Khun Jiraporn is 45 and a managing partner at an accounting firm in Sathorn with 22 staff and more than 180 corporate clients. Thai law requires accounting records to be kept for at least five years; in practice the firm keeps them far longer.

The archive room is 45 square metres, with nine rows of steel shelving to the ceiling holding over 6,000 files going back to 2014. There are no windows and one air conditioner, switched on only when someone goes in.

In May 2025 three members of the audit team, who enter the archive almost daily during closing season, fell ill in the same week with persistent cough, chest tightness and low fever. One was diagnosed with hypersensitivity pneumonitis.

The story

Three people ill in one week

Khun Jiraporn assumed at first it was a cold going round the office, normal enough during closing season when everyone works long hours and sleeps little. She sent all three home to rest and work remotely.

What was not normal was that all three clearly improved within three or four days at home, then deteriorated again within two days of returning. That cycle repeated twice.

The doctor treating the worst affected employee was the one who raised it. He asked whether the workplace had an old archive room or a basement, and whether there was regular handling of old paper.

Khun Jiraporn says that when she answered yes, she went cold — because she had noticed herself that spending time in the archive made her nose itch and made her sneeze, and had never once connected the two.

What they saw when they pulled files from the bottom shelf

The following Saturday she and another partner inspected the archive themselves, in N95 masks, starting with the bottom shelf of the row against the building's external wall.

They found grey-green spotting along file spines and paper edges. Some files opened to reveal small black specks scattered across the pages inside, and a distinct musty smell whenever a file was opened.

When they pulled the shelving away from the wall, they found a broad band of moisture staining behind it — condensation, because that wall faces the afternoon sun outside while the room is air-conditioned cold inside.

Her partner summed the whole problem up in one sentence: we have kept six thousand files of paper in what amounts to an incubator for twelve years, and sent our juniors in to fetch things from it every day.

Why an archive room is harder to fix than any other space

She brought in three cleaning companies for advice. All three said essentially the same thing: they could wipe the steel shelving and mop the floor, and there was nothing they could do about the files and the paper.

The reason is straightforward. Paper cannot be wetted. Disinfectant cannot be sprayed on it — it smears ink, wrinkles paper and damages accounting records that must stand up legally. Steam is impossible, because added humidity is the cause of the problem in the first place.

One company proposed scanning everything to digital and destroying the paper. That is a good long-term answer, but scanning 6,000 files was quoted at around 900,000 baht and several months of work.

More importantly, some categories of accounting record must legally be kept in original paper form, so destruction is not an option for all of it. She was left with a clearly identified problem and no usable method.

The answer came from a client's question

One of the firm's clients owns a mid-sized hotel in Hua Hin. During a closing meeting, Khun Jiraporn mentioned the archive room while complaining about her week.

The client immediately asked why she did not use ozone treatment, explained that his hotel treats function rooms and guest rooms routinely, and stressed that it is a gas, not a liquid, so nothing gets wet.

She says that sentence — it is a gas, not a liquid — was the moment she understood why it was the answer for an archive, because every constraint she had was rooted in the fact that paper cannot get wet.

The team surveyed the following week, recommended pulling the shelving away from the wall to address the moisture at source first, then treated the whole room with files spaced apart as far as practical. It was finished in one Saturday night.

What it actually costs

Why the usual fixes fail

10 reasons ozone treatment solves it

1. It is a gas, so nothing gets wet

This is the first and most important reason for an archive. Ozone is not a liquid and not steam, so it does not dampen paper, smear ink, or wrinkle and embrittle files.

2. It penetrates between pages and into file spines

Ozone goes wherever air goes, which includes the gaps between files, the file spines, the recesses of steel shelving, and the space behind shelves standing against a wall — exactly where the mould is and where nobody can reach.

3. No chemical residue on legally significant documents

Ozone decomposes back into ordinary oxygen after reacting, leaving no deposit, no discolouration and nothing on records that may have to be produced in evidence years later.

4. It adds no humidity to a room whose whole problem is humidity

Chemical fogging and steam add water to a room where moisture is the cause. Ozone is a dry gas, so it removes the organisms without making the room's condition worse.

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. In a room that has already made three people ill, that is the level required.

6. It treats the air conditioning that has become the spore distributor

With the air conditioning set to recirculate during the cycle, ozone travels through the coil and duct work — which in a spore-laden archive typically becomes both a reservoir and the mechanism spreading spores around the room.

7. Records never leave the room

Clients' accounting records are confidential and should not be transported off site. Ozone works in place, so there is no confidentiality risk and no transport cost.

8. Far cheaper than scanning and destroying

Scanning 6,000 files was quoted at around 900,000 baht over months. Ozone treatment of the whole room was completed in one night at a fraction of the cost, and the original paper records were all retained.

9. About thirty minutes per cycle, done outside working hours

The automatic digital timer cycle runs roughly thirty minutes — two to start, twelve to disperse, twelve to purify, twelve to rest. The team worked on a Saturday night and the office opened normally on Monday.

10. 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 usable in a workplace safety report.

Before and after

How we get you started, in 3 steps

What a customer says

Why paper is the material mould likes best

Paper is made of cellulose, an organic material that many fungal species digest directly as food. Six thousand files are not merely stored objects; they are an enormous food supply in a sealed room.

Paper is also highly porous and draws moisture out of the air into the material itself. Once relative humidity in a room exceeds about 65 percent, the moisture content of the paper reaches the level at which mould can grow.

In Thailand relative humidity averages 70 to 85 percent for most of the year, so an archive room without humidity control sits above that threshold almost permanently, with nobody aware of it.

Add a dark, windowless room with no air circulation that is only used occasionally, and an archive becomes one of the highest-risk spaces in any office building — and almost the least inspected.

Which other rooms have exactly this problem

School and university libraries meet every one of the same conditions: large quantities of paper in a poorly humidity-controlled sealed room, used daily by students. An old book with a musty smell is a book with mould, not merely an old book.

Medical records rooms in hospitals and clinics are another category that meets every condition, and are especially sensitive because the records cannot be destroyed and staff retrieve from them every day.

Archive rooms in law firms, audit firms, insurance companies, banks and government agencies are all the same: vast quantities of legally mandated paper in rooms never designed for humidity control.

The same applies to collection stores — art storage, rare book and record collections, and costume or textile stores — where every material is organic, digestible by fungi, and equally impossible to wet.

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