Somchai, 47, lives in a two-storey house in Bang Ban district, Ayutthaya. Last year the ground floor flooded to roughly sixty centimetres for eleven days. The family moved upstairs and lived there throughout.

When the water receded they cleaned urgently: floors washed, mud scrubbed off, badly damaged items thrown out, the soaked sofa and mattresses left in the sun for three days and then carried back inside, and the lower walls repainted. It was all finished within two weeks. Somchai says he felt enormous relief.

One thing never went away: the smell. He describes it as musty, like laundry that never quite dried, mixed with wet earth. It was strongest in the morning before the windows were opened, and grew stronger every time it rained.

The family used air fresheners, charcoal deodorisers and moisture absorbers in every corner. The smell faded briefly and always returned.

In the third month after the flood, their eleven-year-old daughter started coughing at night. The cough became audible wheezing, and one night they took her to hospital at two in the morning. The doctor diagnosed hyperreactive airways and asked the question that left Somchai silent: has the house ever flooded, and is furniture that was submerged still in use?

This is where most people are mistaken. A sofa or mattress being dry does not mean it is safe.

• Flood water is not clean water. It carries soil, sediment, organic matter and microorganisms from outside, all of which penetrate fibres and internal foam.
• Sun drying dries the surface while the core stays damp. Foam and synthetic fill several inches thick need far more time and airflow than people assume.
• Residual core moisture is exactly what mould requires. Mould spores can begin growing within 24 to 48 hours of a material getting wet.
• Residual organic matter feeds dust mites. Flooded homes commonly show a rising mite population in the months that follow.
• A musty smell is a signal, not just a nuisance. It comes from volatile compounds produced by microorganisms, indicating that growth is actively occurring.

In plain terms, a dry, clean-looking surface is covering an environment that is still damp and still alive underneath, and the family sleeps on it every night.

Further reading on post-flood health and indoor air quality is available from the World Health Organization and the Department of Disease Control.

What keeps this problem hidden is the delay. Symptoms usually emerge in month two to four, exactly when everyone believes the flood is behind them.

1. A persistent cough, especially at night and before dawn
2. Wheezing in a child with no previous history
3. Continuous morning congestion and sneezing
4. Itchy rashes that flare in cycles
5. Unexplained headaches and chronic fatigue
6. Improvement after several days away, deterioration on returning home

That last point is the clearest signal. In Somchai case, his daughter went on a three-day scout camp and improved noticeably, then coughed all night on her first night home. That was the evidence that stopped him treating it as coincidence.

Families who have just been through a flood are usually already exhausted financially and emotionally, and then they face a second round.

First-round repair costs. Repainting, replacing door frames, electrical repairs and discarded furniture, a large sum already spent.

Second-round medical costs. Medication, consultations and emergency visits arriving in months three to six.

Furniture discarded twice. Many families eventually throw out the sofa or mattress they tried to save, because it was left until the mould was beyond recovery.

The emotional cost. Somchai says the hardest part was knowing he had genuinely tried, finishing faster than his neighbours, only to find he had carried the problem back into the house with his own hands.

The cost of waiting. The longer it is left, the deeper mould and organic matter penetrate, and the lower the chance of saving the furniture.

Three days in the sun dries the surface but cannot extract the sediment and organic material lodged in the fibres.

Spraying general disinfectant works only where it makes contact, and cannot reach the core of foam or fill several inches thick.

Moisture absorbers and charcoal reduce ambient humidity somewhat but do nothing about moisture held inside the material.

A standard vacuum not only fails to lift fine sediment, it disperses mould spores throughout the house.

Air fresheners mask the odour temporarily without addressing the cause, which can lull a family into leaving the problem far longer.

What is actually required is professional-grade extraction of sediment and organic matter from the deep layers, using a filtration system that does not release spores back into the house, followed by disinfection.

Somchai engaged the dust mite removal, mattress, sofa and carpet cleaning service with full disinfection from World Health Disinfection, stating clearly that the house had flooded and the musty smell persisted.

The team used the SIRENA System dust mite vacuum, water filtration with HEPA down to 0.02 microns, which suits post-flood work particularly well because mould spores and fine sediment sink into water instead of being blown back into the house.

Scope covered the three-seat sofa, the two mattresses that had been submerged, the carpets, the floor-length curtains and the dining chair cushions, followed by medical-grade disinfection across the ground floor.

Somchai says the tank water after the sofa was extracted came out thick brown with a heavy layer of fine mud-like sediment, from a sofa that had been sun-dried for three days and wiped twice with disinfectant.

The fastest change was the smell. The family reports the morning mustiness was gone from day two and has not returned, even on rainy days.

For families currently putting a home back together, this sequence saves the most.

1. Separate unsalvageable items immediately. Heavily waterlogged, deformed materials should go. Do not spend time and do not take the risk.
2. Ventilate as much as possible for the first two weeks. Windows open and fans running all day.
3. Clean hard surfaces first. Floors, walls, door frames and timber furniture.
4. Book deep extraction for fabric furniture as early as possible. The sooner the better. Do not wait for odour or symptoms to appear.
5. Disinfect the whole area to address mould and bacteria carried in by the water.
6. Re-check after two to three months, because the dust mite issue tends to arrive later.

We already dried everything in the sun. Is this still needed?

How soon after the water recedes should this be done?

Can a sofa that was submerged for a long time be saved?

Will the musty smell really go?

Do we need to leave the house during the work?

Do you serve flood-affected provincial areas?

The question flooded families ask most is whether an item can be kept. Getting it wrong in either direction has a cost: discard too soon and you buy replacements you did not need; keep too long and you take a health risk. These are workable criteria.

Discard immediately: materials that absorbed water and lost their shape permanently, such as foam mattresses that no longer recover, plywood that has swollen and delaminated, upholstery with mould covering more than a third of the surface, and anything submerged in sewage-contaminated water.

Usually salvageable: hardwood-framed sofas whose foam still recovers, spring mattresses with no rust in the frame, synthetic carpet that has not begun to disintegrate, curtains where only the hem was submerged, and removable chair cushions.

Needs a professional assessment on site: mattresses submerged for less than twenty-four hours, leather sofas with foam inside, and pieces of sentimental value the owner wants to keep.

Warning signs that an item should not be kept: odour that intensifies when the item is pressed or sat on, grey-green or black staining that spreads week by week, a damp cool feel to the touch even in dry weather, and clearly worsening respiratory symptoms in anyone near that piece.

Somchai says the team told him plainly that two dining chair cushions were not worth recovering, while the sofa and mattresses were. That honesty gave him more confidence than being told everything could be saved.

Many families judge the situation on the wrong timescale. They assume that if nobody is ill in the first fortnight, the house is safe. Biologically, that is not how it works.

Weeks one to two. Materials are still very wet. Microorganisms begin growing but numbers remain low. The odour is so faint it is usually masked by cleaning products.

Weeks three to six. The surface is dry, so families believe the episode is over, but the core is still damp and this is when mould grows fastest. Odour becomes noticeable in the morning and on rainy days.

Months two to three. Spores are released in quantities capable of provoking the respiratory system. The most sensitive person in the household shows symptoms first, usually a child or an older adult.

Month four onwards. Residual organic matter becomes a food supply for dust mites, adding a second layer to the problem. By this point most families no longer connect the symptoms to a flood that happened long ago.

This is why the team recommends deep cleaning within the first two to four weeks rather than waiting for symptoms. That window is when the most can be salvaged and long-term problems best prevented.

In a flooded home, using a vacuum that exhausts particles means distributing mould spores through the house with your own hands.

Our flood was a year ago. Is it still worth doing?

Does insurance cover cleaning costs?

How long does a single-storey house that flooded throughout take?

Should we do this before or after repainting the walls?