A typical scenario (an illustrative story based on situations we see often, not a review from a specific customer)

Three weeks after the water left, the house in Ayutthaya started to smell

The two-storey wooden house sits beside a canal in Bang Ban district, Phra Nakhon Si Ayutthaya. The flood reached almost a metre and stayed for more than ten days. Wilai, a retired teacher of sixty-three, moved upstairs with her daughter and four-year-old granddaughter while the ground floor sat under brown water. When the road finally dried out, the whole family went to work: shovelling mud, hosing the floors, scrubbing the walls with bleach and running electric fans day and night. After a week the house looked clean again. Everyone felt the worst was over.

In the third week, things began to change. First came a grey-green patch the size of a coin in the corner of the gypsum wall behind the wardrobe. Then a brown water ring appeared on the kitchen ceiling, dotted with black specks as if someone had shaken pepper over it. The parquet floor in the living room lifted slightly at the joints, and walking across it released a smell somewhere between stale bread and wet soil. Worst of all, the little girl started sneezing every morning with a constantly running nose, and Wilai, who already had allergies, developed a dry night-time cough that kept her awake.

Her daughter tried everything she found online. Vinegar on the stains. Lavender air freshener. Bamboo charcoal in every corner. The stains faded for a few days, then returned. The smell disappeared for an hour, then came back stronger. The family kept asking the same question: we cleaned everything, so why is the mold still here?

The answer is that mold in a house after a flood does not live only on the surfaces you can see. It lives inside materials that are still wet, in the cavity behind the wall, on the top side of ceiling boards and under floorboards, places no cloth or spray bottle can reach. This article explains the two mold groups most often found after flooding, Aspergillus and Penicillium, why common fixes fail, and the correct order of recovery, in which ozone treatment is the final step that reaches the hidden spaces.

Know the enemy: what are Aspergillus and Penicillium?

Mold spores are everywhere, indoors and outdoors, all year round. Normally they cause few problems because they lack the moisture they need to germinate. A flood changes that overnight. Walls, floors, ceilings, furniture and anything made of absorbent material turn into a giant culture dish with the three things mold needs: water, food and the warm temperatures typical of Thailand.

Aspergillus: the mold that loves building materials and house dust

Aspergillus is a genus with many species. It often appears as yellow-green, grey-green or black growth and thrives on the paper facing of gypsum board, curtains, upholstery and the dust that collects in corners. Its spores are extremely small, stay airborne for a long time and can travel deep into the lungs. Healthy people who inhale small amounts usually have no symptoms. People with allergies or asthma, and people with weakened immune systems, such as patients on chemotherapy, people taking immune-suppressing medication or those with chronic lung disease, can develop allergic reactions to the mold or fungal lung infections that need medical treatment.

Penicillium: the blue-green mold behind the musty smell

Penicillium is another genus found in almost every flooded home. It usually looks like fuzzy blue-green or dull green growth on damp walls, wallpaper, carpets, mattresses and wood. It grows even in cooler conditions and releases huge numbers of spores whenever it is disturbed, for example when someone wipes it with a cloth or sweeps near it. It also releases volatile compounds responsible for the familiar musty, basement-like, wet-earth smell. If you can smell that odour but cannot see any stains, mold is almost certainly growing somewhere hidden.

Why mold spreads so quickly after a flood

Public health agencies such as the World Health Organization and the US Centers for Disease Control and Prevention give the same advice: clean and dry a flooded building within 24 to 48 hours to limit mold growth. In real Thai floods, especially in the low-lying central provinces like Ayutthaya, Pathum Thani and Nonthaburi, water often stands for days or weeks. By the time owners can get back inside, mold is already established, and many materials keep holding water long after their surface looks dry.

• Gypsum and drywall partitions: the gypsum core soaks water like a sponge, the paper facing is ideal food for Aspergillus, and moisture wicks upward well above the flood line.
• Ceilings: even when water never reached them, humidity rising from the wet floor collects in the dark space above the boards.
• Wood and laminate floors: water trapped under planks and in the underlay evaporates very slowly, so mold grows out of sight until boards cup or discolour.
• Plastered brick walls: they absorb water into their pores and release it for weeks, causing paint to blister alongside white salt deposits and mold.
• Plywood, particle-board cabinets and built-ins: they swell and hold moisture on the back side against the wall, a reservoir that is easy to forget.

Symptoms to watch, and who is most at risk

Living with indoor mold can irritate the airways and cause a blocked or runny nose, sneezing, coughing, sore throat, itchy or watery eyes, skin rashes and, in some people, wheezing or asthma attacks. Headaches, tiredness and poor sleep are also common complaints. Take extra care with:

• Young children, whose lungs and immune systems are still developing and who crawl and play close to the floor where spores settle.
• Older adults, who often have chronic conditions and lower resistance.
• Pregnant women, who should reduce exposure to any irritant.
• People with chronic illness such as asthma, allergies, COPD, diabetes or immune deficiency.

Important: if anyone has breathing difficulty, high fever, a persistent cough or clearly worsening allergies, see a doctor straight away and mention that your home was flooded. Anyone who waded through floodwater and then develops fever and muscle pain, especially in the calves, must seek medical care promptly because of the risk of leptospirosis. Ozone treatment improves the home environment. It is not a treatment for any illness.

The real cost: more than a black stain on the wall

Health

Breathing spores every day keeps the body in a constant state of irritation. Family members may not become seriously ill, but they never quite get well. Children miss school, adults take time off for doctor visits, and the cost of antihistamines, inhalers and clinic fees keeps growing while the source stays in the house.

Money

Mold does not stay still. It digests gypsum paper, wood fibre and flooring adhesive. Damage that could once be fixed by replacing the lower strip of a wall can, after a few months, mean tearing out entire walls, ceilings or floors. Repair costs multiply. Furniture, mattresses and clothes stored in wardrobes are damaged too, because spores drift through the whole home.

Time and peace of mind

Scrubbing stains every week, moving bedrooms to escape the smell and worrying about the children wear a family down physically and emotionally. Many people say their home no longer feels like a safe place. Stress after a disaster is real and deserves attention.

Property value and reputation

For landlords, dormitory owners, homestays and shops, a musty smell and visible mold are the first things tenants and customers notice. A few reviews saying "the room smells damp" can hurt income for a long time. For owners planning to sell, signs of mold give buyers a reason to negotiate hard on price.

Family

The people hit hardest are usually those who stay home all day: grandparents and small children. Working family members may barely notice, until someone at home is sick far too often. Restoring the house properly is a direct way of looking after the people you love.

Why the usual fixes don't finish the job

Most families are not careless. Many clean harder than ever. The trouble is that common methods deal with what is visible while the source stays intact.

Bleach fades the stain, not the roots. Bleach works on hard, non-porous surfaces like tiles and plastic. On gypsum, wood or plaster it lightens the colour on top while hyphae buried in the material survive, and the water in the solution adds fresh moisture that helps mold regrow. Mixing bleach with ammonia or acidic cleaners can also release toxic gas.

Fans dry the surface and spread the spores. Ventilation matters, but a fan aimed at a moldy wall blows spores into bedrooms, curtains, mattresses and air-conditioner coils. One problem wall becomes a whole-house problem, and the wall core and the space under the floor stay wet.

Air fresheners and charcoal only mask. Fragrance covers the odour, and charcoal or baking soda absorbs a limited amount of it, but neither affects the mold that keeps producing the smell. Some people also react to the chemicals in fresheners.

Painting over mold seals it in. This is the most common and most expensive mistake. Paint traps moisture inside a damp wall, mold keeps growing underneath, and within months the paint blisters and black spots bleed through. You pay twice.

Cleaning only what you can see. Behind wardrobes, under stairs, above the ceiling, under the sink, inside AC ducts and beneath floorboards are dark, damp and rarely touched, which is exactly why mold thrives there and keeps releasing spores into the air.

Effective recovery needs three things: remove badly damaged materials, dry the structure all the way through, and disinfect the entire space, including the air and the corners hands cannot reach. That last step is where ozone is especially useful.

WHD residential ozone disinfection: the step that completes flood recovery

World Health Disinfection (WHD) provides residential and building ozone disinfection for homes, condominiums, hotels, offices, factories, schools and hospitals. Ozone (O3) is a strongly oxidising gas. Released into a closed space, it spreads in every direction: through the air, into corners, behind furniture, under beds and into gaps no cloth or spray can reach.

Ozone breaks down the cell structures of viruses, bacteria and mold, and breaks apart odour molecules such as those behind musty smells. It then reverts naturally to oxygen (O2), leaving no chemical residue on surfaces, belongings or clothes. According to the company, the process has been tested and certified by Thailand's Department of Medical Sciences and Intertek (UK), eliminating viruses, bacteria and mold by 99.99% according to the test results certified to the company. You can view the certification documents.

Safety first: ozone treatment is carried out only in an empty, closed space. All people, pets and plants must leave before work begins, and nobody re-enters until the rest and ventilation period set by the team is complete. High concentrations of ozone irritate the airways, so the work must be done by trained staff.

Setting the right expectations: ozone does not replace mud removal, washing, tearing out ruined materials or drying. If walls are still soaked, mold will come back because the moisture that feeds it is still there. Ozone is the finishing step once the house is clean and dry, targeting what remains in the air, in hidden corners and on surfaces, and eliminating the musty odour.

How the WHD team handles a flood-damaged, moldy home

1. Consultation and assessment: flood depth, how long water stood, where mold and odours appear, and which materials should be removed first.
2. Readiness check: mud gone, surfaces washed, structure reasonably dry. If it is still very damp, the team advises further drying so results last.
3. Preparation: cupboards, drawers, wardrobes and storerooms opened so the gas can reach inside; windows and vents closed; people, pets and plants moved out.
4. Ozone fill: generators placed zone by zone to reach the target concentration, typically about 30 minutes per zone depending on size and condition.
5. Contact time: concentration is held so the ozone can react with microbes and odour molecules everywhere.
6. Rest and ventilation: machines off, ozone allowed to revert to oxygen, then doors and windows opened before anyone returns.
7. Handover: advice on moisture control and early warning signs of regrowth.

Jobs of 15,000 baht or more receive a free medical-grade disinfectant spray for ongoing surface care. For a wider view of our services, see ozone cleaning service.

10 reasons to finish flood recovery with ozone

1. It reaches hidden cavities above ceilings, under built-ins and behind wardrobes, where Aspergillus and Penicillium hide.
2. It treats airborne spores, the part of the problem your family actually breathes.
3. It removes musty odour at the source instead of covering it with perfume.
4. No chemical residue, because ozone reverts to oxygen. Read more about ozone and chemical residue.
5. No re-wiping afterwards of furniture, clothes or books that survived the flood.
6. Backed by testing from the Department of Medical Sciences and Intertek, 99.99% according to the test results certified to the company.
7. Fast turnaround, about 30 minutes per zone plus ventilation, so families usually return the same day.
8. Lower risk of regrowth after repairs when done after drying and before repainting or new walls go up.
9. Handled by specialists who plan zones and control concentration and timing safely. See our residential ozone disinfection service.
10. Peace of mind, knowing the whole home has been treated. Check available dates.

Example case: a Pathum Thani townhouse with mold that kept coming back

Example case to illustrate the process, not a testimonial from an identified customer.

Ton, a 38-year-old office worker, lives in a three-storey townhouse in Khlong Luang, Pathum Thani. The ground floor flooded to about 60 cm for a week. Eager to make the house look normal, he had the walls repainted as soon as the water left. Two months later the paint was blistering, black spots lined the skirting, and a grey-green patch spread across the ceiling under the stairs. His wife, five months pregnant, had a blocked nose and stinging eyes whenever she came downstairs.

The first advice he received was not "ozone right away" but "fix the cause." A contractor removed gypsum up to about 1.5 m, replaced the rotten ceiling boards and scraped off the blistered paint. Dehumidifiers ran with open windows for several days until plaster and timber were dry. Then the WHD team treated the house floor by floor while the family spent the day with relatives: the ground floor thoroughly, the upper floors to deal with spores that had drifted into bedrooms and wardrobes.

After the ventilation period the family returned that evening, and the smell that had greeted them at the door was gone. Only then were new walls installed, primed with a moisture-resistant primer and painted. The lesson: order matters as much as method. Painting before drying and disinfecting doubled the cost. Ton's wife continued regular check-ups with her obstetrician, because home care never replaces medical advice.

Recovery checklist: before and after ozone

Before ozone

• Wear rubber gloves, boots, goggles and a fine-particle mask when handling moldy areas.
• Switch off power to flooded circuits and have an electrician inspect them.
• Remove all mud and debris; wash hard surfaces with clean water and detergent.
• Discard soaked or visibly moldy absorbent items: mattresses, carpets, lower drywall, rotten ceiling boards, wet insulation.
• Remove skirting boards and lift a few floorboards to check underneath.
• Dry with open windows, dehumidifiers and fans blowing air outward, not at moldy walls.
• If possible, confirm with a moisture meter that walls and timber are truly dry.

On treatment day

• Everyone, including pets and plants, leaves before work starts.
• Open cupboards, drawers and internal doors; close windows and external vents; switch off AC and fans.
• Do not re-enter until the team confirms ventilation is complete.

After ozone

• Ventilate further for the first few hours.
• Clean AC filters or book an AC service; coils and drain pans harbour mold.
• Keep indoor humidity under control, especially in closed rooms.
• Use moisture-resistant primer and materials when repairing.
• Watch for musty smells, spots or blistering paint, and track down any leak.
• Anyone with ongoing breathing symptoms should see a doctor.

For frequently touched surfaces, a suitable disinfectant adds protection: see Chemgene disinfectant or our disinfection service.

Frequently asked questions

1. If I use ozone, can I keep the moldy drywall?

Not recommended. Drywall that was submerged for long, has gone soft or has mold into the core should be replaced. Damaged material holds moisture and keeps feeding mold. Ozone is for what remains after removal and drying.

2. How long should the house dry before ozone treatment?

It depends on flood depth, duration and materials. Some homes are ready in a few days with dehumidifiers; thick masonry can take weeks. The goal is dry all the way through, and our team can advise on timing.

3. Can the family stay upstairs during treatment?

No. Treatment happens in an empty, closed space. Ozone can travel through stairwells and gaps, so everyone, including pets and plants, must leave the area defined by the team and return only after ventilation.

4. Will the mold never come back?

Ozone deals with what is present at the time; it is not a permanent shield. Leaks, trapped humidity or another flood can bring mold back, so moisture control afterwards is the key to lasting results.

5. Will ozone damage furniture or electronics?

The team sets concentration and duration for residential spaces, and normal household items are left without stains or residue. Tell us in advance about artwork, fine leather or other special items so we can decide whether to move them out.

6. How long does it take and what does it cost?

Roughly 30 minutes per zone, plus ventilation. Price depends on the actual area; send photos and room sizes via LINE for a preliminary quote. Jobs from 15,000 baht include a free medical-grade disinfectant spray.

Still smelling damp or seeing mold after the flood?

Let the WHD team assess your home and complete your recovery with ozone disinfection that reaches every corner and leaves no chemical residue.

Book Residential Ozone Disinfection

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

Related pages

• Residential and building ozone disinfection
• Ozone cleaning service
• Contact us

References

• CDC: Floods, cleanup and mold guidance
• World Health Organization fact sheets