recovery checklist by role

What tenants can do now at little or no cost

• Wear boots or closed shoes and rubber gloves when cleaning, and wash hands before eating.
• Do not plug in sockets or extension cords that were underwater until someone checks them.
• Throw away food, drinking water and medicines touched by floodwater.
• Set aside submerged mattresses, pillows and blankets; do not sleep on them if the core is still damp.
• Wash flood-soaked clothes with detergent and dry them fully outdoors.
• Open windows and doors during the day for airflow.
• Keep wet important documents in a bag and contact the issuing office about replacements.

What landlords should take responsibility for

• Inspect electrical systems, water pipes and drains.
• Clean and disinfect shared bathrooms, washing areas and corridors.
• Fix leaks and wall damp before repainting.
• Arrange removal of bulky waste such as mattresses and furniture.
• Communicate plans and dates clearly to every tenant, including those less comfortable in Thai.
• Book whole-building ozone treatment once all rooms are clean and dry.

Treatment day and after

• Check every room for people, pets and plants, including night-shift workers sleeping during the day.
• Post "do not enter" signs on doors and the stairway.
• Return only when technicians confirm rest and ventilation are complete.
• Dry laundry outside when possible and open windows daily so damp does not build up again.
• Anyone with fever, calf pain, diarrhoea or a rash that will not clear should see a doctor promptly.

Frequently asked questions

Can a single tenant book ozone for one room?

Yes, but if neighbouring rooms, corridors and shared bathrooms are untreated, smells and mold may return. Talking to the landlord about treating the whole building together is usually more effective and better value.

Do we have to move everything out?

No. The team will advise what to pack or move. Typically food and medicines go into sealed boxes, pets and plants leave, and cupboards are opened. Most important of all, nobody stays in the room during treatment.

Can a flooded mattress be used again after ozone?

If it was submerged in contaminated floodwater and the core is still damp, do not use it, even after ozone. Ozone reduces germs and odour in clean, dry materials, but does not make a wet mattress core safe.

How do we schedule around night-shift tenants?

Arrange times in advance and ask them to rest elsewhere during treatment, or place their rooms in a round that suits when they are awake. Technicians check every room before starting to make sure no one is inside.

How long do tenants need to stay out?

Treatment typically takes about 30 minutes per zone plus preparation, rest and ventilation. A medium-sized rental building can usually be completed in a day when rooms are grouped systematically, and technicians give each group a clear return time.

What should a tenant do if they fall ill after the flood?

See a doctor promptly, especially after wading through floodwater and then developing fever, calf pain, red eyes or diarrhoea. Workers with social security or universal coverage can use their entitled hospital or clinic. Ozone treatment improves the environment; it is not medical care.

Will mold stay away after ozone?

Ozone deals with the mold and odour present at the time. If the damp source remains, such as leaking walls, broken gutters or constant indoor laundry drying, mold can return. Always fix moisture at the same time.

Make your rental rooms a safe home for working people again

Talk to our team about a whole-building ozone disinfection plan, grouped so tenants are out of their rooms for as short a time as possible.

Book Ozone Disinfection

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

Related services and reading

• Residential and building ozone disinfection
• Ozone removal of chemical residue after repainting rooms
• Chemgene HLD4H medical-grade disinfectant
• Contact World Health Disinfection

References

• 2026 Bangkok floods – Wikipedia
• Al Jazeera: Disaster zone declared across Bangkok
• CDC – Floods and cleanup
• Department of Disease Control, Thailand