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

The office floors are open again, but every time the lift doors open in the lobby, the basement smell comes up

Ake manages a multi-tenant office tower of more than twenty storeys in a central Bangkok business district. The building has two basement levels. The upper basement holds part of the car park, tenant storerooms, the housekeeping room, the security team's locker room and a document store rented by several tenants for years of accounting files. The lower basement holds the plant room, pumps and the building's sump.

In late September 2026 relentless rain left the surrounding streets under water. Public drains could not keep up, water ran down the vehicle ramp into the basement, and more pushed back up through the car-park channel drains. The engineering team ran the pumps flat out and sandbagged the ramp, but water still reached the lowest storerooms and the document store, deep enough to soak the boxes on the bottom shelves.

Once the water was pumped out, housekeeping and maintenance spent days shovelling silt, washing floors and hauling wet boxes into the car park. The office floors reopened, yet tenants began calling about a musty smell in the lobby, especially whenever a lift that serves the basement opened its doors. Security guards using the basement lockers complained of itchy noses and coughs. Several cleaners developed rashes on their arms. When Ake opened the document store a week later, he found greenish-white mold on file spines on the second and third shelves, which had never touched the water.

Basements and storerooms are places most office workers never see. After a flood, though, they become reservoirs of damp, mold and bacteria that affect the whole building. This article looks at why basements recover so slowly, where germs and mold hide, and how after-hours ozone disinfection can help building management.

The late-September 2026 Bangkok floods and buildings with basements

According to news reports, roughly 300 mm of rain fell on Bangkok in about 48 hours between 24 and 27 September 2026, and on 26 September the Bangkok Metropolitan Administration declared all 50 districts a disaster zone. Eastern areas such as Khlong Chan and Bueng Kum were reported as hardest hit, but a city-wide declaration shows how widely Bangkok was affected. Governor Chadchart said canals were full and 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 business report cited a Chamber of Commerce estimate of about 12,300 million baht in damage. Lives were also lost, and our sympathies go to the families. It is a reminder that electrical safety comes first anywhere water has pooled, particularly basements and car parks.

For buildings with basements, the problem does not end when the pumps fall silent. Basements get no sunlight, rely on mechanical ventilation, and their below-grade concrete keeps taking on moisture from the surrounding ground, so damp lingers far longer than on upper floors.

To be clear from the start: ozone does not prevent flooding and it is not medical treatment. It is a step that helps reduce germs and mold in the air and on surfaces, carried out only after water has been pumped out and the space cleaned and dried.

Why office basements and storerooms recover slowly and seed mold through the building

No sun, no natural breeze

A flooded house can at least open its windows. An office basement has almost no openings to the outside, so it depends on extract fans and ductwork. If part of the ventilation system was damaged, or switched off while electrics are repaired, storeroom air goes completely still, humidity stays high, and mold has everything it needs.

Document archives are mold food

Paper files, corrugated boxes, building drawings, ledgers and contracts are all cellulose, which mold loves. Boxes on high shelves may never touch water, but paper readily absorbs moisture from the air. After several days of humid air, the paper is damp too, and mold can appear on spines and box corners. That is why many buildings find mold on documents well above the flood line.

Storerooms nobody clearly owns

Basement storerooms hold a bit of everything: spare ceiling tiles and carpet tiles, old office furniture, uniforms, event equipment, signage and boxes left by departed tenants. Much of it is absorbent. When it gets wet, nobody rushes to remove it because nobody is sure whose job it is, and within days it is a mold nursery.

Below-grade concrete keeps drawing moisture

After days of heavy rain, groundwater around the building rises. Basement walls and slabs can keep taking on moisture through cracks and joints even after all the water inside has been pumped out. Damp lines along joints and white salt deposits on the concrete are signs that moisture is still moving.

Lift shafts and fire stairs carry air to the lobby

In a tall building, lift shafts and fire stairs run from the basement upward, and air moves through them with temperature and pressure differences. Each time a lift that serves the basement opens in the lobby, basement air can come up with it. Musty odours and spores do not stay downstairs; they reach the reception area tenants and visitors use every day.

Basement floodwater is dirty water

Water that pours down a ramp or backs up through drains has crossed roads, sewers and manholes, so it may carry wastewater, vehicle oil, rubbish and rodent waste. When it recedes it leaves silt and sediment laden with bacteria on floors, shelf legs and wall bases. If cleaning misses these, they remain a source of germs for a long time.

Basement germs and mold after a flood, and who is most at risk

The WHO, the US CDC and Thailand's Department of Disease Control agree that flooded buildings should be cleaned and dried as quickly as possible, ideally within 24 to 48 hours, to limit mold. Basements usually take much longer because pumping must finish and electrics must be made safe first, so their risk is higher.

Mold and airborne spores

Mold on paper, wood, fabric and gypsum releases tiny airborne spores. Breathing them can cause allergic reactions, a blocked nose, sneezing, coughing, eye and throat irritation, and can trigger asthma. People who work in the basement daily, or spend long periods searching through files, receive a much bigger dose than others.

Bacteria from sewage and silt

Faecal bacteria can remain on floors and surfaces that touched the water. Not washing hands after contact, or eating in a basement staff room that has not been properly cleaned, can lead to diarrhoea, nausea and vomiting. Cuts that touch dirty silt risk skin infection.

Leptospirosis

Basements connected to drains and sumps are easy for rats to reach. Leptospira bacteria from rat urine enter through cuts or mucous membranes. Anyone who waded or cleaned the basement and then develops a fever, headache, muscle pain (especially in the calves) or red eyes should see a doctor promptly and mention flood exposure.

People who need extra care

• Cleaners and housekeeping staff, who shovel silt and carry wet items by hand.
• Security guards and technicians, who use basement lockers, rest rooms or plant rooms every day.
• Admin and accounts staff, who go down to retrieve files.
• Staff with asthma, allergies or chronic lung disease, who are sensitive to spores.
• Pregnant staff and people with weakened immunity, who should avoid cleaning contaminated areas.

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

What building management has to deal with

Staff health: Housekeepers, guards and technicians who spend every shift underground are affected first. Coughs, congestion, rashes and fatigue lead to sick leave, which hits teams already stretched by the recovery.

Money: Moldy documents may need professional recovery or re-creation, neglected storeroom contents may have to be thrown out, and walls and ceilings where mold spreads must be stripped and replaced. The longer it waits, the higher the bill.

Time: Management is answering tenants, coordinating contractors and handling insurance claims all at once. Re-cleaning the same rooms because mold keeps returning steals time from critical system repairs.

Reputation and leases: Tenants who smell damp in the lobby every morning, or discover their stored boxes have gone moldy, start questioning the quality of building management, and that can surface at lease renewal.

Common approaches, and why basements need more

Washing floors with bleach and stopping there: Properly diluted bleach disinfects smooth floors, but does nothing about spores in the air, inside archive boxes or behind steel racking. In a poorly ventilated basement the fumes linger, and bleach must never be mixed with other products.

Running extract fans only: Fans lower humidity somewhat but do not reduce germs already on surfaces, and if airflow is misdirected they can push storeroom air into lift shafts and corridors.

Locking the storeroom and dealing with it later: Common when teams are overloaded, but sealing a damp room creates an ideal mold incubator. The longer it stays shut, the worse it gets.

Fragrance or charcoal in the lobby: This masks the smell for a while but leaves the source untouched and does not reduce the spores rising from below.

WHD ozone disinfection for office basements and storerooms

World Health Disinfection (WHD) provides residential and building ozone disinfection for homes, condos, hotels, offices, factories, schools and hospitals. According to the service information, ozone (O3) breaks down the cell structures of viruses, bacteria and mold, as well as odour molecules, then reverts to oxygen (O2) with no chemical residue. According to test results certified for the company by Thailand's Department of Medical Sciences and Intertek (UK), it eliminates viruses, bacteria and mold at 99.99%.

In a storeroom, the advantage of a gas is that it reaches the gaps between boxes, behind steel racking, under pallets and up into ceiling corners where wiping is impractical.

How the work runs in an office basement

1. Joint survey with building management: storerooms, the document archive, housekeeping room, lockers and basement lift lobby are inspected for tide marks, wall moisture, mold and ventilation, so zones can be sized correctly.
2. Readiness check: water pumped out, silt removed, surfaces washed and dried; badly damaged or moldy items removed; electrics cleared by an electrician.
3. Scheduling with the building: work is planned outside office hours, such as nights or weekends; tenants with stored goods are informed; engineering isolates the zone's ventilation.
4. Preparing the space: everyone, plus any animals or plants, leaves the zone; doors are shut and vents linking to other areas are closed so the gas stays in place for the required time.
5. Ozone release: a generator distributes ozone through the room; a typical cycle is about 30 minutes per zone, according to the service information.
6. Rest and ventilation: ozone is allowed to revert to oxygen, the area is ventilated, and staff re-enter only when the team advises.

Safety rule, every time: Ozone treatment takes place in an empty, closed space. People, pets and plants must leave the zone and may return only after the rest and ventilation period. In an office building the zone entrance is barricaded and every security shift is briefed so nobody opens the door during treatment.

10 reasons to ozone-treat office basements and storerooms after a flood

1. It reaches between boxes and behind racking, where cloths and sprays cannot.
2. It helps tackle archive mold without spraying liquid onto files.
3. No chemical residue, because ozone reverts to oxygen according to the service information, which suits rooms holding several tenants' goods.
4. It cuts musty odour at the source before it rides the lift shafts to the lobby. See the ozone disinfection service.
5. It fits outside office hours, at about 30 minutes per zone according to the service information.
6. It protects the workplace of cleaners, guards and technicians, the first people exposed to damp and mold.
7. It is backed by testing from the Department of Medical Sciences and Intertek (UK), according to test results certified for the company. View certificates.
8. It gives management something concrete to tell tenants: recovery followed proper steps rather than masking smells.
9. It prepares rooms for repair crews, who then work in better air. Book a basement survey.
10. It is good value across multiple rooms. According to the service information, jobs of 15,000 baht or more include a free medical-grade disinfectant spray for lift buttons and door handles.

Before and after ozone treatment in a basement

Note: results depend on residual moisture and fixing the cause. If basement walls are still seeping or ventilation is not running, mold can return, so leaks should be repaired as well.

Example case: an accounting firm's basement archive in a rented office tower

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

Prae is the admin manager of an accounting firm renting space in a central office tower. The firm also rents a basement store for years of client files. When water entered the basement in late September, the bottom-shelf boxes were soaked while the upper shelves looked fine, so the team took only the wet boxes upstairs to dry.

A week later, a staff member who went down to fetch a file came back sneezing non-stop and reported spots of mold appearing on second-shelf spines. Prae contacted the building manager. The building had an electrician check the power and restart basement ventilation; clearly moldy boxes went to a document specialist for assessment, and the floor and steel racks were washed.

After the WHD survey, the archive and the corridor outside were scheduled for a Friday night. Management briefed every security shift, posted no-entry signs and closed the vent linking the area to the lift lobby. After the cycle and the rest and ventilation period, the room was back in use on Monday morning.

Prae says the musty smell dropped noticeably and staff no longer come back up sneezing. The firm has also switched bottom-shelf storage to lidded plastic boxes and raised the lowest shelf further off the floor. Staff who waded in to carry boxes on the first day were advised to see a doctor if they developed a fever or calf pain.

Basement and storeroom recovery checklist

Safety first

• Nobody enters a basement with standing water until an electrician has isolated power and confirmed it is safe.
• Ventilate rooms that have been shut for a long time before anyone works inside.
• Cleaners wear boots, rubber gloves and suitable masks, and wash hands and treat scratches promptly.

Clean and dry

• Pump out water and remove all silt, especially under racking, in channel drains and in corners.
• Send wet critical documents to a specialist and list damaged items for insurance.
• Remove absorbent and moldy items such as cardboard, carpet tiles and ceiling boards.
• Wash floors, walls and racking with detergent, then dry.
• Restart ventilation and run dehumidifiers in the archive.
• Inspect below-grade walls and pipes for leaks and plan repairs.

On treatment day

• Notify tenants with stored goods and agree access times.
• Brief every security shift, post no-entry signs and barricade the zone.
• Coordinate with engineering to close vents to other areas.
• Confirm no people, animals or plants are in the zone, and re-enter only after the team confirms rest and ventilation are complete.

After treatment

• Keep controlling humidity and check room corners weekly.
• Raise the lowest shelves and consider lidded plastic boxes for key records.
• Keep cleaning lift buttons, stair rails and storeroom door handles.
• Monitor basement staff and send anyone with fever, calf pain or breathing trouble to a doctor.

Frequently asked questions about ozone disinfection for basements after a flood

1. Do documents have to be removed before treatment?

Not all of them, but wet or visibly moldy documents should be assessed by a document specialist first. Discuss critical records or special materials with the team during the survey.

2. Can it be done during office hours?

After hours is recommended, because nobody may be in the zone and access must stay closed for the whole cycle, typically about 30 minutes per zone according to the service information, plus rest and ventilation.

3. The basement is still damp. Can we treat it now?

Dry it first. Ozone comes after pumping, silt removal, washing and drying. If walls are still very wet, mold can return. The team will assess readiness during the survey.

4. Will the lobby smell go too?

Once the basement source is dealt with, the smell rising upstairs usually drops as well. If needed, the lift lobby or other areas can be added to the plan.

5. Does the whole building need to be evacuated?

No, but nobody may be in the treated zone, and routes by which air could reach occupied areas must be closed. The team plans this with building engineering.

6. A staff member who helped carry boxes now has a fever. What should we do?

They should see a doctor and mention flood exposure, especially with calf pain, as this can indicate leptospirosis. Ozone treatment cares for the environment; it is not medical treatment.

Fix the source in the basement before smell and mold reach the whole building

Talk to WHD about surveying your building's basement, storerooms and archives, planning zones and scheduling ozone treatment outside office hours.

Book a Basement Ozone Survey

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

Related links

• Residential and building ozone disinfection
• Disinfection services for buildings and businesses
• Ozone removal of chemical residue and odours
• Contact us

References

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