A Story from the Field: Cool Air That Brought a Musty Smell With It

Khun Ek is forty-five and lives in a two-storey townhouse in a housing estate in Nong Chok district, on the far eastern edge of Bangkok. He shares it with his wife, their eight-year-old son, who has allergies, and his seventy-year-old mother. The ground floor is a small grocery shop with one wall-mounted air conditioner. Upstairs there are two bedrooms, each with its own unit. All three outdoor condensing units sit on the ground at the back and side of the house.

When floodwater spread across eastern Bangkok in early October 2026, it rose almost half a meter inside the estate. Khun Ek's ground floor sat underwater for several days, and the outdoor units were almost completely submerged. The family moved upstairs and kept every door and window shut against mosquitoes and the smell of the water outside.

When the water finally drained away, Khun Ek scrubbed the mud off the ground floor until it looked clean. The weather was hot and sticky, so he tried the air conditioner in one of the upstairs bedrooms, a room the water had never reached. Cool air came out, and with it a stale, musty smell, like laundry that had never quite dried. Within an hour his son was sneezing over and over, his nose blocked and his eyes itching. His mother had a dry cough all night. When Khun Ek lifted the front panel of the unit, he saw small black specks clinging to the fan blades and the air outlet. Downstairs in the shop, the filter was caked with dried mud and the drain pan held a pool of cloudy water.

Khun Ek had assumed that rooms the flood did not touch would be fine. The reality is that the whole house had been sitting in very high humidity for days, sealed shut, with no air conditioning running. Those are ideal conditions for mold to grow on the cooling coil, the fan and the drain pan inside an AC unit. The moment the unit was switched on, it blew spores straight out into the room.

This article is for homeowners, landlords and shop owners in Nong Chok, Min Buri, Lat Krabang, Khlong Sam Wa and nearby areas who are facing the same problem after the 2026 floods. It explains why air conditioners and ducts become hiding places for mold, why common quick fixes do not work, and how to restore clean, comfortable air safely.

Why Air Conditioners and Ducts Hide Mold After a Flood

The cooling coil and drain pan: dark, damp and full of food

An air conditioner works by pulling heat and moisture out of the air. The water that condenses on the cooling coil runs into a drain pan and out through a pipe. After a flood, the drain pipe that leads outside may be blocked with silt, so water sits in the pan instead of draining away. Dust and grime trapped in the coil become food for mold. Add several days of humid, still air inside a closed house and mold can spread very quickly.

The blower wheel: out of sight, but blowing at you all night

A wall-mounted unit has a long, cylindrical fan, often called a blower wheel or squirrel-cage fan, tucked behind the louvers. Once mold settles on its blades, every time the unit runs it sends spores and musty odor directly toward whoever is sleeping in the room. That is why wiping the front panel or rinsing the filter on its own rarely solves the problem.

Ducts and ceiling voids in shops and commercial buildings

Many shops, offices and commercial buildings in Nong Chok use ceiling-suspended units, concealed ducted units above a false ceiling, or ventilation fans connected to ductwork. If floodwater or humidity reaches the ceiling void, or the ceiling boards were soaked by roof leaks during the storms, the inside of the ducts and the insulation wrapped around them can become a reservoir of mold that is invisible from the room below. Air from those ducts carries spores and odor to every outlet in the building.

Submerged outdoor units: an electrical risk that comes first

An outdoor condensing unit that sat in floodwater contains circuit boards, a compressor and wiring. Switching it on while the inside is still wet or full of mud can cause a short circuit or serious damage. It must be inspected by a qualified air-conditioning technician or electrician before it is used again. This step comes before any discussion of smells or mold.

The impact on health, money and business

• Health. Mold spores blown out of an air conditioner can trigger allergies and asthma, a blocked nose, coughing and eye irritation, particularly in children, older people and anyone with an existing health condition. Rooms that were flooded may also carry bacteria from sewage-contaminated water, such as E. coli, and the organisms that cause leptospirosis on surfaces.
• Money. A unit clogged with mold cools less effectively and uses more electricity. Running a submerged outdoor unit without an inspection can turn a cleaning job into an expensive repair or a full replacement.
• Business. A shop where the air conditioning smells musty drives customers away within seconds. Stock on the shelves, especially fabric, paper and packaged goods, can also absorb the odor.
• Sleep and recovery. Families forced to choose between sleeping in the heat or breathing musty air do not get the rest they need during the exhausting weeks of post-flood recovery.

This is general information. Anyone who develops breathing difficulty, wheezing or a high fever after flood exposure should see a doctor and follow guidance from the Department of Disease Control.

What Many Households Try, and Why It Falls Short

• Washing the filter and carrying on. The filter is cleaner, but the mold on the coil, the blower wheel and in the drain pan is still there. The musty smell comes back within days.
• Spraying air freshener into the vents. This masks the smell briefly, but some fragrances irritate the airways, and none of them kill mold.
• Spraying bleach into the unit. Bleach can corrode the aluminum fins and metal parts, leaves a chlorine smell that is blown back into the room, and should not be used inside an air conditioner by anyone without the right training.
• Running the fan on full to dry things out. If the inside of the unit has not been cleaned, this simply spreads more spores around the room.
• Repainting over mold stains. The room looks new, but if the air conditioner keeps blowing spores, mold will settle on the walls and curtains again.

The key point is that solving air-conditioner mold after a flood has two parts that must go together. The first is the unit itself, which needs to be inspected, cleaned and, if necessary, repaired by an air-conditioning technician. The second is the room and the air around the unit, where spores have already been blown and settled. That second part is where disinfection fogging and ozone treatment make a real difference.

The Solution: A Thorough AC Clean, Then Disinfection Fogging and Ozone Treatment With WHD

World Health Disinfection (WHD) handles the room and the air. Our disinfection fogging service uses Chemgene HLD4H applied through ULV equipment, which produces a very fine mist that drifts into room corners, under beds, behind wardrobes and onto the surfaces where spores have landed. This is followed by our ozone treatment service, which helps reduce airborne mold spores and breaks down the odor molecules trapped in curtains, carpets and upholstered furniture. For the air conditioner itself, we recommend an experienced AC technician inspects and cleans the unit before or alongside our work, and our team plans the order of work with you and your technician so the two jobs support each other.

Our step-by-step process

1. Survey. We check how high the water rose, where the indoor and outdoor units are, the drain pipes, the ceiling and any spots with obvious odor or mold.
2. Clean-up and electrical advice. Mud is removed, heavily molded materials are discarded, and an electrician checks the system before any submerged appliance is switched on.
3. Coordinating the AC clean. We recommend your technician cleans the cooling coil, blower wheel and drain pan, and clears blocked drain pipes. Any outdoor unit that was submerged should be checked before it is run.
4. Preparing the room. Food, medicines, pets and plants are taken out. Sensitive electronics such as computers and televisions, and artworks or valuable collections, are covered or removed.
5. Chemgene HLD4H fogging. We concentrate on floors, the lower parts of walls, door frames, under beds, behind furniture and the surfaces around the air conditioner.
6. Closed-room ozone treatment. Using a Master Ozone Generator, we apply a controlled dose matched to the room size. No people, pets or plants may be in the room during treatment.
7. Ventilation. Doors and windows are opened so the ozone breaks down back into oxygen. Our team tells you when it is safe to go back in.
8. Re-check. We run the cleaned air conditioner to test for odor, check high-risk spots for mold and advise on ongoing care.

Warning Signs That Your Air Conditioner Has Mold After the Flood

• A musty, wet-laundry or earthy smell in the first few minutes after switching the unit on.
• Black specks or slimy deposits on the blower wheel, the air outlet or the swing louvers.
• Water dripping from the indoor unit because the drain pipe is blocked or the drain pan is full.
• People sneezing, getting a blocked nose or coughing more often only when the air conditioner is running.
• The room taking longer to cool down at the same temperature setting.
• In shops or buildings with ducts, a musty smell from ceiling diffusers or water stains on the ceiling around them.

If you notice any of these signs, stop using that unit for now, have a technician inspect and clean it, and then plan disinfection fogging and ozone treatment for the room.

10 Reasons Homes and Shops in Nong Chok Choose WHD to Restore Air-Conditioned Rooms

1. Fewer airborne spores. Ozone treatment helps reduce the mold spores an air conditioner has already blown around the room.
2. Odors broken down, not masked. Musty smells trapped in curtains, carpets, mattresses and sofas are tackled at the source.
3. Every corner reached. Disinfection fogging with a ULV mist gets under beds, behind wardrobes and into gaps you cannot wipe.
4. Gentle on bedrooms. Chemgene HLD4H has no sharp chlorine smell, which matters in rooms where children and older people sleep.
5. Planned around your AC clean. We sequence the work with your technician so nothing is done twice.
6. Controlled ozone dosing. The dose is matched to room size, with care taken over electronics and rubber parts.
7. Clear ventilation timing. You know exactly when it is safe to go back into the room.
8. Homes and shops alike. Suitable for family bedrooms and for ground-floor shops that need to reopen quickly.
9. Bedding covered too. Add our dust mite removal service for mattresses and sofas in the same room.
10. Everyday follow-up. Chemgene HLD4H spray is handy for wiping AC front panels and frequently touched surfaces.

Before and After Restoring the Air-Conditioned Rooms

In Our Client's Words

"At first I thought washing the filters would be enough, but the smell did not go away at all. Once the technician cleaned the whole unit and the team came in to fog and ozone-treat the room, my son slept through the night without waking up sneezing. What I liked most was that the team explained clearly how long we had to stay out and what we needed to put away."

Townhouse and grocery shop owner, Nong Chok, Bangkok (illustrative scenario from the field)

Checklist Before You Switch the Air Conditioning Back On

• Do not run any outdoor unit that was submerged until an AC technician or electrician has checked it and confirmed it is safe.
• Check the breakers and wiring for every unit, especially where plugs or cables sit low on the wall.
• Have the technician clean the cooling coil, blower wheel and drain pan, and flush any drain pipe blocked with mud.
• Replace or wash the air filters and wipe the front panels with a suitable product.
• In shops and buildings with ducts, ask the technician to inspect the inside of the ducts, the insulation and the ceiling around the diffusers.
• Have the room fogged and ozone-treated, and ventilate fully before using the air conditioner normally.
• After you start using the unit again, pay attention to smells and to how people in the room feel over the first two or three days.

Ducted Systems in Shops, Offices and Commercial Buildings

Ducted and ceiling-concealed systems need extra thought after a flood, because so much of the system is hidden. In a typical shophouse or small office, a single indoor unit above the ceiling may feed several rooms through a network of flexible or rigid ducts. If that ceiling void stayed damp for days, mold can grow along the inside of the ducts and in the fibrous insulation wrapped around them. You may only notice it as a faint mustiness from the diffusers, or as dark rings on the ceiling boards around each outlet.

Before any disinfection work in the rooms, it is worth asking an air-conditioning or ventilation contractor to open inspection hatches and look inside the ducts. Sections of flexible duct or insulation that are badly contaminated or have been soaked are often best replaced rather than cleaned. Ceiling boards that sagged or stained after getting wet should be replaced as well, because they hold moisture and spores.

Once the hidden parts of the system have been dealt with, our team can fog and ozone-treat the occupied spaces below. In a shop, that means the sales floor, stockroom, staff area and restroom. Stock that easily absorbs odors, such as fabric goods, paper products and anything without sealed packaging, should be moved out or covered before treatment. Electronics such as point-of-sale terminals, CCTV recorders and network equipment should be switched off and covered.

Rooms the Flood Never Reached Can Still Need Help

One of the most common surprises after a flood is that upstairs rooms, which stayed completely dry, still develop musty air conditioners. The reason is humidity. When the ground floor of a house is full of water for days, the moisture evaporates and rises through the whole building. If the family has kept every door and window shut and the air conditioners have been off, there is nothing to remove that moisture. Relative humidity inside can stay very high for a long time.

The cooling coil inside an idle air conditioner is a cool, sheltered surface coated with a thin layer of dust. In those conditions it can collect moisture and become a place where mold takes hold. The first time the unit is switched on, everything that has grown inside is pushed out into the room.

That is why we recommend checking every air conditioner in a flooded house, not just the units on the ground floor. If an upstairs unit smells musty when you switch it on, treat it the same way: technician's clean first, then room treatment if the smell or symptoms persist.

Keeping Mold Out of Your Air Conditioner for the Rest of the Rainy Season

Once your rooms have been restored, a few simple habits greatly reduce the chance of the musty smell returning.

• Run fan mode for a short while before switching off. This helps the cooling coil dry out and reduces moisture left inside the unit. Do this only after the unit has been cleaned.
• Clean the filters every two to four weeks during the rainy season and after a flood, because dust and moisture build up faster than usual.
• Watch the drain. If water drips from the indoor unit, the drain pipe may be blocked and should be cleared by a technician.
• Air the room on dry, sunny days by opening doors and windows to let accumulated humidity escape.
• Raise outdoor units if your home is in an area at risk of flooding again. Ask a technician about mounting brackets or a higher platform.
• Consider periodic ozone treatment for rooms that stay humid all year, or for shops that want the air to feel fresh for customers.

Nong Chok is a low-lying district with many canals, rice fields and orchards, and humidity stays high for much of the year. Small routines like these are the cheapest form of mold prevention there is.

A Note on Health for Families With Children and Older Relatives

In homes like Khun Ek's, the people most affected by moldy air conditioning are usually the youngest and the oldest. Children breathe faster than adults relative to their body size and spend long hours asleep in air-conditioned bedrooms. Older relatives may already have reduced lung function, heart conditions or diabetes that make any respiratory irritation harder to shake off.

If anyone in the family has asthma or allergies, it is sensible to keep them out of rooms with musty air conditioning until the unit has been cleaned and the room treated, and to keep their usual medicines close at hand. Watch for coughing at night, wheezing, a persistent blocked nose or eye irritation that gets better when they are away from the room. These are useful clues that the indoor air is the problem.

Flood exposure itself carries other risks. Anyone who waded through floodwater and later develops a high fever, headache or muscle pain, especially in the calves, should see a doctor promptly and mention the flood exposure, as this can be a sign of leptospirosis. Reliable general advice is available from the Department of Health and the World Health Organization.

What to Expect on the Day of Service

If you have never booked a professional disinfection service before, here is how a typical day looks for a home with two or three air-conditioned rooms.

• Walk-through. Our team arrives at the agreed time and walks through the house with you to confirm which rooms are included and what needs to be moved or covered.
• Preparation. Food, medicines, pets, plants and personal valuables are taken out. Sensitive electronics are switched off and covered.
• Fogging. The team fogs each room in turn with Chemgene HLD4H while the family waits outside or on another floor.
• Ozone. Doors and windows are closed and the ozone generator runs for the time calculated for each room. A sign is placed on the door and nobody enters.
• Ventilation and handover. Doors and windows are opened. When ventilation is complete, the team walks through again with you, checks for odor and explains how to keep the rooms fresh.

Every home is different, so exact timings are confirmed during the survey. That lets you plan where the family will spend those hours and when your AC technician should visit.

Frequently Asked Questions

Q: Does WHD clean air conditioners?
A: Our core work is disinfection fogging and ozone treatment of rooms and surrounding areas. For cleaning and repairing the air conditioner itself, we recommend an experienced AC technician, and our team will help sequence the work so both jobs fit together.

Q: Should the AC be cleaned first, or the room ozone-treated first?
A: In general, have the technician clean the unit and fix any drain problems first, to remove the source of the spores. Then fog and ozone-treat the room to deal with the spores and odors that have already spread.

Q: Will ozone damage my air conditioner or other appliances?
A: We use a controlled ozone dose and treatment time matched to the room size, and we ask for appliances to be switched off and sensitive electronics to be covered or removed first, to reduce any effect on rubber and plastic parts.

Q: Do rooms that were not flooded need treatment?
A: If the air conditioner smells musty when switched on, or people in the room are sneezing or congested, it is worth including. High humidity in a closed house can let mold grow inside an AC unit even if the room never flooded.

Q: How long do we need to stay out of the room?
A: It depends on the room size and the ozone dose. Our team will confirm exact timing after the survey and will tell you when ventilation is complete and it is safe to go back in.

Q: Do you serve areas near Nong Chok?
A: Yes. We serve Nong Chok, Min Buri, Lat Krabang, Khlong Sam Wa, Khan Na Yao and other parts of Bangkok and its surrounding provinces, as well as nearby provinces such as Chachoengsao. Call 065-556-6294 or LINE @whd268.

Switch On the AC and Breathe Easy Again

Homes and shops in Nong Chok whose air conditioning still smells musty after the flood can talk to the WHD team today.

Book Ozone Treatment, Click Here

See Our Disinfection Service, Click Here

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

Related Links

• Residential ozone disinfection
• Ozone treatment for odors and chemical residue
• Dust mite removal service
• All articles

Health references: World Health Organization (WHO): Floods · Department of Health, Thailand