Khun Boonlert is 58 and owns a scheduled coach operation running Bangkok to Ubon Ratchathani: twelve first-class air-conditioned coaches, six round trips a day, a business he took over from his father and has run for 31 years between them.

In 2025 the company invested over 9 million baht replacing the seats in all twelve coaches, fitting individual screens at every seat, adding USB power and upgrading the sound system — to compete with the low-cost airlines taking their market share.

Three months after that investment, online bookings had fallen by almost half. He had his niece, who runs the online marketing, go back through the reviews. Of 61 one-star reviews, 43 said the same thing: the smell from the onboard toilet.

The story

Forty-three reviews all saying the same thing

His niece printed every review and marked them up. The result silenced the whole family, because not one review complained about the seats. Nobody mentioned the screens. Nobody mentioned the price.

What people wrote, over and over, was some version of: the coach is new and the seats are very comfortable, but halfway through the journey the toilet smell fills the whole vehicle and I could not eat the food I had brought.

There was one review Khun Boonlert says kept him awake all night. A passenger wrote that she was taking her 78-year-old mother home, and her mother vomited during the journey because she could not tolerate the smell.

What hurt most, he says, was that he had spent nine million baht so passengers would feel his coaches were better than the competition — and passengers remembered only one thing, and it was the thing he thought he already had under control.

Why the smell persists despite cleaning after every trip

He was not neglecting it. Every coach goes to the depot for a toilet clean after every trip, with two dedicated staff using disinfectant, scrubbing the floor, cleaning the bowl and dosing the holding tank according to the manual.

He brought in a technician to inspect the toilet system. The technician explained that the problem in a coach toilet is not the bowl or the floor — it is three places that cannot be cleaned: the waste pipe running from the bowl to the holding tank, the walls of the tank itself, and the toilet's own air vent.

But the thing that made him understand the real problem was when the technician pointed at the air conditioning. A coach air conditioning system is a closed loop recirculating the same body of air through the whole vehicle for eight hours. When odour comes out of the toilet at the rear, the system draws it in and distributes it through the entire coach within minutes.

Worse, the ducting and coil of a vehicle running sixteen hours a day in Thai humidity carry condensation continuously. Once organic material from the toilet accumulates in there, it becomes a breeding site producing an odour of its own on top of the original one.

What halved bookings actually cost

The coaches have 32 first-class seats, tickets are 620 baht, and they run six trips a day. With bookings down by almost half, lost revenue was around 1.4 million baht a month.

But a coach's costs do not fall with passenger numbers. Fuel, drivers, insurance, finance payments and maintenance are identical whether the vehicle is full or half empty. A half-full coach goes straight into loss.

The company was also servicing the debt on nine million baht of seat and system upgrades — debt taken on the assumption that bookings would rise, not halve.

In the worst month, Khun Boonlert says, he paid staff wages out of his personal savings. That night he read the one-star reviews again and realised his entire problem was worth exactly the smell inside a twelve-metre steel box.

The answer came from one of his drivers

A senior driver had previously worked for an international tour operator collecting tourists from the airport. He told Khun Boonlert that at his old company every vehicle was ozone-treated after each trip before taking on a new group.

He explained it is not chemical spraying: it is a gas released into the sealed vehicle with the air conditioning recirculating, so it reaches the air conditioning system, the seats, the carpet and the toilet all in one cycle.

Khun Boonlert tried it on the two coaches with the worst reviews. The team took about thirty minutes per vehicle following the machine's cycle, working at the depot during the midday layover.

He got on board the first coach after ventilation and sat there alone for about ten minutes. What convinced him, he says, was not that the toilet smell had gone. It was that the faint mustiness in the seats — which he had lived with for thirty years without noticing — had gone too.

What it actually costs

Why the usual fixes fail

10 reasons ozone treatment solves it

1. Toilet, air conditioning and seats treated in one cycle

Ozone fills the vehicle's whole air volume, so it solves all three linked problems at once rather than fixing one and letting the others bring the problem straight back.

2. It reaches the roof ducting that cannot be washed

With the air conditioning recirculating during the cycle, ozone travels through the coil and the full length of the ducting — the mould reservoir that scheduled servicing cannot reach.

3. It reaches the toilet waste pipe and vent

The parts of a coach toilet that actually produce odour are the waste pipe, the tank walls and the vent, none of which can be cleaned after every trip. A gas reaches all of them.

4. It penetrates seat foam without wetting it

Odour embedded in seats sits in the foam beneath the upholstery, where wiping cannot reach and extraction shampooing only adds moisture. Ozone is a dry gas, so it reaches it without leaving damp foam behind.

5. 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 vehicle where passengers share air for eight hours, that is the appropriate level.

6. No chemical residue on seats passengers sit in

Ozone decomposes back into ordinary oxygen, leaving no deposit or film on seats, armrests or seatbelts, and no chemical smell for passengers to endure — unlike spraying, which leaves vapour in a sealed vehicle for hours.

7. About thirty minutes per vehicle, done during a layover

The automatic digital timer cycle runs roughly thirty minutes — two to start, twelve to disperse, twelve to purify, twelve to rest. It fits into a depot layover without taking the coach out of the schedule.

8. Whole fleets treated consecutively

For operators with several vehicles, the team works through them in sequence at the depot, scheduling around the timetable so not a single departure is affected.

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.

10. It gives you something to put on the booking page and in review replies

Stating that every coach is ozone-disinfected before departure with certified equipment is a concrete answer to an odour review, and a selling point almost no competitor on the same route is making.

Before and after

How we get you started, in 3 steps

What a customer says

Why long-distance coaches are the most concentrated sealed space of all

Of all the enclosed spaces Thais use daily, a long-distance coach combines the three most severe factors at once: small air volume, high occupancy per unit of volume, and long shared duration.

A 32-seat coach has an air volume of roughly 60 to 70 cubic metres. A meeting room seating 32 people has many times that, plus ventilation from outside. A coach mainly recirculates the same air for the whole journey.

Coach air conditioning also works far harder than a building system, because it must cool a cabin absorbing sunlight all day. In Thai humidity, the coil is condensing continuously.

The result is a damp, dark system accumulating organic material from passengers and from the toilet every single day — the recipe for a perfect breeding site, which then blows its contents back at the passengers every time the fan runs.

The lesson about investment order for transport operators

Khun Boonlert says his most expensive lesson was about sequence. He invested nine million baht in what passengers can see — seats, screens, power sockets — and nothing in what passengers perceive before anything else: smell.

If he could go back, he says, he would have ozone-treated the whole fleet first and replaced the seats afterwards, because treating the fleet costs a small fraction of the seat budget and moves the reviews far more.

The company now has ozone treatment in the maintenance schedule alongside oil changes — a routine item, not something done when there is a problem.

He also uses it directly in marketing, stating it on the booking page and using it to respond to older reviews about smell by describing what has actually been done. Replying to an old review with a concrete action, he says, is completely different from simply apologising.

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