The short version: paper is one of mould's favourite foods. It is made of cellulose fibres that soak up water and feed many kinds of fungi. When bookshelves, filing cabinets or boxes of files are flooded, or simply sit in a damp room after the water recedes, black, green or fluffy white patches appear within days. The spores released from those piles do not just ruin pages; they float in the room's air and reach the lungs of anyone reading or working there. This guide covers how to salvage books and papers correctly, what not to do, and how the WHD Residential and Building Ozone Disinfection service helps disinfect libraries and archive rooms so the mould does not come straight back.

A typical scenario: a retired teacher's book room in Sukhothai

Aroonee is a retired Thai-language teacher who lives in a half-timber, half-concrete house on the Yom River in Sukhothai. On the ground floor is a small room she calls her book room. It holds three teak cabinets full of Thai literature collected since her teacher-training days, more than thirty lesson-plan notebooks, her wedding album, and a blue plastic folder containing her land title deed, house registration, bank passbooks and the certificate of honour she is proudest of.

The year the Yom burst its banks, the water rose faster than anyone expected. She and her grandson managed to lift only part of her belongings. The ground floor stood under about a metre of water for ten days. When it receded and she opened the book-room door, a basement-like mustiness rushed out. The books on the bottom shelves had swollen until they cracked the cabinet doors. Hard covers had warped and pages had fused into blocks. The blue folder on the middle shelf had escaped the water, but inside she found small grey-green spots at the corner of the title deed, and the edges of her wedding photos carried a white bloom that looked like talcum powder.

What she did first is what most people do. She carried every book outside to dry in the sun, wiped the mould off the covers with a damp cloth, and pointed a fan at the open pages. Three days later many books were brittle, pages had rippled into waves, the ink in her lesson notebooks had faded, and the mould marks on the covers had become permanent stains. Worse, she developed a dry cough and a blocked nose, and began waking at night short of breath. Every time she went into the room where the cabinets stood, it got worse.

This picture repeats in homes, schools, temples, public libraries and offices wherever floods are frequent, from Chiang Mai and Ayutthaya to Ubon Ratchathani and Hat Yai. Books and documents are not just objects. They are legal evidence, family memory and organisational history, so losing them hurts in a particular way, and salvaging them the wrong way can do more damage than the water did.

Why books and papers go mouldy so quickly after a flood

Paper is made from plant fibres that are mostly cellulose. Book bindings often use starch or animal glue, and some are covered in cloth or leather. All of it is food for fungi. Once paper absorbs water, moisture wicks along the fibres well beyond the actual waterline, so a book that stood half-submerged is usually wet almost all the way through. Packed tightly on a shelf, it has almost no way to release that moisture.

Public-health bodies including the CDC and the World Health Organization agree that wet materials should be cleaned and dried within 24 to 48 hours to limit mould. In a real flood, though, most families wait days or weeks before they can get back inside, so books and papers sit damp far longer than that safe window.

The problem does not stop with the books that got wet. A room that flooded stays humid for a long time; walls, floors and wooden cabinets keep releasing moisture. Books on upper shelves that never touched the water can grow mould within weeks. Office archive rooms, which are often windowless and stacked to the ceiling with boxes of files, are close to ideal conditions for fungal growth.

The paper-eating moulds, and what they do to the people in the room

Several groups of mould turn up on flood-damaged books and documents. Common ones include Aspergillus, often yellow-green or black; Penicillium, blue-green; Cladosporium, black or dark green; and Chaetomium, a strong cellulose digester frequently found on paper that has been wet for a long time. You may also see small reddish-brown spots scattered across pages, the result of moisture, trace metals in the paper and fungal growth acting together.

These moulds reproduce through spores too small to see. Every time someone opens a book, turns a page, lifts a box of files or dusts a shelf, millions of spores are released, stay suspended in the air for a long time, and are breathed deep into the airways.

Common symptoms near mouldy books

• Sneezing, blocked or runny nose, itchy red eyes, much like hay fever
• Dry cough, sore throat and chest tightness, especially on entering the room
• Asthma flare-ups in people who already have asthma
• Itchy rashes on the hands and arms after handling mouldy papers
• Headache, fatigue and poor concentration after time in a musty room

In people with very weak immunity, such as those on chemotherapy, organ-transplant recipients or people with chronic lung disease, breathing large amounts of spores from moulds like Aspergillus can lead to a fungal lung infection that needs medical care. Some people exposed repeatedly over long periods develop an allergic inflammation of the lungs, with episodes of fever, cough and breathlessness.

Who is most at risk

• Older adults, who often own old books and papers and spend long hours with them.
• Young children reading in school libraries or home reading corners, with lungs still developing.
• Pregnant women, who should avoid heavy spore exposure and strong cleaning chemicals.
• People with allergies, asthma or lung disease, whose symptoms flare easily.
• Librarians, clerks and accountants who handle large volumes of paper every day.

The damage goes beyond stained paper

Health: a library or archive room with mouldy books becomes a constant source of spores. Regular users are exposed every day, and allergies and a lingering cough that seem minor build up until they disturb sleep and work.

Legal and financial: title deeds, leases, share certificates, loan agreements, tax receipts and a business's supporting accounting documents are all evidence you may need later. If the text fades or the paper disintegrates, requesting copies or replacements from the issuing agency takes time, money and repeated trips.

Business: accounting firms, law offices, clinics and schools must keep records for set periods under law or regulation. A mouldy archive room means files cannot be found, staff avoid going in, and if an audit asks for records that no longer exist, the consequences can be serious.

Time: sorting thousands of books or hundreds of boxes takes a very long time, and the longer it waits, the further the mould spreads from one volume to the next. A one-week job can stretch into months.

Memory and wellbeing: family albums, letters from people who have passed away, a parent's diary and signed first editions cannot be bought again. Many people say watching mould destroy them hurts more than losing all the furniture.

Reputation: a school or community library that reopens while it still smells musty and has mould on its shelves worries parents and loses users.

Why familiar book-rescue methods so often backfire

1. Drying in strong sun. Harsh sunlight dries the surface of the paper while the inside is still wet, so the sheet shrinks unevenly, cockles into waves, hard covers warp and spines crack. UV light also fades ink and photographs. Crucially, the sun does not kill mould deep inside the book; once it goes back into a damp room, the mould can return.

2. Wiping mould with a wet cloth. Wiping damp or powdery mould smears spores and fungal threads across the page, pushes them into the paper, and leaves blotches that will not come out.

3. Hot air or an iron. Heat melts spine adhesives, permanently fuses coated pages together and ruins photographs.

4. Spraying alcohol or bleach straight onto paper. Bleach weakens paper and fades ink, and spraying any liquid onto paper that is trying to dry simply adds moisture back. For important documents and valuable old items, consult a paper conservator before using any chemical.

5. Putting cleaned books straight back on the old shelves. Shelves and wooden cabinets that stood in water still hold spores and moisture. Cleaned books are recontaminated, and mould spreads to volumes that were never affected.

6. Air freshener or charcoal deodorisers. They improve the smell for a while, but they do not reduce the spores in the air or on shelf surfaces, and some fragrances leave an oily film on paper.

7. Shutting the cabinet tight with a few silica sachets. A handful of sachets cannot absorb the moisture held in a cabinet that stood in floodwater. Sealing it just creates a humid incubator where mould grows even faster.

How WHD ozone treatment helps libraries and archive rooms

To be clear from the outset: ozone treatment does not repair books, and it does not replace removing mud, washing, cleaning and drying. Wet books must be sorted and dried correctly first, and the room must be washed and fully dry. After that, ozone deals with what other methods cannot reach: mould spores floating in the air, lodged in shelf grooves, behind cabinets, inside filing drawers and between boxes, plus the musty odour soaked into the room's materials.

Ozone (O3) is a strongly oxidising gas. Released into a closed room, it spreads through the whole space and into gaps that cloths and sprays miss, breaking down the cell structures of mould, bacteria and viruses and splitting the molecules that cause mustiness. Afterwards it reverts to oxygen (O2), leaving no liquid film or chemical residue on the shelves. According to test results certified for the company by Thailand's Department of Medical Sciences and Intertek (UK), the WHD process eliminates 99.99% of viruses, bacteria and mould.

A note specific to paper collections: because ozone is an oxidiser, prolonged exposure at high concentration may affect some materials. The team will plan with you and recommend removing items of high historical value, old photographs, rare manuscripts and anything irreplaceable from the room first, and consulting a conservation specialist about those items specifically. The room itself, the shelves, cabinets and air, is the main target of ozone treatment.

The process in a library or archive room

1. Survey and assessment. The team checks the extent of damage, the densest mould areas and the moisture in walls and shelving, and asks which books or documents need to be set aside.
2. Preparation. People, pets and plants leave; items to be set aside are moved out; cabinets and filing drawers are opened and boxes spaced so the gas can circulate; doors and windows are closed and air-conditioning switched off.
3. Ozone release. Generators are sized to the room, typically about 30 minutes per zone. Large libraries are divided into zones along the rows of shelving.
4. Contact time. The gas works in the sealed, unoccupied room.
5. Rest and ventilation. Doors and windows are opened so ozone reverts to oxygen. Nobody re-enters until the time the team specifies has passed.
6. Check and handover. The team checks the odour with you and advises on ongoing humidity control. Jobs of 15,000 baht or more receive a free medical-grade disinfectant spray.

Safety first: ozone treatment takes place only in an empty, closed room. No people, pets or plants may be inside during treatment, and re-entry is allowed only after the rest and ventilation period the team gives you.

10 reasons libraries and document rooms should be ozone-treated after a flood

1. It reduces airborne spores, the cause of the sneezing, coughing and asthma that room users suffer, and something wiping surfaces cannot do.
2. It reaches shelf grooves and the backs of cabinets, where dust and mould collect most and nobody can clean thoroughly by hand.
3. It helps stop mould jumping from book to book. Returning salvaged books to a clean room lowers the chance of recontamination.
4. No liquid is sprayed onto the shelves. It is a gas, so it adds no moisture to a room that has only just dried.
5. No chemical residue. Ozone reverts to oxygen, leaving no film on shelves and no chemical smell on pages. Read more about ozone and chemical residue.
6. It removes that basement mustiness that makes a library unwelcoming and drives users away.
7. It scales to large spaces. School libraries and office or government archive rooms can be treated systematically, zone by zone.
8. It is quick. About 30 minutes per zone plus ventilation, so the room is back in use sooner.
9. It is tested and certified, according to test results certified for the company. See details on the ozone disinfection service page.
10. The team plans with you, telling you clearly which items should be set aside so you can decide with confidence. Ask WHD for an assessment.

Example case: an accounting office archive in Ayutthaya

This example case is an illustrative scenario based on the kind of job that comes up often, to show how the work is sequenced.

Ae runs a small accounting practice in Phra Nakhon Si Ayutthaya, keeping the books for dozens of local shops and companies. Her archive room is on the ground floor, windowless and packed with boxes of tax invoices and supporting documents going back several years. Floodwater entered the office to about half a metre, soaking every box on the bottom row.

After the water drained, her staff carried out the bottom boxes and sorted them. Papers that could be saved were separated sheet by sheet, laid on racks in a room with fans moving air without blowing directly on them, and photographed or scanned one at a time. Documents too damaged to read were listed so the firm could request copies from clients and the relevant agencies. Cleaners washed the floor, scrubbed the walls, took the steel shelving out to wipe down, and then ran dehumidifiers until the walls were dry.

The room looked clean, yet two staff members with allergies still sneezed every time they went in, and the mustiness lingered. Ae contacted WHD. The team suggested moving the most important original files out temporarily, opening every drawer of the steel cabinets and spacing the top-row boxes apart, and then treated the room in two zones. After the rest and ventilation period, the musty smell was clearly reduced and staff could work there without wearing masks all day. Ae says the advice that helped most was to scan key documents to cloud storage, which has been standard practice in her office ever since.

Salvage checklist for books and documents, before and after ozone

In the first 48 hours you have access

• Wear gloves, a mask that filters fine dust and eye protection before touching mouldy books or boxes.
• Prioritise legal and financial records, family photos and anything irreplaceable.
• Photograph or scan every important document as soon as you can, even while wet, to preserve the information.
• If you cannot deal with everything in time, bagging wet books and freezing them is a method conservators commonly use to pause mould growth.

Drying

• Air-dry in shade with good airflow, away from strong sun and hot air.
• For damp books, stand them fanned open, interleave clean absorbent paper every 20 to 30 pages, and change it often.
• Lay single sheets flat on racks or clean cloth, never stacked.
• Do not pull apart photos that are stuck together as they dry; ask a photo specialist.
• Brush dry, powdery mould off covers gently outdoors with a soft brush, or use a vacuum with a HEPA filter.

Before ozone day

• Remove the mud and wash floors, walls, shelves and cabinets, then let the room dry completely.
• Take out historically valuable items, old photos and rare originals as the team advises.
• Open cabinets and drawers, space out boxes, and move plants out.

After ozone

• Re-enter only after the rest and ventilation period the team specifies.
• Keep humidity low with a dehumidifier, and position shelving away from walls.
• Return fully dry, checked books to the shelves in batches, without packing them tightly.
• Store key documents in waterproof boxes on high shelves, with digital copies kept off-site.
• If coughing, wheezing or chest tightness continue, especially in older people or those with lung disease, see a doctor.

Frequently asked questions

Will ozone remove the mould stains from my books?
No. Ozone disinfects and reduces odour, but the pigment mould leaves on paper is permanent. Treating stains on valuable items is a job for a conservation specialist.

Can all the books stay in the room during treatment?
Ordinary, dry working books can usually stay, based on the team's assessment. Historically valuable items, old photographs and irreplaceable originals should be moved out first.

The books are still wet. Can we ozone now?
No. Books and the room must be dry first, or mould will simply grow again. Ozone comes after drying; it does not replace it.

How long must we stay out?
About 30 minutes per zone plus the ventilation period. The team will give you a clear re-entry time. No people, pets or plants may be inside during treatment.

When should a school library be treated before term starts?
Finish washing and drying first, then schedule ozone at least a day before reopening to leave time for ventilation and a final check.

An important document has rotted beyond reading. What now?
Keep what remains of the original, photograph it, and contact the issuing agency about copies or a replacement. Ozone cannot recover lost text.

I have a cough and chest tightness after handling mouldy books. What should I do?
Stop handling them and leave the area. If it does not improve, or if you have an underlying condition, see a doctor. Ozone treatment manages the environment; it is not a treatment for illness.

Library or archive room still musty? Let WHD clear the mould

Free consultation and site assessment, a joint plan for what to set aside, and ozone disinfection for homes, offices, schools and public agencies.

Book Ozone Mould Treatment

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

Related reading

• Residential and Building Ozone Disinfection
• Ozone cleaning service
• Ozone disinfection machines
• Contact WHD

References

• CDC: Floods on mould and clean-up after flooding
• World Health Organization fact sheets on health and the indoor environment