The Detrimental Effects of Humidity and Moisture on Library Materials

Walk into any old library in India during the monsoon, and you can often smell the problem before you see it. That musty, earthy odour rising from the stacks is the signature of moisture quietly attacking the collection. Humidity is one of the most underestimated threats to library materials precisely because it works slowly and invisibly. A small amount of moisture is actually necessary to keep paper supple, but once levels climb too high, that same moisture becomes the single biggest cause of physical, chemical, and biological decay. Understanding how humidity behaves, and how to keep it in check, is one of the most important skills in preservation work.

What humidity actually means for library materials 🔗

When preservation experts talk about humidity, they almost always mean relative humidity (RH), which is the amount of moisture in the air compared to the maximum it could hold at that temperature. The warmer the air, the more water vapour it can carry. This matters because paper, parchment, leather, textiles, and adhesives are all hygroscopic materials, meaning they constantly absorb and release moisture to stay in balance with the surrounding air. As paper-based materials respond to relative humidity, they swell when the air is damp and shrink when it is dry.

This balancing act is the root of the danger. Materials are not damaged simply by being wet at one moment; they are damaged by the repeated expansion and contraction that comes with changing moisture levels. High relative humidity encourages mould and pests, while very low humidity causes desiccation and embrittlement. Neither extreme is safe, which is why the goal is a stable, moderate level rather than the elimination of moisture altogether.

How humidity weakens paper fibers 🔗

Paper is made primarily of cellulose fibers, and these fibers are naturally fibrous and absorbent. In prolonged high-humidity conditions, paper becomes soggy and the moisture gradually weakens the bonds that hold these fibers together. Over time, the sheet loses its strength and structure, and pages that were once firm begin to feel limp and fragile.

The damage does not stop at the paper itself. Moisture also weakens the adhesives used in book binding, causing covers to loosen and spines to come apart. Ink is another casualty. In damp conditions, ink can run, smudge, or spread across the page, blurring text that may be centuries old. Documents that experience repeated swings in moisture are far more vulnerable, because the correct preservation of old and rare books needs suitable and stable values of temperature and relative humidity.

The opposite problem: too little moisture 🔗

It is tempting to assume that the drier the environment, the better. This is a mistake. When relative humidity drops too low, paper loses the small amount of internal moisture that keeps it flexible. The cellulose fibers dry out, and the paper becomes brittle to the point where it can crack or crumble at the slightest touch. This is a particular risk in air-conditioned reading rooms or in regions with very dry winters. The lesson is that paper needs a moderate, controlled amount of humidity to remain durable, which is why preservation aims for a middle range rather than the lowest possible reading.

The relationship between moisture and biological growth 🔗

The most visible and feared consequence of high humidity is biological attack. Mould, fungus, and other microorganisms are present everywhere as dormant spores, and they need only one thing to spring into action: moisture. Library materials are an ideal target because paper, cloth, and leather are organic substances. The sugars and starches in cellulose provide a ready food source for fungi.

Research consistently links sustained high humidity to mould outbreaks. Most molds will germinate at around 65 percent relative humidity, and the risk rises sharply when warmth is added to the equation. Studies show that fungi grow between 15 and 35 °C, with growth becoming much faster above 23 °C and 65% RH. For libraries in India’s warm, humid climate, this combination is an everyday concern rather than a rare event.

Why duration matters as much as the number 🔗

An important and often overlooked point is that brief spikes in humidity are far less dangerous than sustained dampness. A short jump to 65% RH for a couple of hours carries much less biological risk than a steady reading held at slightly lower levels for several weeks. It is the cumulative exposure to moisture that allows spores to germinate and colonies to spread. This is why hidden pockets of stagnant, humid air, such as the space behind a bookshelf pushed against an outer wall, are such common starting points for mould.

The wider damage caused by microorganisms 🔗

Once active, mould does more than look unpleasant. It produces coloured staining that is extremely difficult to remove, and it will keep consuming a material until its food source is exhausted, meaning an untreated collection could be progressively destroyed. The predominant cause of mould is excess water, whether from leaks or from high relative humidity caused by poor storage or faulty climate systems. Damp conditions also attract insects and other pests, compounding the threat. There is a human cost too, since fungal spores can act as allergens and cause respiratory problems for library staff and readers.

Controlling humidity to prevent physical and chemical deterioration 🔗

Moisture is not only a biological problem. It is also a powerful driver of chemical deterioration. Water participates directly in the chemical reactions that break paper down, particularly the hydrolysis of cellulose, which severs the long fiber chains and is a major reason old paper turns weak and yellow. Dust and dirt make this worse, because solid particles attract and hold moisture, creating tiny pockets where acids can form and accelerate decay. This is why environmental control sits at the heart of preventive conservation.

The recommended humidity range 🔗

There is no single magic number, but authoritative institutions converge on a fairly narrow band. The recommended standard for paper-based collections is generally around 30 to 50 percent relative humidity, paired with cool temperatures. Many practitioners aim for a target near 45 to 55 percent so that paper stays flexible without inviting mould. For a warm country like India, keeping RH reliably below the roughly 60 to 65 percent mould threshold is the priority, since the climate naturally pushes moisture levels upward.

Stability over perfection 🔗

Perhaps the most important principle is that stability matters more than hitting an exact figure. Keeping humidity as stable as possible is often more valuable than achieving an ideal reading that constantly fluctuates. Because materials expand and contract with every change, a collection held at a slightly imperfect but steady level will fare better than one swinging between extremes. Where seasonal change is unavoidable, conservators recommend that any shift in humidity happen slowly and gradually rather than in sudden jumps.

Practical control measures 🔗

Several straightforward steps help keep moisture under control. HVAC systems that combine cooling, heating, and ventilation are the most reliable tool, since they manage temperature and humidity together while keeping air circulating. Where full air conditioning is not affordable, dehumidifiers can pull excess moisture from the air, and good ventilation prevents the stagnant pockets where mould thrives. Simpler structural habits matter too: book racks should be kept around 15 cm away from walls to stop damp from migrating into the collection, roofs and plumbing should be checked for leaks, and windows should be sealed against rain. Storing collections in basements or against outer walls should be avoided wherever possible.

Tools and techniques for monitoring humidity levels 🔗

You cannot control what you do not measure. Monitoring humidity is therefore a daily part of preservation work, and a range of instruments make this possible. The traditional tool is the hygrometer, which reads the relative humidity of the air. A thermohygrometer combines this with a temperature reading, since the two factors must always be considered together. A psychrometer, which uses wet and dry bulb measurements, is another classic method for calculating relative humidity.

Data loggers and continuous monitoring 🔗

Modern preservation relies heavily on data loggers, small electronic devices that record temperature and humidity automatically at set intervals over long periods. Instead of relying on occasional manual readings, a logger captures the full pattern across days, weeks, or months, revealing the fluctuations and seasonal trends that single readings would miss. In a study of a historic library, researchers carried out a six-month monitoring campaign using data loggers to analyse the spatial and temporal variations of temperature and relative humidity. Many modern loggers connect wirelessly and can send email alerts when conditions cross a set threshold, allowing staff to respond before damage begins.

Reading the data and acting on it 🔗

Collecting numbers is only useful if they guide action. European preservation guidance, such as the EN 15757 standard, sets out a procedure for judging whether seasonal patterns of humidity and temperature are safe for hygroscopic materials on display or in storage. Placing loggers at several points across a building is wise, because conditions near a window or an exterior wall can differ sharply from those in the centre of the stacks. By tracking these patterns, librarians can identify problem zones, fine-tune their climate systems, and catch dangerous trends early. Regular visual inspection of the collection for the first signs of warping, staining, or that telltale musty smell remains an essential companion to instrument readings.

Bringing it all together 🔗

Humidity sits at the centre of nearly every form of library deterioration. Too much moisture weakens fibers, loosens bindings, spreads ink, fuels acid-forming chemical reactions, and invites mould and pests. Too little leaves paper brittle and prone to cracking. The solution is not to chase a single perfect reading but to maintain a moderate, stable environment within a sensible range and to watch it closely with reliable instruments. In a climate as warm and humid as much of India’s, this vigilance is not optional. It is the quiet, ongoing work that decides whether a collection survives for decades or crumbles within years.

What do you think? If your local library or college collection had to choose between buying a single expensive HVAC system or several affordable data loggers and dehumidifiers, which investment do you think would protect the books better? And how might India’s monsoon season change the way you would design a preservation plan?

References

1. https://www.amnh.org/research/research-library/conservation/preserving-personal-books-and-papers
2. https://siarchives.si.edu/what-we-do/preservation/environment
3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6446128/
4. https://preservation.library.harvard.edu/managing-mold-contamination
5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11508409/
6. https://www.library.illinois.edu/preservation/collections-care/mold-pest-and-water-damage-response/
7. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8619385/