The short version: when water rises, rats are among the first refugees to move upstairs. They shelter above ceiling panels, inside kitchen cabinets and behind the warm back of the refrigerator. When the water goes down, they leave behind far more than gnaw marks: droppings, urine trails, saliva, fur, nests, and sometimes a carcass hidden where nobody can see it. Those residues can carry hantavirus, the bacteria that cause leptospirosis, and gut bacteria such as Salmonella. This guide explains the risks, how to clean up without breathing contaminated dust, and where the WHD Residential Ozone Disinfection service fits as the finishing step that reaches the air and the corners a cloth cannot.

A typical scenario: the first night back in Pathum Thani

Nok is in her forties and lives in a two-storey townhouse in Lam Luk Ka, Pathum Thani. Knee-deep water sat in her street for almost three weeks, so she moved her seven-year-old daughter and her seventy-three-year-old mother to a relative's flat in Bangkok. On the day the water finally drained, she unlocked the front door expecting the smell of mud. What hit her instead was a sickly-sweet, sharp stench that made her step straight back onto the porch.

Walking through the house, she found small black pellets, each about the size of a grain of rice, scattered along the edges of the stairs, across the kitchen counter, in the cutlery drawer, and on her mother's mattress, which had been carried upstairs for safety. The rice sack had a hole chewed through it. The cable behind the fridge had bite marks. The hem of the living-room curtain carried yellow stains. Worst of all, the rotten smell grew stronger near the upstairs bathroom. Her husband eventually lifted a ceiling panel and found two dead rats lying on top of it.

Nok grabbed a broom because she wanted her home back as fast as possible. She swept the dried droppings into a dustpan, and fine dust floated up through the sunlight coming in from the window. She had no idea that this completely ordinary action is exactly what public-health guidance tells people not to do, because sweeping or vacuuming dry rodent droppings can lift contaminated particles into the air where they are easily inhaled.

Two days of scrubbing later, the smell was still there. She had opened every window, sprayed air freshener, and mopped with a floral floor cleaner. Her daughter complained she could not sleep. Her mother refused to go upstairs. This is not the story of one unlucky household; it repeats wherever floods linger, from Ayutthaya and Nonthaburi to Ubon Ratchathani and Hat Yai. When the ground goes under water, rats that lived in drains, rubbish heaps and rice fields head for higher ground, and a closed, quiet house is the best shelter they can find.

Why rats invade homes during a flood

Rats are strong swimmers and excellent climbers. The large brown or sewer rat common in Thai cities lives in drains and burrows; when those flood, it swims to anything dry. Smaller roof rats and house mice climb power lines, downpipes and overhanging branches, then slip in through vents, eaves, and gaps around air-conditioner pipes.

While a family is away, a dark, silent house with food left in the kitchen is ideal habitat. Rats nest where it is warm and dry: above the ceiling, in upstairs storage cupboards, under the sink, behind the refrigerator where the compressor gives off heat, or inside cardboard boxes of old clothes. A single rat can leave dozens of droppings a day and dribbles urine along every route it travels to mark its territory. Within a few weeks, a whole house can become an invisible map of urine trails.

When the water recedes, some rats leave. Others die inside from hunger, exhaustion, drowning, or poison bait put out by neighbours. A carcass inside a hollow wall, above a ceiling, or in the void under the stairs is the source of the most intense odour, and it is almost always in the hardest place to reach.

The germs rats bring: hantavirus, leptospirosis and more

1) Hantavirus

Hantaviruses are a family of viruses carried by rodents. Infected rats usually look perfectly healthy, yet they shed virus in urine, droppings and saliva. Most people are infected by breathing in dust contaminated with rodent excreta, typically while sweeping, dusting or vacuuming an area with dried droppings. Less often, infection follows touching contaminated material and then the nose or mouth, or a rat bite.

In Asia, the hantavirus linked with the brown rat is associated with haemorrhagic fever with renal syndrome. Early symptoms resemble influenza: high fever, headache, back and abdominal pain, nausea, blurred vision and a flushed face. Severe cases can involve low blood pressure, abnormal bleeding and reduced kidney function. Some hantaviruses in the Americas instead cause serious lung disease. The practical lesson is the same for every type: never let dried droppings become airborne, and if anyone develops a fever after cleaning a rodent-infested space, see a doctor and mention the exposure.

2) Leptospirosis

This is the flood disease Thai families know best. Leptospira bacteria leave the body in rat urine and survive in standing water and mud. They enter through cuts, scratches, the lining of the eyes, nose and mouth, and skin softened by long soaking. The warning signs to memorise are sudden high fever, headache and severe muscle pain, especially in the calves, sometimes with red eyes, reduced urine or yellowing skin. Thailand's Department of Disease Control repeatedly advises anyone who waded through floodwater or cleaned a flooded home and then develops these symptoms to see a doctor promptly and say they were exposed, because early treatment greatly reduces severity.

3) Gut bacteria from droppings

Rodent droppings can contain Salmonella and other intestinal bacteria. When rats run across counters, plates and chopping boards or chew into food packets, those germs reach what the family eats, causing diarrhoea, cramps, nausea, vomiting and fever. Any food a rat could reach, including rice, snacks, instant noodles and fruit, should be thrown away.

4) Bites, mites and fleas

Rat bites can introduce bacteria into the body. Fleas and mites living on rats look for a new host once the rat dies, so people may wake up with itchy bites. Rotting carcasses also attract flies, which then carry contamination onward to food.

Who is most at risk

• Young children, who crawl on floors, put things in their mouths and have developing lungs.
• Older adults, whose immunity is lower and who often have chronic conditions.
• Pregnant women, because a high-fever infection affects both mother and baby.
• People with chronic illness such as diabetes, kidney or lung disease, or those on immune-suppressing medication.
• Anyone cleaning without protection: no gloves, no mask, no boots.

What a rodent-occupied house costs a family

Health: beyond direct infection risk, allergens from rat fur, dander and dried urine trigger sneezing, congestion and asthma in sensitive people. A carcass odour that lingers for days brings headaches, nausea, loss of appetite and broken sleep to people already exhausted by flood clean-up.

Money: rats chew electrical cables, creating short-circuit and fire risk, and damage air-conditioner insulation, plastic water pipes and furniture. Some homes must open ceilings to recover a carcass, discard an entire pantry, and throw away pillows, blankets and storage boxes that became nests. That damage stacks on top of the water damage itself.

Time: an odour that will not fade stops the family moving back on schedule. People take extra leave, pay for more nights of temporary accommodation, or leave children with relatives for longer.

Family wellbeing: feeling disgusted and unsafe in your own home is an invisible but real stress. Children become afraid of their bedroom, grandparents lie awake, and arguments start over who should clean which corner.

Reputation and income: for landlords, shop owners and homestay hosts, a single guest who notices a rodent smell can post a review that never goes away, and tenants may simply not renew.

Why the usual fixes do not work

1. Sweeping or vacuuming dry droppings. This is the most common mistake and the most dangerous one, because it throws contaminated dust into the air. Guidance from public-health bodies such as the CDC is to open doors and windows and leave the space to air for at least thirty minutes, put on rubber gloves, then spray droppings and urine spots with a disinfectant or diluted bleach until they are thoroughly wet, wait about five minutes, and only then wipe them up with paper towels and bag them. Never sweep them dry.

2. Air freshener. Fragrance does not destroy the molecules produced by decay; it simply layers perfume on top. The result is often a cloying sweet-and-rotten mix that makes people feel sicker than before, and when the perfume evaporates the original smell returns.

3. Running fans all day. Ventilation helps, but moving air does not kill germs, and a fan can blow dust from droppings that have not yet been collected into every room. Odour that has soaked into timber, gypsum ceiling boards, curtains and mattresses does not disappear just because there is a breeze.

4. Bleaching only the surfaces you can see. Bleach is useful on smooth, hard surfaces, but it cannot reach the ceiling void, the inside of partition walls, the space behind cabinets, the gaps around sockets, or the fibres of a sofa, which are exactly the places rats travel and nest. Strong bleach in a closed room also stings the eyes and irritates the airways, and it must never be mixed with acidic toilet cleaners or ammonia-based products.

5. Painting over urine stains. A fresh coat of paint is not a permanent seal. On humid days, ammonia from urine that soaked into plaster slowly evaporates back into the room.

6. Putting down poison and walking away. Poisoned rats usually crawl away to die somewhere hidden, such as inside a wall or on top of the ceiling, which creates a fresh carcass-odour problem days later. Rodent control works best with traps plus sealing entry points, and a professional pest controller should be called if there are many rats.

How WHD ozone treatment helps, and where it belongs in the recovery plan

Let us be clear from the start: ozone treatment is not rodent control, and it does not replace removing carcasses, collecting droppings, and cleaning. The right order is to find and remove every carcass, collect droppings and nests using the wet method above, discard materials too contaminated to clean, wash and wipe surfaces with disinfectant, and let the house dry. Once that is done, ozone treatment performs the finishing step that ordinary methods struggle with.

Ozone (O3) is a molecule made of three oxygen atoms and is a powerful oxidiser. Released into a sealed space, the gas spreads through the whole room, into corners, behind cupboards, under furniture and into fabric fibres. It breaks down the cell structures of viruses, bacteria and mould, and splits the odour molecules produced by urine and decay. Afterwards, ozone reverts to ordinary oxygen (O2), leaving no chemical residue, unlike perfumes and sprays that leave a film behind. According to test results certified for the company by Thailand's Department of Medical Sciences and Intertek (UK), the WHD process eliminates 99.99% of viruses, bacteria and mould.

The process in a home with a rodent problem

1. Survey and assessment. The team checks for signs of rats and the strongest odour points, and asks whether carcasses have been removed and the house cleaned. If a clear odour source remains, they will recommend dealing with it first, because ozone cannot stop a carcass that is still there from rotting.
2. Preparation. People, pets and plants leave the house. Cupboards, drawers and internal doors are opened so the gas can reach inside; external doors and windows are closed; and the house is divided into zones.
3. Ozone release. Generators are run at a concentration suited to each room, typically about 30 minutes per zone, working through the kitchen, bedrooms, storerooms and any accessible ceiling void.
4. Contact time. The gas works in the sealed, unoccupied space.
5. Rest and ventilation. Doors and windows are opened so ozone can revert to oxygen. Nobody re-enters until the rest period the team specifies has passed.
6. Odour check and handover. The team walks through with the owner and gives aftercare advice. Jobs of 15,000 baht or more receive a free medical-grade disinfectant spray for ongoing use.

Safety first: ozone treatment is carried out only in an empty, closed space. No people, pets or plants may be inside during treatment, and re-entry is allowed only after the rest and ventilation period the team gives you.

10 reasons to ozone-treat a home that sheltered flood rats, after cleaning

1. It tackles what is left in the air. Even careful wet clean-up can leave particles suspended in the air and on surfaces nobody can see. Gas treats the whole room at once.
2. It follows the rats' routes. Behind wardrobes, under beds, inside drawers and between furniture: places a cloth and spray bottle simply do not reach.
3. It destroys odour at the molecular level. Instead of masking with fragrance, it breaks down ammonia and decay compounds.
4. It reaches smell trapped in soft materials. Curtains, sofas and rugs that were worth keeping lose much of their absorbed odour.
5. It leaves no chemical residue. Ozone reverts to oxygen, which matters in homes with toddlers and grandparents. Read more on using ozone to reduce chemical residue.
6. It covers several kinds of germs in one pass. Viruses, bacteria, and the mould that tends to follow once a flooded house stays damp.
7. It is fast. About 30 minutes per zone plus ventilation, so the family can return sooner than if they wait for a smell to fade on its own.
8. It is tested and certified. Based on test results certified for the company by the Department of Medical Sciences and Intertek; the certificates are available to view.
9. The team understands post-flood work. They assess the odour source first and will tell you honestly if a carcass still needs to come out. See the full process on the Residential Ozone Disinfection page.
10. It gives peace of mind before children and parents come home. Knowing the house has had a thorough disinfection step eases a lot of worry. Ask WHD for an assessment.

Example case: a canal-side house in Nonthaburi

This example case is an illustrative scenario based on the kind of job that comes up often, to show how the work is sequenced.

Somchai owns a detached two-storey house beside a canal in Bang Bua Thong. Five days after the water left, workers had shovelled out the mud, washed the floors and thrown away the ground-floor furniture. Yet a rotten smell in his upstairs study refused to go, even though that room had never been flooded. When WHD surveyed the house, the team noticed the odour was strongest in the corner next to the rainwater downpipe and suggested he have a builder open the ceiling there first. Behind the panel was a dead rat and a nest made of shredded paper.

Once the carcass was removed and the spot cleaned with disinfectant using the wet method, the team returned to ozone-treat the whole house in six zones: kitchen, living room, three bedrooms and the study, with every cupboard and drawer open and the family waiting at a relative's home. After the rest and ventilation period, Somchai walked through with the team. The smell that had forced him to keep the study shut was barely noticeable. What stayed with him, he said, was that nobody rushed to sell him a service; they pointed him to the source first.

The lesson: rodent odour usually comes from somewhere unexpected. Ozone works best once the source has gone, as the step that deals with the odour and germs left across the house.

Recovery checklist: before and after ozone

Before you clean

• Wear thick rubber gloves, a mask that filters fine dust, eye protection and boots. Cover every cut with a waterproof plaster.
• Open doors and windows for at least 30 minutes before starting, and stay out while the space airs.
• Keep children, older relatives, pregnant family members and pets away until the work is finished.

While you clean

• Never sweep or vacuum dry droppings. Soak them with disinfectant, wait about five minutes, then wipe up with paper towels.
• Spray a carcass with disinfectant, pick it up with an inverted plastic bag, and double-bag it before disposal.
• Throw away any food rats could reach, including chewed rice sacks.
• Wash dishes and utensils in hot soapy water, then disinfect again.
• Hot-wash contaminated bedding, curtains and clothes; discard heavily nested materials.
• Remove gloves carefully and wash your hands with soap every time.

Before ozone day

• Make sure every carcass is out. If one spot still smells of decay, tell the team.
• Let the house dry fully, leave cupboards and drawers open, and move plants outside.
• Put away important documents and valuables.

After ozone

• Re-enter only after the rest and ventilation period the team specifies.
• Seal entry points around pipes, vents and under doors with metal mesh or other gnaw-proof material.
• Store food in sealed containers, take rubbish out daily, and trim branches touching the roof.
• Watch everyone's health for two to three weeks. Fever, calf pain, red eyes, reduced urine or breathing difficulty mean an immediate visit to a doctor, mentioning rodent and floodwater exposure.

Frequently asked questions

Will ozone drive the rats away?
Do not expect that. Ozone treatment is for disinfection and odour reduction, not for killing or repelling rodents. Prevention means sealing entry points, managing food and rubbish, and calling a specialist pest controller if there are many rats.

We have not found the carcass yet. Can we ozone first?
Ozone will reduce odour in the air, but a carcass left in place keeps decomposing and producing new smell. The team will recommend locating the source first. The smell is usually strongest near it, and flies often circle the spot.

How long do we have to stay out?
It depends on the number of zones and the size of the home. Allow about 30 minutes per zone plus the ventilation period; the team will tell you exactly when you can go back in. No people, pets or plants may remain inside during treatment.

If we ozone, can we skip cleaning up the droppings?
No. Ozone comes after removing carcasses, collecting droppings, washing and disinfecting surfaces. It does not replace those steps.

Someone has a fever after cleaning a rat-infested house. What should we do?
See a doctor quickly and mention contact with rat droppings, urine or floodwater. Leptospirosis and hantavirus infection need medical diagnosis and treatment. Ozone treatment is not a medical treatment.

Will ozone damage furniture or electronics?
The team controls concentration and duration to suit the space. Let them know in advance about valuable items or special materials so you can plan together.

Will we need a second treatment?
If the odour source is gone and the house is dry, one treatment is usually enough. If materials hold a lot of smell, or another carcass turns up later, a repeat treatment of that zone may be worthwhile.

Rodent smell still there? Let WHD finish your flood recovery

Free consultation, site assessment and an ozone disinfection and deodorising plan for homes, condos, rentals and buildings of any size.

Book Ozone Disinfection Now

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

Related reading

• Residential and Building Ozone Disinfection
• Disinfection services for homes and organisations
• Certificates and test results
• Contact WHD

References

• CDC: Floods for safe clean-up after flooding
• Department of Disease Control, Thailand for leptospirosis and flood-related disease