A typical scenario: three weeks in a classroom that became a bedroom

(Example case. The people and events in this section are illustrative, not real individuals.)

Auntie Lek lives in a two-storey wooden house on the bank of the Bang Pakong River. In late September 2026 the water rose faster than she could remember it ever doing. The ground floor went under in a single night, so her son moved her, her five-year-old grandson and two elderly in-laws in their eighties to a hilltop school, where dozens of neighbouring families were already settling in.

At first it was bearable. Sleeping mats were laid edge to edge until there was barely a walkway. A handful of toilets served the whole building. Several windows stayed shut against rain and mosquitoes, and one fan pushed the same air round and round. By night, coughing echoed from every corner. Within a week, the grandson had a low fever, a blocked nose and a sore throat, followed by a dry cough that would not stop. A few days later, several people in the same room had the same symptoms.

Visiting health staff advised testing for anyone with symptoms, masks, a separate sleeping corner for the sick and every door and window open for airflow. When the water finally went down and families began heading home, Auntie Lek asked her son a question that thousands of people are asking right now: "Is our house safe? Will the old folks bring something home with them? And what about the school my grandson has to go back to?"

The honest answer: respiratory viruses travel mainly through the air, so spraying is not a magic shield. It is one layer that reduces germs on frequently touched surfaces and helps a building be handed back in an organised way.

The 2026 Chachoengsao flood: all 11 districts affected

According to Thai PBS, flooding reached all 11 districts of Chachoengsao province. The Bang Pakong River could not drain to the sea fast enough, while water released from the Khun Dan Prakan Chon Dam added to the volume. Route 3121 between Bang Khla and Plaeng Yao became impassable, and riverside homes suffered heavy damage. Figures from the Department of Disaster Prevention and Mitigation (DDPM) dated 2 October 2026 put the number of affected households at 47,959.

When that many households are affected, people inevitably move together into safer places: temporary shelters, temple pavilions, school buildings, relatives' homes, or the upper floor of a neighbour's house shared by several families. There is also a second, often overlooked category of crowded space: relief distribution points, volunteer kitchens and first-aid tents, where large numbers of people breathe the same air and handle the same objects within a few hours.

Crowding is the key condition for fast respiratory spread, which is why this article focuses on the coronavirus family.

Understanding COVID-19 (SARS-CoV-2) and human coronavirus 229E

Coronaviruses are a large family of RNA viruses wrapped in a fatty outer layer called an envelope. Spike proteins stick out of that envelope like the points of a crown, which is where the name "corona" comes from. Several members of the family infect people. Some cause mild illness; others can be severe.

SARS-CoV-2, the cause of COVID-19

SARS-CoV-2 is the virus behind COVID-19, which the World Health Organization (WHO) characterised as a pandemic in 2020. The global emergency has passed, but the virus keeps mutating and new sub-variants keep emerging, so infections continue to circulate, especially when people gather in large numbers. Illness ranges from no symptoms at all to fever, cough, sore throat, runny nose, body aches, fatigue and changes in smell or taste, and in some people shortness of breath or pneumonia.

HCoV-229E, a seasonal coronavirus

Human coronavirus 229E was making people sick long before COVID-19 appeared. Together with HCoV-OC43, HCoV-NL63 and HCoV-HKU1, it is responsible for a share of ordinary colds. Healthy adults usually get a stuffy nose, runny nose and sore throat. In young children, older adults and people with weakened immunity, though, these viruses can cause bronchitis or lower respiratory tract infection. In a crowded shelter, a cold that is trivial for one person can become a real problem for the person sleeping next to them.

The weak spot of enveloped viruses

The good news is that enveloped viruses such as coronaviruses are among the easier microbes to inactivate. Soap, alcohol and most standard disinfectants disrupt the fatty envelope, and once that envelope is broken the virus can no longer enter cells. That is why handwashing with soap and correctly applied surface disinfectants work well against this group, provided surfaces are clean and the disinfectant stays wet for the contact time the manufacturer specifies.

How COVID-19 and other coronaviruses spread in crowded post-flood spaces

Let us be clear from the start. Coronaviruses do not arrive in floodwater the way sewage bacteria do. The main route for SARS-CoV-2 and HCoV-229E is person to person, through droplets and smaller aerosols released when people cough, sneeze, talk, sing or simply breathe, especially in poorly ventilated rooms. Touching a contaminated surface and then touching the eyes, nose or mouth is a possible secondary route. International health agencies therefore recommend ventilation, masks for people with symptoms or in crowded settings, and cleaning of frequently touched surfaces, all at the same time.

Floods create several conditions that favour respiratory spread at once:

• Density. People from several villages sleep in one room, with less than an arm's length between mats.
• Poor ventilation. Windows are closed against rain, mosquitoes or cold nights; large halls may have few openings.
• Shared objects. Cups, ladles, kettles, the power strip everyone uses to charge phones, stair rails and toilet door handles are touched by hundreds of hands a day.
• Exhaustion and stress. Short sleep, irregular meals and worry about home all weaken the body's defences.
• Harder access to care. With roads like Route 3121 between Bang Khla and Plaeng Yao cut, getting to hospital takes longer.
• Vulnerable people close together. Older adults, small children, pregnant women and people with chronic illness sleep near others who may be infected without knowing it.

The cycle does not end the moment the water recedes. People infected in a shelter can carry the virus home, and the shelters themselves, usually schools, temples or community halls, must be handed back to students, monks and regular users. That transition is exactly when systematic cleaning and disinfection matter most.

Where coronaviruses linger in shelters and returning homes

Viruses cannot multiply outside cells but can persist on surfaces for a time that depends on material, temperature and humidity. Prioritise high-touch surfaces, not every square inch.

In shelters, temples and school buildings

• Door handles and knobs on classrooms, toilets and storerooms
• Stair rails, ramp rails and table edges people push on to stand up
• Taps, flush handles, water dippers and handbasins
• Dining tables, registration desks and distribution counters
• Light switches, fan remotes and shared phone-charging power strips
• Plastic chairs that are moved around all day, and pavilion bench backs
• Shared refrigerator doors and water dispensers

In homes people are returning to

• Bags and belongings carried to the shelter and back
• Door handles, wardrobe doors and drawers everyone opened while moving things
• Mobile phones, remotes and children's tablets
• Beds, hammocks and chairs used by sick or elderly family members
• The bathroom, which everyone in the house shares

Floods also leave sewage bacteria and mould behind, so look at the whole picture.

Symptoms to watch, and who is most at risk

Early COVID-19 and a seasonal coronavirus such as HCoV-229E look very similar and cannot be told apart by eye. Common symptoms include fever, cough, sore throat, blocked or runny nose, headache, muscle aches and tiredness, sometimes with diarrhoea. A change in smell or taste occurs in some COVID-19 cases. A rapid test or an assessment by health staff is the reliable way to tell the difference.

People who need extra protection

• Older adults, especially those over 60 with underlying conditions
• People with chronic disease such as diabetes, high blood pressure, heart, kidney or chronic lung disease, and obesity
• People with weakened immunity, for example those on immunosuppressive drugs or chemotherapy
• Pregnant women, whose bodily changes can make illness more severe
• Young children, who cannot manage their own hygiene and constantly put hands in mouths
• Volunteers and officials who meet large numbers of people every day

For vaccination, talk to local health staff or your sub-district health promoting hospital, who will advise according to Department of Disease Control guidance and each person's health status.

The impact goes beyond feeling ill

When a respiratory virus moves through a community that has just been flooded, the effects pile up on each other.

• Health. Older and chronically ill people, worn down by evacuation, may need hospital care just as local services are stretched.
• Money. Sick breadwinners miss work just when repairs and livelihoods need them, and detours make doctor visits costlier.
• Time. Restoring homes, shops and farms slows down when several working adults are sick at once.
• Education. Schools used as shelters reopen later, or reopen to classrooms full of coughing children.
• Confidence. Parents hesitate to send children back, older relatives avoid the temple, and shop owners worry that customers will stay away.

Prevention from day one is cheaper than handling an outbreak later.

Why the usual cleaning habits may not be enough

Hard work does not always bring the expected result:

1. Wrong product or wrong dilution. Bleach diluted too much does little; too strong, it damages surfaces and irritates airways. Mixing bleach with acidic or ammonia-based toilet cleaners can release dangerous gases.
2. Wiping and rinsing straight away. Every disinfectant needs a contact time. Wipe it off immediately and some germs survive.
3. Cleaning only what looks dirty. Light switches, stair rails and the underside of table edges look clean and get skipped.
4. One cloth for the whole building. A cloth used in the toilet and then on the dining table simply moves germs around.
5. Relying on air fresheners. A pleasant smell is not the same as clean, and fragrance does nothing to germs on surfaces.
6. Doing it once. Busy buildings are recontaminated daily, so a routine is needed too.
7. Forgetting the air. However well surfaces are wiped, a stuffy, crowded room still lets respiratory viruses spread through the air.

Surface spraying cannot replace ventilation, masks, isolation and handwashing. It completes the picture.

WHD disinfection spraying: filling the gap in post-flood recovery

World Health Disinfection (WHD) provides a disinfection spraying service carried out by trained technicians, using spraying and/or ULV (Ultra Low Volume) fine-mist fogging so the disinfectant reaches surfaces and corners that are hard to wipe by hand. According to the service information, the disinfectant is CHEMGENE HLD4H, a medical-grade disinfectant from the United Kingdom containing benzalkonium chloride and didecyldimethylammonium chloride, both quaternary ammonium compounds.

What the efficacy information says about coronaviruses

According to WHD's product and service information, the organisms the product is stated to cover include SARS-CoV-2 (COVID-19) and human coronavirus 229E by name, along with RSV, influenza H1N1, avian influenza H5N1, hand-foot-and-mouth virus, norovirus and other bacteria, viruses and fungi. The service page states 99.99% efficacy, and the product page states 99.85% against COVID-19 within one minute. The service page also references the Thai Department of Medical Sciences, Mahidol University, NHS England and EN 14476:2013+A2:2019, the European standard for testing the virucidal activity of chemical disinfectants.

These figures come from tests under defined conditions; real-world results depend on surface cleanliness, coverage and contact time. WHD describes the product as broad-spectrum, killing many types of germs, without giving a number of strains. Ask WHD for the test report for any specific organism before you book.

How the service works

1. Book and assess. Share the floor area, building type, number of rooms and where people gathered most.
2. Prepare the area. Remove mud and rubbish, wash surfaces with water and detergent, put away food, food containers and personal items, and make sure everyone leaves the zone to be sprayed.
3. Technicians spray, starting with high-touch surfaces and the highest-risk rooms.
4. Wait for the time the technicians specify, typically around 30 minutes according to the service information, then ventilate.
5. Return to the space when the technicians say so, and keep up daily cleaning of high-touch points.

WHD lists hospitals, schools, malls, airports, offices, airlines, banks and residential buildings among its customers.

10 reasons to spray for COVID-19 and coronaviruses before reopening a crowded space

1. The target viruses are named. According to the product information, SARS-CoV-2 and HCoV-229E are on the list, not merely implied by a germ group.
2. Enveloped viruses are a good match for quaternary ammonium disinfectants when surfaces are clean and contact time is respected.
3. Fine mist reaches awkward places such as table legs, under counter lips and complex shapes.
4. One visit covers several germ groups. Beyond respiratory viruses, the service information lists bacteria and fungi that follow floods.
5. It saves exhausted volunteers' energy for other recovery work.
6. Trained technicians wear protective equipment and control the dose, avoiding the risks of mixing chemicals at home.
7. It scales to large buildings, from temple pavilions and assembly halls to canteens and multi-storey classroom blocks. See the service details.
8. It rebuilds confidence. Parents, temple visitors and customers know the space has been handled systematically.
9. Documentation and tax invoices are available, which helps schools and agencies with reimbursement.
10. Follow-up supplies are included. Under the current promotion, customers receive a CHEMGENE spray bottle and WELLGIENIC wet wipes for daily touch-ups. Ask about the promotion.

Before and after spraying a former shelter

Example case: a sub-district school that served as a shelter

(Illustrative example, not a verified customer review.)

Somjai looks after the buildings at a school in Chachoengsao that sheltered villagers in two buildings. Once everyone left, she had a few days to prepare for classes, amid news that some evacuees had caught colds and some had tested positive for COVID-19.

Together with the school board, she planned three stages. First, volunteers removed mats, mattresses and rubbish, washed the curtains and scrubbed floors with water and detergent. Second, every window was opened so the buildings could dry. Third, the school booked WHD to spray both buildings, the canteen and all the toilets. Nobody was inside during spraying, and staff returned only at the time the technicians gave.

When classes resumed, the school added a handwashing point, asked children with fever or cough to stay home and see a doctor, and had the caretaker wipe high-touch points daily. Somjai posted a step-by-step summary to the parents' LINE group. Anxiety dropped, not because anyone promised zero illness, but because parents could see a plan carried out.

Checklist: before and after post-flood spraying

Before spraying

• Remove mud, rubbish, mattresses and soaked materials that cannot be saved.
• Wash surfaces with clean water and detergent until visible dirt is gone; disinfectants work best on clean surfaces.
• Open windows and dry the building as much as possible.
• Store food, drinking water, dishes, cups, children's toys and personal items.
• Switch off non-essential electrical equipment and tell technicians about anything that must stay dry.
• Make sure everyone, including pets, leaves the zone to be sprayed.
• Point out where sick people stayed and which areas saw the heaviest use.

After spraying

• Wait the full time the technicians specify before re-entering, then ventilate.
• Wipe food-contact surfaces with clean water before use, as the technicians advise.
• Set up soap-and-water handwashing and alcohol gel at entrances.
• Wipe handles, rails, taps and switches every day.
• Keep doors and windows open for cross-ventilation, or use extractor fans.
• Anyone with fever, cough or sore throat should wear a mask, rest apart from others and see a doctor.
• Watch older relatives and young children closely for the first two weeks.

People-first measures that must go alongside spraying

For completeness, here are the basic measures WHO and Thailand's Department of Disease Control recommend against respiratory infections, all the more important in crowded settings.

• Ventilate. Open doors and windows on opposite sides so air flows through; avoid closed rooms full of people.
• Space out where possible. Arrange sleeping mats head to toe and set aside a separate area for people with symptoms.
• Wear masks when you have respiratory symptoms or care for sick or elderly people.
• Wash hands with soap and water, especially before eating, after using the toilet and after coughing or sneezing.
• Cover coughs and sneezes with a tissue or your elbow, and bin tissues in a lidded container.
• Do not share personal items such as cups, spoons and towels.
• Test when symptomatic and follow health staff advice.
• Get vaccinated as advised by health professionals, particularly if you are in a high-risk group.

Spraying handles surfaces; these measures handle airborne spread. Do both.

FAQ: spraying for COVID-19 and coronaviruses after a flood

1. Will spraying guarantee that nobody catches COVID-19?

No. Spraying reduces germs on surfaces at the time of treatment, but COVID-19 spreads mainly through the air between people. It must be combined with ventilation, masks for people with symptoms, isolation of the sick and handwashing.

2. Does CHEMGENE HLD4H really kill coronaviruses?

According to the product information, SARS-CoV-2 and human coronavirus 229E are among the organisms it is stated to cover. The product page states 99.85% against COVID-19 within one minute and the service page states 99.99%. You can ask WHD for the test documents.

3. Do we have to clean before spraying?

Always. Mud, dust and grease reduce disinfectant performance. The correct order is remove mud, wash with water and detergent, let surfaces dry, then disinfect.

4. Can people stay inside during spraying?

No. Everyone should leave the sprayed zone and return at the time the technicians specify, typically around 30 minutes according to the service information, followed by ventilation.

5. How often should a former shelter be sprayed?

The key moment is before handing it back to normal users; repeat if there is an outbreak. In between, wipe high-touch surfaces daily.

6. Is there any residue afterwards?

Disinfectants of this type can leave a residue; rinse food-contact surfaces and toys with clean water as technicians advise.

7. What if someone at home is already sick?

They should see a doctor first and follow medical advice. Spraying is not treatment, and no room should be sprayed while people are inside.

Reopen shelters, schools, temples and homes after the flood with confidence

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

Related links and references

WHD services and information

• World Health Disinfection spraying service
• CHEMGENE HLD4H medical-grade disinfectant
• Residential ozone disinfection (a complement for musty air)

External references

• Thai PBS on the Chachoengsao (11 districts) and Prachin Buri floods: thaipbs.or.th/news/content/558982
• World Health Organization fact sheets: who.int/news-room/fact-sheets
• CDC, floods and your health: cdc.gov/floods
• Department of Disease Control, Thailand: ddc.moph.go.th