What Really Happened in Chiang Rai in September 2024

This article does not begin with an invented story. It begins with something that really happened, and that thousands of Thai families lived through.

On 11 September 2024, heavy rain caused flooding and flash floods in Mae Sai district, Chiang Rai. The Department of Highways reported that floodwater cut a road bridge on Route 1338, and engineers had to install a temporary Bailey bridge to reopen traffic. Source: Thansettakij, 11 Sep 2024

A few days later the water reached the city itself. THE STANDARD visited the Koh Loi and Terd Phra Kiat communities in Mueang Chiang Rai, where flooding hit between 11 and 14 September 2024 and about 430 households were affected. A resident's son said the water reached the rooftops, far higher than the famous 1994 flood. A 72-year-old resident said he had never seen anything like it. Source: THE STANDARD, 24 Sep 2024

When residents were finally allowed back on day 10, they found mud and water lines across ground floors and single-storey houses. Cleaning crews were washing walls, floors, doors and windows with pressure washers. Furniture and appliances were being thrown out or repaired. Volunteers from the Mirror Foundation and inmates from the Department of Corrections helped shovel out the mud.

That is the visible part of a flood. The part nobody photographs is what happens over the next weeks: the house looks clean, but it does not smell clean.

The Smell That Stays After the Mud Is Gone

Anyone who has cleaned up after a flood in Thailand knows this smell. It is damp, earthy, sour and heavy. It sits in the bedroom at night. It clings to curtains, mattresses, wardrobes and the inside of kitchen cabinets. You open the front door after a day out and it greets you.

In October 2026, Thai media reported a warning from paint-industry researchers that mold left behind after floodwater recedes can affect health even when the walls look dry and the house has been cleaned. They listed a persistent musty smell as one of the first warning signs, together with dark spots and walls that stay damp, especially behind cabinets, under furniture and in corners with poor airflow. Source: Naewna, 16 Oct 2026

That musty smell is not just unpleasant. It is information. It tells you that something in the house is still wet, still growing, or still releasing gases from organic material that the floodwater carried in: river silt, sewage, rotting plant matter, soaked wood and fabric.

Why Flood Odor Is So Hard to Remove

Many families try the usual fixes first. They are understandable, but they rarely work on their own.

• Air fresheners and scented sprays only cover the smell for an hour or two. When the perfume fades, the musty odor returns, sometimes mixed with the fragrance into something worse.
• Opening the windows helps, and you should do it, but outdoor humidity in the Thai rainy season is often high. Damp air coming in does not dry a soaked wall cavity.
• Bleach on every surface can remove visible stains, but it does not reach the inside of porous wood, the back of plasterboard or the foam of a sofa. Mixing bleach with ammonia or other cleaners also creates toxic gas, a mistake researchers specifically warn against.
• Painting over the walls hides the stain for a few weeks. The same October 2026 warning says not to paint over mold, because it causes peeling and regrowth.
• Charcoal bags and baking soda absorb a little odor in a small cupboard, but they cannot treat a whole house.

The real problem is that flood odor lives in three places at once: in the air, on surfaces, and deep inside porous materials. A single product only touches one of these.

What the Smell Can Do to Your Family's Health

The health risk from post-flood mold is well documented. According to the researchers quoted in the October 2026 report, mold spores can cause:

• A stuffy or runny nose
• Coughing and wheezing
• Eye irritation
• Skin rashes
• Worse symptoms in people with allergies or asthma

People with weak immune systems or chronic lung disease face a higher risk of infection, so they should stay away from contaminated areas and should not try to remove mold themselves.

Think about who usually lives in a Thai family home: grandparents with long-term lung or heart conditions, small children who crawl on floors, a mother who is pregnant, someone who already has a dust allergy. These are exactly the people the warning is about. For them, a musty house is not just unpleasant. It can be the reason for weeks of coughing and repeat visits to the clinic.

The Cost Nobody Talks About

The financial side of a flood is usually counted in mud removal, repairs and replaced appliances. The smell adds hidden costs that come later:

• Furniture thrown away too early. Many families discard sofas, mattresses and wardrobes simply because they still smell, even after they are dry.
• Rental homes and shophouses that cannot be let. A tenant who walks in and smells damp will walk out again.
• Small businesses losing customers. A café, salon, clinic or guesthouse cannot operate if the first thing customers notice is the smell.
• Repeat cleaning. Paying for deep cleaning two or three times because the smell keeps coming back.
• Time. Weeks of living with windows open, fans running, and nights of poor sleep.

These costs add up long after the water has gone.

Step One Is Always the Source, Not the Smell

Before we talk about ozone, it is important to be honest about the order of work. Odor treatment only works properly when the source has been dealt with. This is what reputable restoration specialists follow, and it matches the advice in the October 2026 warning:

1. Remove water and mud quickly. The first 24 to 48 hours after the water recedes matter most. Drain, ventilate and dry wet materials as fast as possible.
2. Fix the moisture source. Leaks, blocked drains and wet wall cavities keep feeding mold.
3. Remove what cannot be saved. Carpets and ceiling panels that hold mold and cannot be dried properly usually need to be replaced.
4. Clean visible mold with protection. Wear a fitted N95 mask, gloves and goggles. Call a professional for large areas or sewage contamination.
5. Dry everything completely. Use fans and dehumidifiers until walls and floors are dry to the touch and do not feel cold and damp.
6. Then treat the remaining odor. This is where ozone treatment fits.

Skipping steps one to five and going straight to odor treatment is like spraying perfume on a wet towel. It will not last.

How Ozone Treatment Works on Flood Odor

Ozone (O₃) is a form of oxygen with three atoms instead of two. It is very reactive. When a professional ozone generator fills a closed, empty room with ozone, the gas reaches places a cloth and spray bottle cannot: the gap behind a wardrobe, the inside of a drawer, the fibers of a curtain, the air inside a cupboard.

Ozone reacts with many odor-causing compounds and breaks them down. When the treatment is finished and the room is ventilated, the remaining ozone naturally turns back into ordinary oxygen (O₂). There is no perfume left behind and no chemical liquid to wipe off.

This is why restoration companies around the world use ozone as a final step after floods and fires. It is a way of treating the air and the surfaces of a whole room at once, after the cleaning has been done.

Safety First: The Honest Rules for Ozone

Ozone is powerful, and that is exactly why it has to be used correctly. The US Environmental Protection Agency (EPA) warns that ozone can irritate and damage the lungs, cause chest pain, coughing, shortness of breath and throat irritation, and worsen asthma. The EPA also states that no US federal agency has approved ozone generators for use in occupied spaces, and that high-concentration ozone should only be used in unoccupied spaces under controlled conditions. Source: US EPA

The EPA also points out that ozone does not remove contaminants embedded deep in porous materials, and that some indoor chemicals react with ozone very slowly. That is another reason why cleaning and drying must come first.

In practice, a safe professional ozone treatment means:

• Nobody inside. No people, pets or plants in the treated area during the process.
• Doors and windows closed during treatment, with clear warning signs.
• Correct machine size and time for the room volume, set by a trained technician.
• Full ventilation afterwards before anyone goes back in.
• Never running an ozone machine in a bedroom while someone sleeps.

Why Master Ozone Generator Is the Right Tool for Post-Flood Odor

World Health Disinfection provides professional ozone treatment using the Master Ozone Generator for homes, condos, warehouses, offices and hotels. Our ozone odor service page explains how the treatment works on paint, chemical and musty odors in closed spaces. See the ozone service

Here are ten reasons it fits the last stage of flood recovery:

1. It treats the whole room at once. Gas reaches corners, cupboards and gaps that hand cleaning cannot reach.
2. It breaks down odor instead of covering it. No fragrance is added, so the smell does not come back when a perfume fades.
3. It works on soft furnishings in place. Curtains, sofas and mattresses that have been cleaned and dried can be treated in the room rather than thrown away.
4. No liquid chemical residue. Ozone turns back into oxygen after ventilation, so there is nothing to wipe off tables or children's toys.
5. Fast turnaround. A typical room can be treated and ventilated in a few hours, so families can move back in sooner.
6. Suitable for many spaces. Bedrooms, living rooms, kitchens, shophouses, storage rooms and vehicles can all be treated.
7. Measured, not guessed. WHD's service page shows on-site air measurements taken at multiple points in real treated areas.
8. Carried out by trained technicians. Our team controls dose, time and ventilation and keeps people out of the area for safety.
9. Helps businesses reopen. Cafés, salons, clinics and guesthouses can remove the musty smell before customers return.
10. Fits after professional disinfection. WHD also provides disinfection services, so cleaning, disinfection and odor treatment can be planned together.

Before and After: What Changes in a Treated Room

Before treatment (after cleaning and drying):

• The room looks clean but still smells damp and sour.
• The smell is strongest inside wardrobes and drawers.
• Curtains and cushions hold the odor and release it when touched.
• Family members avoid sleeping in the room.

After ozone treatment and ventilation:

• The heavy musty odor is significantly reduced or gone.
• Cupboards and drawers no longer release a sour smell when opened.
• Fabrics that were cleaned and dried smell neutral, not perfumed.
• The room can be used again without windows open all night.

Results depend on how well the source was removed and dried first. If the smell returns after a few days, it usually means there is still hidden moisture or mold, and that needs to be found and fixed.

Which Spaces Benefit Most After a Flood

• Ground-floor bedrooms and living rooms where water reached the walls
• Kitchens, especially cabinets made of particle board
• Wardrobes and storage rooms where clothes and boxes were soaked
• Shophouses and small businesses that need to reopen quickly
• Warehouses storing goods that absorbed odor
• Cars and vans that were partly submerged, after professional cleaning and drying
• Guesthouses and hotels where every room must smell fresh for guests

A Room-by-Room Checklist for the First Two Weeks

The October 2026 warning stresses speed: the first 24 to 48 hours after the water recedes decide how much mold you will be fighting later. If you are returning to a flooded home, this practical checklist helps you work through the house in a sensible order before any odor treatment.

Living room

• Lift sofa cushions and stand them on their sides so air reaches every face.
• Pull furniture at least 30 cm away from the walls so the wall surface can dry.
• Remove skirting boards if water reached them; moisture hides behind them.
• Check the underside of rugs. If a rug smells sour after drying, it usually needs to go.

Bedrooms

• Take mattresses outside on a sunny day if they were only splashed. Fully soaked mattresses, especially from sewage-contaminated water, should be replaced.
• Empty wardrobes completely. Wash clothes in hot water where the fabric allows, then dry them fully before putting anything back.
• Leave wardrobe doors and drawers open for several days with a fan blowing in.

Kitchen

• Open every cabinet. Particle board soaks up water like a sponge and swells; swollen boards often cannot be saved.
• Throw away any food, spices or dry goods that touched floodwater, even if the packaging looks sealed.
• Clean and disinfect plates, pots and utensils before using them again.

Bathroom and drains

• Pour clean water down floor drains and check that they flow. Blocked drains keep sending sewer odors back into the house.
• Look for cracked tiles or grout where water may still be trapped underneath.

Walls and floors

• Mark the highest water line with tape. Everything below it needs checking.
• Press on plaster and gypsum board. Soft, crumbling or bubbling areas usually need to be cut out and replaced.
• Use a low-cost moisture meter if you can. A wall that looks dry can still hold water deep inside.

Electrical safety

• Do not switch on appliances or sockets that were underwater until an electrician has checked them.

When this checklist is complete and the house has stayed dry for several days, odor treatment has the best chance of working. Many families find the musty smell drops noticeably at this stage alone. What remains is the deep, stubborn odor in fabrics, cabinets and the air itself, and that is what professional ozone treatment is designed for.

Signs You Should Call a Professional

Some situations are beyond a family weekend clean-up. Contact a professional if:

• Mold covers a large area, roughly more than a single door's worth of wall.
• The floodwater was mixed with sewage, which is common in urban floods.
• Someone in the home has asthma, a weak immune system or a chronic lung condition.
• The smell returns within days, even after careful cleaning and drying.
• You run a business and need the space to be ready for customers quickly.
• The building is large, such as a warehouse, guesthouse or shophouse with several floors.

How a WHD Ozone Service Visit Works

1. Contact and assessment. Tell us the size of the area, what flooded, and what cleaning has already been done. Photos on LINE help.
2. Site check. Our technician checks for remaining moisture and advises if more drying is needed first.
3. Preparation. Everyone, including pets, leaves the area. Plants and open food are removed. Windows and doors are closed.
4. Treatment. The Master Ozone Generator runs for the time set for the room volume.
5. Ventilation. Windows are opened and fans are used to clear the remaining ozone before re-entry.
6. Check and handover. We walk through the space with you and confirm the result.

Frequently Asked Questions

Can ozone remove mold? Ozone can help reduce odor and treat surfaces, but mold growth must be removed physically and the moisture fixed. Ozone is the final odor step, not a replacement for cleaning.

Is it safe for my children and pets? Yes, when done correctly: nobody is in the room during treatment, and the room is fully ventilated before anyone returns. Never stay in a room while ozone is running.

How long does it take? Most single rooms take a few hours including ventilation. Larger homes or warehouses take longer. Our technician will give you a time plan before starting.

Will the smell come back? If the source was removed and the house is dry, the musty smell should not return. If it does, there is likely hidden moisture that needs attention.

Do I need to throw away my sofa and mattress? Not always. Items that can be cleaned and fully dried may be saved with ozone odor treatment. Items soaked with sewage or that stay damp should be replaced.

Related Reading

• World Health Disinfection homepage — our full range of disinfection products and services
• Ozone odor and chemical residue service — how Master Ozone Generator works
• US EPA: Ozone generators sold as air cleaners — safety facts about ozone
• THE STANDARD: Chiang Rai 10 days after the flood — the real situation in 2024

Bring Your Home Back, Not Just Its Walls

The 2024 floods showed how fast water can take over a home, and how slow the recovery is. Mud can be shoveled out in days. The smell, if ignored, can stay for months and affect the health of the people you are trying to protect.

Do the work in the right order: remove water and mud, fix moisture, clean and dry, then treat the odor professionally and safely. When you reach that last step, World Health Disinfection is ready to help with the Master Ozone Generator and a trained team that puts safety first.

Call 065-556-6294 or add LINE @whd268 to send photos of your space and get advice on the right treatment.

Call 065-556-6294 | LINE: @whd268