Protecting Library Materials from External Damage: Factors You Must Know

Every book, manuscript, and newspaper in a library is slowly reacting to the world around it. Even when a volume sits untouched on a shelf, the air, the heat, the light, and the living organisms in the room are quietly working against it. These influences are grouped together as external causes of deterioration – agents that come from the environment rather than from the chemistry of the paper itself. Understanding them is the first real step in preservation, because most damage from these sources is gradual, cumulative, and often irreversible by the time it becomes visible. This post walks through the main external threats and what can be done to keep them in check.

Temperature and humidity: the most important pair 🔗

Temperature and relative humidity are the two conditions that influence the lifespan of library materials more than any other. They also work together, which is why conservators almost never discuss one without the other. Given a fixed amount of moisture in the air, relative humidity rises as temperature falls and drops as temperature rises, so a change in one immediately shifts the other.

Heat is damaging because it speeds up chemistry. Higher temperatures accelerate the chemical reactions that break down paper and media, while very high humidity encourages mould and pest activity and very low humidity dries materials until they crack. Paper, parchment, leather, and the adhesives used in bindings are all hygroscopic, meaning they absorb and release moisture from the air, so they respond directly to these swings.

Why fluctuation is worse than a steady “wrong” level 🔗

A constant but slightly imperfect environment is usually less harmful than one that keeps changing. Most books are made of several materials layered together – paper, board, cloth, glue, leather – and each expands and contracts at its own rate. Rapid changes in humidity cause these materials to move against one another, producing physical stress that leads to cracking, warping, or delamination. Repeated cycling slowly weakens the structure of an item even when no single reading ever looks dangerous.

Recommended conditions 🔗

For paper-based collections, a cool and stable environment is the goal. The Smithsonian Institution Archives recommends storage at roughly 35-65°F with relative humidity around 30-50 percent for paper, with cooler conditions for sensitive photographic and film materials. A widely used rule of thumb is that the rate of most deterioration reactions roughly doubles for every 10°C rise in temperature, which is why even a few degrees of difference matters over decades. The seasonal heat and monsoon humidity common across much of the country make stable storage especially difficult, and especially important, for libraries here.

The dangers of light exposure 🔗

Light feels harmless, but it is one of the most persistent threats to a collection. Light is energy, and when paper and inks absorb it, that energy drives chemical reactions that degrade the material. This process is called photochemical deterioration, and its most important feature is that light damage is cumulative and irreversible – once a page has faded or yellowed, storing it in the dark afterwards cannot undo the harm.

Ultraviolet radiation and visible light 🔗

Sunlight and fluorescent tubes both emit ultraviolet (UV) radiation, an invisible band of energy that sits just beyond violet in the spectrum. Because UV has shorter wavelengths, it carries more energy than visible light and is therefore more damaging. UV is associated with the embrittlement and weakening of cellulose fibres, while visible light tends to fade inks, dyes, and pigments. Both contribute to paper yellowing and darkening, so neither can be ignored.

The intensity-plus-time formula 🔗

The total damage light causes depends on how bright it is multiplied by how long the material is exposed. An object displayed for one hour in direct sunlight at 100,000 lux receives the same dose as 200 hours under typical 50-lux display lighting. This means damage can be controlled either by lowering the light level or by shortening the exposure time. For sensitive items such as manuscripts and works on paper, conservators often keep display lighting around 50 lux and eliminate UV entirely from light sources.

Practical controls include fitting windows with blinds or UV-filtering film, switching lights off when storage areas are empty, keeping materials in boxes when not in use, and never displaying fragile originals in permanent sunlight.

Environmental pollutants 🔗

The air inside a library is rarely clean, and the contaminants it carries fall into two broad groups: gaseous pollutants and particulate matter. Both attack collections, and both become more aggressive when humidity is high.

Gaseous pollutants 🔗

The main gaseous threats are sulphur dioxide, nitrogen oxides, ozone, and formaldehyde, which trigger chemical reactions and dye discolouration in paper. Sulphur dioxide and nitrogen oxides largely come from the burning of fuels and from vehicle and industrial emissions – an obvious concern for libraries in busy urban areas. Inside paper, these gases combine with moisture to form acids. Acids absorbed from polluted air repeatedly cut the cellulose chains into shorter lengths through a reaction called acid hydrolysis, which itself generates more acid and feeds further decay. Book pages that are browner and more brittle along their edges than at the centre are a classic visual sign of this absorption.

Dust and particulates 🔗

Solid particles cause a different kind of harm. Dust, soot, and tarry matter settle on surfaces and stain them, but they also act as abrasives that scratch paper and bindings during handling. Worse, dust particles often carry adsorbed acidic pollutants and can attract pests and mould. As one museum conservation guide notes, dust and dirt can lead to a combination of chemical, biological, and mechanical damage at once. Regular, careful cleaning of shelves and items is therefore a preservation activity, not just housekeeping.

Biological agents 🔗

Library materials are made largely of cellulose, starches, proteins, and adhesives – in other words, food. This makes collections attractive to a range of living organisms. The warm, humid climate across much of the region makes this threat particularly serious, since tropical conditions support termites, cockroaches, and other pests that cause even greater damage to wood and paper than in cooler regions.

Fungi and mould 🔗

Mould is the most common biological problem and is driven almost entirely by humidity. Spores are always present in the air, waiting for the right conditions. When relative humidity stays high – generally above about 65 percent – mould begins to grow on glue, leather, and paper, appearing first as a dusty grey or whitish bloom that later turns brown or green. As it grows, it digests and stains the material and weakens its structure. Visible mould is also a warning sign that the humidity has crossed the safe limit.

Insects 🔗

Several insects feed directly on collections. Termites, “bookworms,” cockroaches, silverfish, booklice, and carpet beetles are the main culprits. Silverfish graze on paper surfaces and the starch in sizing and bindings. The larvae of certain beetles – the true “bookworms” – tunnel directly through pages and spines, leaving channels packed with powdered excrement and small round exit holes. Termites are perhaps the most destructive, since cellulose makes up most of their diet and they can consume both the structure of a building and the collections inside it. Many of these pests, including silverfish and booklice, depend on relative humidity of 60 percent and above, and warm temperatures around 20°C encourage insect breeding, which again ties biological damage back to climate control.

Rodents 🔗

Mice and rats cause damage by gnawing materials to obtain nesting fibre and by contaminating shelves and items with urine and droppings, which stain paper and create health hazards. They are especially likely to settle in neglected storage areas where collections are seldom inspected.

Practical steps for mitigation 🔗

Because these agents are interconnected, controlling a few core conditions protects against several threats at once. The most effective preservation approach focuses on prevention rather than repair.

Control the climate. Aim for a cool, stable temperature and moderate humidity, and prioritise stability over chasing a perfect number. Air conditioning, dehumidifiers, and good air circulation all help, and keeping humidity in check simultaneously discourages mould and many insects.

Manage light. Reduce light levels in storage and display areas, fit UV-filtering film or sleeves on windows and fluorescent tubes, use blinds, and turn off lighting when spaces are unoccupied. Keep valuable originals out of direct sunlight permanently.

Reduce pollutants. Filter incoming air where possible, store materials in good-quality boxes and enclosures that act as a buffer against gases and dust, and clean shelves and items regularly to remove abrasive, acid-carrying particles.

Adopt integrated pest management. Rather than relying on chemical spraying, IPM combines prevention and monitoring. Sticky blunder traps placed in a grid against wall-floor junctions, at doorways, and near vulnerable collections allow pests to be detected early, while sealing entry points, keeping food out of storage areas, controlling humidity, and inspecting collections regularly removes the conditions pests need. Anything newly received should be checked, and suspected infestations isolated quickly.

Taken together, these measures show why preservation is mostly about managing the environment. A clean, cool, stable, dim, and well-monitored space slows nearly every external cause of deterioration at the same time.

What do you think? If a library in your area could afford to fix only one environmental problem first – heat, humidity, light, pollution, or pests – which would give the greatest protection for the money spent? And how should libraries balance the public’s desire to see rare materials on display against the cumulative light damage that every viewing causes?

References

1. https://www.amnh.org/research/science-conservation/preventive-conservation/agents-of-deterioration/temperature-and-relative-humidity-rh
2. https://siarchives.si.edu/what-we-do/preservation/environment
3. https://www.amnh.org/research/science-conservation/preventive-conservation/agents-of-deterioration/light-ultraviolet-and-infrared
4. https://ccaha.org/resources/light-exposure-artifacts-exhibition
5. https://www.nedcc.org/preservation101/session-4/4external-factors
6. https://www.loc.gov/preservation/care/deterioratebrochure.html
7. https://manual.museum.wa.gov.au/book/export/html/82/index.html
8. https://www.mdpi.com/2075-4450/6/2/595
9. https://www.nedcc.org/free-resources/preservation-leaflets/3.-emergency-management/3.10-integrated-pest-management
10. https://www.archives.org.uk/collections-care-toolkit/integrated-pest-management-ipm-for-paper-archives