Khun Thanakrit is 47 and general manager of a 120-room hotel in Hua Hin. The property has four function rooms, from a 20-seat boardroom up to a ballroom that seats 250.

Corporate business is the hotel's lifeline — about 38 percent of annual revenue — and the largest single client was an insurance company running four executive seminars a year on contracts worth 2.4 million baht.

In February 2025, on the first day of a three-day seminar, six of the 140 delegates had to leave the room within the first two hours, sneezing, streaming and struggling to breathe. One of them was the company's vice president.

The story

The two hours that ended a four-year contract

Khun Thanakrit was in the room himself that morning. He says that when he opened the door at seven he caught a faint musty note, assumed it was normal for a room that had been closed a while, and told staff to run the air conditioning at full power and spray air freshener throughout.

By the time delegates came in at nine the smell had gone. Everything looked in order — until about half past ten, when one woman began sneezing continuously and excused herself.

Within the next forty minutes five more followed her out. One was the vice president, the person who approves the hotel contract every year. Over lunch she told Khun Thanakrit directly that she is allergic to mould, and that this room definitely had mould in it.

He explained that the room had been cleaned the previous day: floor mopped, tables and chairs wiped down. She said she believed him. But what she was reacting to was not on the floor. It was in the air coming out of the vents.

A room sealed shut twenty-six days a month

After the event he had a technician open the ceiling to inspect the ballroom's air conditioning. What he found made him sit down on the floor. The coil and the condensate tray were coated in sheets of black-green growth, and the duct work held thick dust mixed with moisture.

Working backwards, he found a very simple truth. The ballroom was used on average four days a month, which meant it stood sealed with the air conditioning off for twenty-six days a month — in a province where relative humidity averages over 75 percent for most of the year.

A sealed room with no air circulation, thick carpet, hundreds of fabric chair covers, heavy curtains, and a static air conditioning system holding condensate in the coil provides everything mould requires.

And on event day, staff ran the air conditioning at full power for two hours before the guests arrived. What that actually did was blow a month's accumulated spores out at their most concentrated, into a sealed room that 140 people were about to sit in.

The contract that was not renewed, and what followed

The insurance company completed the three days as scheduled, then did not renew for the following year. The letter was very polite: it said only that the company had revised its venue selection policy.

Khun Thanakrit knew exactly what that meant. Two point four million baht a year, gone in a single day, from an incident lasting two hours.

What followed was worse. The vice president sits on the committee of a business association, and within six months the hotel lost seminar bookings from three more organisations that had been regular clients — roughly another 1.7 million baht.

He says he can never prove those three were connected. But in the MICE business, where the decision makers all know each other, he did not need proof to know.

Everything they tried that was not enough

The hotel's first move was to service every air conditioning unit in all four function rooms, at close to forty thousand baht. It genuinely helped to a degree. Two months later the musty smell was back.

Next they replaced the entire ballroom carpet, at over eight hundred thousand baht. The room looked dramatically better and the smell was gone for about three months, until the rainy season brought it back.

They also ran dehumidifiers in the function rooms between events. That helped the air humidity, and was worth doing, but it did nothing to the mould already growing in the coil and the duct work.

The turning point came when a new housekeeping manager, recently arrived from a large chain hotel in Bangkok, asked Khun Thanakrit why they did not ozone-treat the function rooms before every event. At her previous property, she said, it had been standard for five years.

What it actually costs

Why the usual fixes fail

10 reasons ozone treatment solves it

1. It reaches the duct work and coil that cannot be washed

With the air conditioning recirculating during the cycle, ozone is drawn through the entire system — coil, condensate tray, and the duct work above the ceiling along the full length of the room — the true source of spores that servicing cannot reach.

2. 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. For a room seating 250 people, that is the level required.

3. Carpet, curtains and chair covers treated at the same time

Ozone penetrates the fibres of thick carpet, heavy curtains and hundreds of chair covers — the materials that hold the most odour and trap the most spores — without removing them for laundering, replacing them, or adding moisture.

4. 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 your sales team can put in a corporate proposal.

5. No chemical residue on surfaces guests touch

Ozone decomposes back into ordinary oxygen. No deposit on conference tables, no film on chairs, no chemical smell for delegates to ask about, and nothing to wipe off before the event starts.

6. It adds no humidity to a room that already has a humidity problem

Steam treatment and carpet shampooing add water to a sealed damp room, which feeds mould directly. Ozone is a dry gas and leaves no moisture behind as fuel for the next cycle.

7. About thirty minutes per cycle, done the night before

The automatic digital timer cycle runs roughly thirty minutes — two to start, twelve to disperse, twelve to purify, twelve to rest. The team works the night before the event and the room is ready in the morning.

8. Nothing has to be moved out

Ozone is a gas, so it travels under the stage, behind backdrops, into equipment cupboards and around a room already set for the event. Your banquet team can set up in advance as normal.

9. It is the standard at global hotel groups

The Ritz-Carlton, Fraser Suites, Andaz Hotels and Dusit International use ozone treatment on spaces they must hand to paying clients, and over 300 organisations in Thailand use the service. Applying it to your function rooms puts you at the same standard.

10. It is a selling point that actually closes deals

In a seminar proposal, stating that every function room is ozone-disinfected before each event with equipment certified by the Department of Medical Sciences is something most competitors do not have — and something corporate HR departments weigh more heavily every year.

Before and after

How we get you started, in 3 steps

What a customer says

Why function rooms are the worst mould risk in a hotel

Guest rooms are occupied almost daily, run their air conditioning almost continuously, are entered by housekeeping every day and get aired periodically. A function room is the opposite on every count: used occasionally, sealed the rest of the time.

Function rooms are also filled with the most absorbent, spore-trapping materials in the building: wall-to-wall carpet, hundreds of fabric chair covers, heavy floor-length curtains, and acoustic panels on walls and ceiling.

Add a large air conditioning system with long duct runs and residual condensate sitting in the coil after shutdown, and the room provides everything mould needs: dark, damp, still, and rich in organic material.

What makes it more dangerous than any other space is the usage pattern. The room does not release spores gradually every day. It releases the whole month's accumulation at once, on the morning someone switches on the air conditioning to welcome a hundred people.

The pre-event checklist every hotel should have

After the incident, Khun Thanakrit's hotel rebuilt its function-room preparation procedure. He says the single most important change was moving preparation from the day of the event to the night before.

The new steps are: one, run the air conditioning for at least two hours the night before to drive out residual moisture; two, ozone-treat the whole room with the air conditioning recirculating so the gas travels the duct work; three, ventilate fully after the cycle; four, set the room as normal on the morning of the event.

For rooms unused for more than three weeks, the hotel now schedules ozone treatment as a routine even without an event, so mould never accumulates to the level that requires fixing afterwards.

He calculates that the entire annual cost of the new measures is still less than the single carpet he replaced that did not solve the problem, and a tiny fraction of the contracts he lost.

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