A typical scenario: a Chaeng Watthana office after a week-long closure

This is an illustrative example case, not an interview with a real person or a description of any specific building. Flood facts come from a published news report.

According to a Dailynews report on 26 September 2026, Chaeng Watthana Road was under almost 1 metre of water. Small vehicles were barred, the Chaeng Watthana and Muang Thong Thani expressway exits were closed, and many cars were submerged. BTS offered free parking at several stations from 26 September to 2 October, and Nonthaburi province declared disaster zones in Mueang Nonthaburi and Pak Kret districts.

Picture Boy, the manager of an eight-storey office building on Chaeng Watthana Road. With staff unable to travel, the building closed almost entirely for about a week. The basement, home to the car park and the water pump room, flooded. The lobby walls carried a line of mud. The central air-conditioning system was shut down, the rooftop cooling tower stood idle, and the water in the building's pipes was barely used the whole time.

When the water fell, management wanted the building open quickly. Boy organised mud removal, floor washing and electrical checks. But the engineer responsible for the air-conditioning warned him that before restarting the cooling tower or letting staff use the gym showers, the stagnant water in the system had to be dealt with, because those were exactly the conditions that favour Legionella. Boy's question changed from "how fast can we reopen?" to "what needs doing, and in what order?"

What is Legionella?

Legionella is a Gram-negative bacterium that lives naturally in freshwater such as rivers and lakes, where numbers are usually low and rarely cause problems. The species that most often causes illness in people is Legionella pneumophila. Trouble starts when the bacteria get into human-made water systems that let them multiply.

The US Centers for Disease Control and Prevention (CDC) explains that Legionella grows best in warm water, roughly 25 to 45 degrees Celsius. Factors that help it multiply in buildings include stagnant or slow-moving water, low levels of disinfectant such as chlorine, sediment and scale in pipes, and biofilm, the slimy layer microbes build on pipe walls, which shields bacteria from disinfectants.

How people catch Legionella

Unlike many post-flood germs, Legionella is not caught by touching floodwater or contaminated surfaces. People become infected by breathing in small droplets of water that contain the bacteria, for example mist from showers, taps, cooling towers, hot tubs or decorative fountains. Less commonly, it happens when someone accidentally aspirates contaminated drinking water into the lungs. Drinking water normally, without choking, is not an important route, and the disease is generally not spread from person to person.

Can household air conditioners spread Legionella?

This is a common misunderstanding. The CDC states that home and car air-conditioning units generally do not spread Legionella, because they do not use water to cool. What matters is the cooling tower, part of the central air-conditioning system in large buildings. Cooling towers use water to remove heat and release mist into the air. If the water is contaminated, the droplets can drift and be inhaled by people nearby. Filters, cooling coils and drain pans in ordinary air conditioners should still be cleaned to reduce dust and mould, but according to the CDC they are not a main Legionella source.

Why buildings closed by floods face a Legionella risk

Water stagnant for days. With nobody using water, flow stops in pipes, tanks and water heaters, and the chlorine in the water gradually loses strength. The CDC publishes specific guidance for reopening buildings after prolonged shutdowns because stagnation is such an important driver of Legionella growth.

Temperatures the bacteria like. In hot, humid Thai weather, water in rooftop pipes or unventilated shafts can warm into the growth range. Water heaters that have been switched off hold water that has cooled from the temperatures that normally keep bacteria in check.

Cooling towers left idle without treatment. A cooling tower that sits with water in its basin and no chemical treatment is a risk point that needs cleaning and water treatment by specialists before it restarts.

Floodwater in pump rooms and tanks. Many buildings keep pumps and storage tanks in the basement. When floodwater gets in, sediment and contaminated water can enter the system. Technicians must check tanks and pipes for damage and clean them before use.

Sediment and pipe disturbance. Pressure changes, emergency repairs and sediment shaken loose from pipe walls release biofilm and scale into the water, the very material Legionella lives in.

Places in an office building to check for Legionella

• Central air-conditioning cooling towers, the most important point for wide-area spread.
• Basement and rooftop water tanks, plus pumps that may have flooded.
• Water heaters and hot-water systems in gym showers, executive suites or kitchens.
• Shower heads and tap outlets, especially in rarely used rooms.
• Dead legs, pipe runs to outlets that are seldom used, such as closed-off floors or storeroom taps.
• Plumbed-in water dispensers and ice machines, which should be cleaned and have filters changed according to the manual.
• Decorative fountains, lobby water features and misting systems that create spray.
• Bidet sprays and flexible shower hoses that hold water inside.

The main job in these areas is managing the water in the system, which must be done by building engineers or specialist water-treatment companies. Mud-soaked areas and surfaces people touch are where disinfection spraying comes in.

Symptoms of Legionnaires' disease and Pontiac fever

The following is general information based on CDC and WHO material to help you recognise symptoms. It is not a diagnosis.

Legionnaires' disease

A type of pneumonia. Symptoms usually begin 2 to 14 days after exposure, though sometimes later, and can include:

• Cough, shortness of breath or difficulty breathing.
• Fever and chills.
• Muscle aches and headache.
• In some people, diarrhoea, nausea or confusion.

It is a serious illness that needs hospital treatment. Diagnosis requires laboratory tests, such as a urine test or culture of sputum, which a doctor will decide on.

Pontiac fever

A milder, flu-like illness with fever, headache and muscle aches but no pneumonia. Symptoms usually start within a few hours to three days after exposure and generally clear up on their own fairly quickly.

When to see a doctor

• Fever, cough or breathlessness within two weeks of returning to work in a building that has just reopened after a flood.
• Pneumonia-like symptoms in anyone aged 50 or over, a smoker, or a person with chronic illness.
• Tell the doctor you work in a building that was shut for days, or that you used its showers, to help them decide on testing.
• Anyone who waded through floodwater and then develops fever or calf pain should mention the flood exposure, as it could be leptospirosis.

Who is most at risk of Legionnaires' disease?

Most people exposed to Legionella do not get sick. The CDC identifies these groups as being at higher risk of serious illness:

• People aged 50 or older.
• Current or former smokers.
• People with chronic lung disease such as emphysema.
• People with weakened immune systems or taking immune-suppressing medicines.
• People with cancer.
• People with underlying illnesses such as diabetes, kidney failure or liver failure.

Any office building has plenty of staff, executives, cleaners and visitors in these groups, which is why building managers have a duty to reduce the risk before reopening. A Legionnaires' case linked to a building means more than one sick employee: it can mean an investigation with public health authorities, shutting water systems or cooling towers again for repeat cleaning, and anxious staff and tenants. All of that costs far more than doing things properly the first time.

Why the usual fixes are not enough for Legionella

Running the taps briefly and carrying on. Flushing is the right idea, but it needs to be done systematically throughout the building, hot and cold, at every outlet, following an engineer's plan, not just at the taps people use most.

Restarting the cooling tower as soon as power returns. A stagnant cooling tower must be cleaned and its water treated according to specialist guidance first. Restarting immediately could send contaminated mist into the air.

Assuming room spraying covers the water system. Surface disinfection does not reach water in pipes, tanks or cooling towers. They are separate jobs that have to run side by side.

Washing air-conditioner filters and calling it done. Cleaning AC units helps air quality, dust and mould, but it is not Legionella control for cooling towers or hot-water systems.

Masking musty smells with air freshener. A musty odour after a flood signals lingering damp and mould. Covering it up does not fix the cause.

Where WHD post-flood disinfection spraying helps an office building

Let us be clear from the start: surface disinfection spraying is not a way to control Legionella in water systems. That work, including flushing, raising chlorine levels in the system, cleaning cooling towers and testing water quality, must be done by building engineers or water-treatment specialists, following a building water management program of the kind the CDC recommends.

What the World Health Disinfection (WHD) spraying service does help with is the rest of the reopening: lobbies, basements, pedestrian areas of car parks, restrooms, pantries, meeting rooms, desks, lifts and shared touch points that were soiled by floodwater or left closed and damp for days. According to the service information, trained technicians spray and/or ULV-fog (Ultra Low Volume fine mist), which suits large spaces such as offices. The website lists customers including office buildings, banks, shopping malls, hospitals and airports.

The disinfectant is CHEMGENE HLD4H, a medical-grade disinfectant from the United Kingdom containing Benzalkonium Chloride and Didecyldimethylammonium Chloride. According to the product and service information it is broad-spectrum. The organisms listed on the website include SARS-CoV-2 (COVID-19), Human Coronavirus 229E, RSV, influenza H1N1 and H5N1, HIV, norovirus, hand-foot-and-mouth virus, vaccinia virus, mycobacteria, Salmonella, Bordetella pertussis, bacteria in general, and fungi. The service page states 99.99% efficacy and the product page 99.85% against COVID-19 within one minute, and the service page lists approvals or endorsements from the Thai Department of Medical Sciences, Mahidol University, NHS England and EN 14476:2013+A2:2019. See the certificates page.

Straight talk about Legionella

Legionella is not specifically named in the organism list on WHD's website, so we do not claim a tested kill result for it. Legionella is a bacterium, which belongs to the groups the product states it covers for surface disinfection, and WHO and CDC guidance advises cleaning and then disinfecting surfaces contaminated by floodwater. Organisations that need test data for this organism can ask WHD for the documents directly. Whatever surface test data shows, the water system must still be managed by water-system specialists.

The service steps, per the service information

1. Book by phone or LINE with the number of floors, the area and the flooded zones.
2. Prepare: remove mud, wash away residue, and clear documents and food from the area.
3. Technicians spray the agreed zones. Staff and visitors must leave the zone being sprayed; out-of-hours work is recommended.
4. Wait about 30 minutes, per the service information, and re-enter at the time the technicians advise.
5. Use the space normally.

Do not assume nothing remains on surfaces afterwards. Ask the technicians about wiping food-contact surfaces such as pantry tables. Per the promotion in the service information, customers receive a free CHEMGENE spray and WELLGIENIC wet wipes, and tax invoices are available, which is convenient for building juristic persons.

Book Post-Flood Office Disinfection Spraying

A safe reopening order for an office building after a flood

1. Assess structural and electrical safety. Have engineers check electrics, lifts and plant rooms before use.
2. Remove mud and discard unsalvageable materials such as carpets, wet ceiling tiles, soaked gypsum board and swollen furniture.
3. Have specialists manage the water system. Inspect and clean flooded tanks and pumps, flush every outlet hot and cold, and check chlorine levels and hot-water temperatures according to the engineer's plan.
4. Clean and treat the cooling tower through a water-treatment company before restarting central air-conditioning.
5. Clean the rest of the AC system: filters, coils, drain pans and ducts, to reduce dust and mould.
6. Wash and dry the lobby, basement, restrooms and pantries.
7. Spray-disinfect surfaces in every flooded zone and at shared touch points, out of hours.
8. Clean spray-generating fixtures such as showers, water dispensers and fountains before use.
9. Communicate with staff. Ask them to watch for fever, cough or breathlessness over the first two weeks and to see a doctor if symptoms appear.
10. Record every step as evidence of the building's risk management.

If meeting rooms or closed rooms still smell musty, you might add the ozone disinfection service, which must be done in closed rooms with no people, animals or plants inside.

10 reasons office buildings should spray-disinfect surfaces after a flood

1. Flooded areas are contaminated with sewage and must be cleaned and disinfected before staff return.
2. Buildings have many shared touch points: lift buttons, handrails, door handles and fingerprint scanners.
3. Fine-mist spraying suits large spaces and is faster than wiping spot by spot.
4. Medical-grade disinfectant: CHEMGENE HLD4H is stated to be broad-spectrum.
5. Out-of-hours service avoids disrupting work.
6. A short wait: about 30 minutes per the service information.
7. It fits the reopening plan, running alongside the engineers' water-system work.
8. Experience with many building types, as listed on the website. See service details.
9. Tax invoices available for corporate accounting.
10. Reassurance for staff and tenants that the building was restored systematically. Request a quotation.

Before and after: a building reopened in the right order

Example case: Boy's reopening plan

An illustrative example case, not a real customer review.

Boy split the work into two tracks. The first was the water system and cooling tower, which he assigned to the building engineers together with a water-treatment company. They inspected and cleaned the flooded basement tank, flushed every floor hot and cold, and cleaned the cooling tower before restarting the air-conditioning. The second track was the occupied space. The housekeeping team removed mud and washed the lobby and basement, and then the spraying team treated the lobby, lifts, restrooms on every floor, pantries and shared touch points on Friday night.

On Monday morning the building opened with a notice telling staff what had been done and asking anyone with fever, cough or breathlessness in the next two weeks to see a doctor and inform building management. Boy's lesson: disinfection spraying and water-system work are not either-or choices. They are two jobs that must run together.

Checklist for building managers

• A building water management plan identifying Legionella risk points.
• Flooded tanks and pumps inspected and cleaned.
• Every outlet flushed, hot and cold.
• Cooling tower cleaned and treated before restart.
• Rest of the AC system cleaned.
• Flooded areas washed, dried and spray-disinfected out of hours.
• Showers and spray-generating fixtures cleaned.
• Staff informed about symptoms to watch for.
• Records and documents kept for every step.

Frequently asked questions

Can you catch Legionella directly from floodwater?

That is not the main route. Legionella is caught by inhaling contaminated water droplets, usually from building water systems with stagnant water, cooling towers or showers. Floods raise the risk indirectly by forcing buildings to close and water to stagnate.

Are split-type air conditioners in homes or offices a source?

The CDC states that home and car air conditioners generally do not spread Legionella because they do not use water for cooling. They should still be cleaned after a flood to reduce dust and mould.

If we spray-disinfect, can we skip the water-system work?

No. Spraying treats surfaces and does not reach water in pipes or cooling towers. The water system must be handled by specialists.

Does CHEMGENE kill Legionella?

According to the product information, CHEMGENE HLD4H is broad-spectrum against bacteria, viruses and fungi, but Legionella is not specifically named in the website's organism list. You can ask WHD for test data, and the water system must be handled separately regardless.

Is Legionnaires' disease contagious between people?

Generally, no.

When should spraying happen in the reopening plan?

After mud removal, washing and drying, and preferably out of hours so staff stay out of the area throughout.

Reopen with confidence after the flood

If your building is on Chaeng Watthana Road in Pak Kret or anywhere else that has just flooded, plan two tracks side by side: engineers for the water system and cooling tower, and a disinfection team for surfaces throughout the occupied space.

See Details and Request a Spraying Quote

Phone 065-556-6294 · LINE @whd268 · Email worldhealthdisinfection@gmail.com · All contact channels

Remember: disinfection spraying is not medical treatment and not water-system treatment. Anyone with fever, cough, breathlessness, or calf pain after wading should see a doctor and mention their exposure.

Related links and references

Internal links

• World Health Disinfection spraying service
• CHEMGENE HLD4H medical-grade disinfectant
• Residential and building ozone disinfection
• WHD certificates

References

• Dailynews, 26 September 2026, flooding on Chaeng Watthana Road, Pak Kret: dailynews.co.th/news/6223597
• World Health Organization (WHO) fact sheets, including Legionellosis: who.int/news-room/fact-sheets
• CDC Legionella information and flood guidance: cdc.gov/legionella and cdc.gov/floods
• Department of Disease Control, Thailand: ddc.moph.go.th

#Legionella #LegionnairesDisease #DisinfectionSpraying #PostFloodDisinfection #FloodGerms #Chemgene #OfficeDisinfection #ChaengWatthanaFlood

Keywords: Legionella after flood, Legionnaires' disease office building, building water system reopening, cooling tower Legionella, office disinfection after flood, Chaeng Watthana flood 2026, Pak Kret, post-flood disinfection spraying, CHEMGENE HLD4H