Head teacher Wanna is 56 and has run a 180-pupil private kindergarten in Bang Khae for 19 years: nine classrooms, three nap rooms, an activity room and a dining room, all air-conditioned and sealed against PM2.5.

In the first semester of the 2025 academic year, sickness absence ran abnormally high — an average of 19 children absent out of 180 every day, over 10 percent, for almost the entire term.

The repeating symptoms were not ordinary colds but persistent cough, congestion, and asthma flare-ups in children with a history of allergy. By mid-term, 22 parents had given notice that they were moving their children for the following semester.

The story

The statistics one parent put on the desk

The parent worked in data. She had kept a log of her four-year-old son's symptoms for two years, because he is allergic and she reports to his doctor at every appointment.

What she showed the head teacher was a very clear graph: her son averaged 3 days a month of coughing and congestion during school holidays, and 14 days a month during term. The pattern repeated identically across both years.

She said she was not there to complain. She was there because she believed the school did not know, and she wanted them to investigate before she had to move her son.

Head teacher Wanna says her instinct was to answer as she had for nineteen years — that small children together simply get sick. But looking at that graph she could not say it, because the graph did not say the child got sick. It said the child got sick only when he was at school.

What they found when they opened all sixteen air conditioners

She had an air conditioning technician inspect the whole school on a Saturday. He opened the face panel of each of the 16 units in turn. Nearly every one showed sheets of black-green growth on the coil and in the condensate tray.

He explained that the school services its units twice a year, which meets the general standard, but scheduled servicing only cleans the coil and removable filters. Nobody reaches inside the duct work, the ceiling void, or the deeper parts of the machine.

What makes a school worse than an ordinary building is the usage pattern: eight hours a day, five days a week, then sealed with no ventilation at all through every weekend and every school holiday.

The two-month holiday is the worst period. Rooms sealed, moisture sitting in the coils, no air conditioning running, no circulation. Then on the first morning of term, every unit is switched on at once — and two months of accumulation is blown out over 180 children simultaneously.

Why small children are affected many times more than adults

The paediatrician the school consulted gave three reasons why kindergarten children are the highest-risk group for air quality.

First, breathing rate relative to body weight. Small children inhale several times more air per kilogram of body weight than adults, which means that in identical air, a child receives a far higher dose of spores and allergens per unit of body mass than their teacher.

Second, height. Kindergarten children stand 90 to 110 centimetres tall and spend most of their time sitting or lying on the floor — the level where dust and settling spores accumulate most.

Third, immune systems and airways that are still developing. A small child's airways are far narrower, so a modest amount of inflammation narrows them enough to trigger wheezing far more easily than in an adult.

What the school had always done, and why it was not enough

Head teacher Wanna stresses that her school was not negligent. Every toy is wiped with disinfectant each evening, nap mats are put out in the sun weekly, floors are mopped twice a day, and every child's temperature is checked each morning.

The school also installed air purifiers in every classroom during the worst PM2.5 seasons, at over four hundred thousand baht. They genuinely helped with particulates and did nothing at all about spores coming out of the air conditioning vents.

After the mould was found, the school had all 16 units serviced immediately at nearly ninety thousand baht. Absence rates did fall for the first two weeks, then climbed back up within about six weeks.

The same technician told her plainly that servicing alone is not enough for a school, because the organisms in the duct work and ceiling void remain — and recommended she find an ozone treatment service, the method hospitals use where airborne contamination must be controlled.

What it actually costs

Why the usual fixes fail

10 reasons ozone treatment solves it

1. It reaches the duct work and ceiling void that servicing cannot

With the air conditioning recirculating during the cycle, ozone travels through the coil, the condensate tray and the whole duct system — the real spore source that twice-yearly servicing never touches.

2. No chemical residue on toys children put in their mouths

This is the decisive reason for a kindergarten. Ozone decomposes back into ordinary oxygen, so there is no deposit or film on toys, tables, nap mats or floors children crawl on, and nothing to rinse off afterwards.

3. 99.99% removal of embedded mould

Testing confirms 99.99% removal of deep-seated mould in air conditioning systems and furnishings, with virus and bacteria kill an order of magnitude beyond that — the level required for rooms where 180 small children share air.

4. Nap mats, soft toys and carpet treated at the same time

Nap mats, plush toys, pillows and activity room carpet are the school's biggest spore and dust mite reservoirs. Ozone penetrates the fibres of all of them in one cycle, with nothing sent out for laundering.

5. It reduces dust mites, the leading allergy trigger in children

A nap mat used daily in a sealed air-conditioned room is perfect dust mite habitat. Ozone reduces both the mites and the allergens they leave — the most common asthma trigger in this age group.

6. It adds no humidity to rooms sealed every weekend

Steam and wet washing add water to rooms about to be sealed for two days, feeding mould directly. Ozone is a dry gas and leaves nothing behind for the next growth cycle.

7. About thirty minutes per room, done at weekends or in the holidays

The automatic digital timer cycle runs roughly thirty minutes — two to start, twelve to disperse, twelve to purify, twelve to rest. A nine-classroom school is completed in a single Saturday with no teaching time lost.

8. It should be standard before every term starts

The most dangerous moment is the first morning of term, when every unit is switched on at once after two months sealed. Treating before term begins cuts the problem at exactly the point it actually happens.

9. The Master Ozone Generator is certified by the Department of Medical Sciences

WHD uses the Master Ozone Generator, the only ozone brand in Thailand certified by the Department of Medical Sciences, Ministry of Public Health, and tested by Intertek UK — documentation a school can show to parents.

10. It gives you a concrete answer when parents ask

When a parent asks what the school is doing, saying that every room is ozone-disinfected before each term and monthly during term, with certified equipment, is completely different from saying you have increased cleaning.

Before and after

How we get you started, in 3 steps

What a customer says

Why air-conditioned classrooms in Thailand are riskier than people realise

Over the past decade many Thai schools have converted from open, fan-cooled classrooms to sealed air-conditioned ones, for good reasons: PM2.5 protection and comfort in the heat.

But that change carries a side effect almost nobody discusses. The classroom becomes a sealed space recirculating the same body of air for eight hours, with no fresh outside air introduced at all.

In Thailand's humidity, averaging 70 to 85 percent, a coil condensing continuously is the ideal environment for mould. And a school's usage pattern — sealed every weekend and every holiday — gives it long uninterrupted periods to grow freely.

The result is that a system installed to keep outside dust away becomes an indoor allergen generator, and children are the group it affects most.

The air quality checklist every school should have

After this, head teacher Wanna's school rebuilt its standards, and she says the most important change was to stop equating cleanliness with what is visible.

The new standard has five points: service every unit at least twice a year; ozone-treat every room before every term; ozone-treat monthly during term; run the air conditioning for at least two hours on Friday evening before the weekend closure to drive out residual moisture; and keep documentation each time for parents.

That last point matters more than people expect, she says, because when parents ask what the school is doing, having documentation and a defined schedule to show is completely different from an assurance in words.

Her advice to other schools is not to wait for a parent to bring the data to you — because by then, several other parents are thinking the same thing and quietly moving their children without saying anything at all.

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