The Five Enemies of Books: Light, Heat, Humidity, Pests, and People

Books are remarkably durable objects. Copies of Gutenberg’s Bible, printed on rag paper in the 1450s, remain in excellent condition after nearly six centuries. Yet books are also fragile, vulnerable to a specific set of environmental and human threats that can degrade or destroy them in months rather than centuries. The conservator’s shorthand for these threats is “the five enemies”: light, heat, humidity, pests, and people. Every decision about storing, handling, and displaying a book collection should be informed by an understanding of these enemies and the damage they cause.

1. Light

Light is the most insidious enemy of books because its damage is cumulative, irreversible, and often unnoticed until severe. Both natural sunlight and artificial light cause photochemical degradation of paper, inks, and dyes.

How light damages books

Fading. Ultraviolet (UV) radiation breaks down the chemical bonds in dyes and pigments, causing colours to fade. Dust jacket spines are the most common victims — because they face outward on shelves and receive the most exposure, jacket spines fade dramatically while the front and back panels retain their original colour. A dust jacket with a vivid front panel and a washed-out spine is the signature of light damage.

Paper degradation. UV radiation accelerates the breakdown of cellulose, the primary structural component of paper. Prolonged exposure causes paper to become brittle, yellowed, and fragile. The “browning” seen on the pages of old paperbacks left in sunlight is photochemical degradation in action.

Bleaching. Extended exposure to intense light bleaches leather bindings, cloth covers, and coloured endpapers, stripping away the colour and character of the binding.

Protection

• Store books away from direct sunlight. Never shelve books on a wall that receives direct sun at any time of day.
• Use UV-filtering glass or film on windows in rooms where books are stored.
• Choose LED lighting over fluorescent lighting — LEDs emit virtually no UV radiation, while fluorescent tubes emit significant amounts.
• Limit display time. If you want to display a particularly beautiful book open to a specific page, rotate the display regularly and keep light levels low.
• Use Mylar dust jacket protectors, which provide a modest additional barrier against UV.

2. Heat

Heat accelerates every form of chemical degradation. The general rule in conservation science is that the rate of chemical deterioration approximately doubles for every 10°C (18°F) increase in temperature. A book stored at 30°C degrades roughly twice as fast as the same book stored at 20°C.

How heat damages books

Accelerated aging. High temperatures speed up the oxidation and acid hydrolysis reactions that cause paper to yellow, become brittle, and eventually crumble. This is particularly damaging for books printed on acidic wood-pulp paper (roughly 1850–1990), which is already prone to self-destruction through acid content.

Adhesive failure. Modern perfect-bound books (most paperbacks and many hardcovers since the 1970s) use hot-melt adhesive to hold pages to the spine. Prolonged heat can soften or degrade this adhesive, causing pages to loosen and fall out.

Binding warping. Sustained heat causes differential expansion of the materials in a book — cloth, paper, adhesive, board — which can warp covers, cock spines, and create structural stress.

Protection

• Maintain room temperature between 16–21°C (60–70°F). Consistency matters more than hitting an exact number — temperature fluctuations cause repeated expansion and contraction that stresses bindings.
• Never store books in attics, which experience extreme temperature swings (freezing in winter, sweltering in summer).
• Keep books away from heat sources: radiators, heating vents, fireplaces, and sunny windows.
• Avoid storing books against exterior walls, which can transfer outdoor temperature extremes.

3. Humidity

Moisture is the most destructive of the five enemies in terms of speed of damage. A flood can destroy a library in hours. But even without catastrophic water events, ambient humidity causes chronic damage that degrades books steadily over years.

How humidity damages books

Mould and mildew. The most feared consequence of excess humidity. Mould spores are ubiquitous — they exist in every indoor environment — but they require moisture to germinate and grow. Above approximately 65% relative humidity, mould growth on paper becomes likely. Once established, mould stains paper, weakens its structure, and produces characteristic musty odours that are extremely difficult to remove.

Foxing. The brown spots that appear on the pages of old books — called “foxing” — are caused in part by the interaction of iron particles in the paper with ambient moisture. Higher humidity accelerates foxing.

Cockling and warping. Paper absorbs moisture from the air and expands. When it dries, it contracts — but not always evenly, resulting in cockled (wavy) pages and warped boards. Repeated humidity cycles cause progressive distortion.

Adhesive failure. Excess moisture can dissolve or weaken water-soluble adhesives used in bookbinding, particularly the paste adhesives used in pre-twentieth-century books.

Conversely, too-low humidity (below 30% RH) is also damaging. Excessively dry conditions cause paper to become brittle, leather to crack, and adhesives to desiccate. The ideal range is narrow.

Protection

• Maintain relative humidity between 30% and 50%, with 40% as the ideal target. Use a hygrometer (available for under $20) to monitor humidity in your storage area.
• Use a dehumidifier in damp climates or during humid seasons. In dry climates, a humidifier may be necessary to prevent excessive dryness.
• Never store books in basements unless the basement is properly climate-controlled. Basements are naturally humid and prone to flooding.
• Ensure adequate air circulation around bookshelves. Stagnant air promotes moisture buildup and mould growth.

4. Pests

Books are made of organic materials — paper (cellulose), leather (protein), cloth (cotton or linen), and adhesive (starch, casein, or synthetic polymers). All of these materials are potential food sources for insects and rodents.

Common book pests

Silverfish (Lepisma saccharina). The most common book pest in domestic environments. Silverfish feed on starch — including the sizing in paper, the adhesive in bindings, and the paste under endpapers. They are nocturnal and prefer dark, humid environments. Damage appears as irregular holes and surface grazing on paper and bindings.

Booklice (Psocoptera). Tiny, pale insects that feed on the moulds and fungi that grow on damp paper. Their presence usually indicates a humidity problem. While they do not eat paper directly, their feeding on mould can damage surfaces, and their presence indicates conditions that threaten the collection.

Carpet beetles (Dermestidae). The larvae of carpet beetles feed on natural fibres, including leather bindings, silk bookmarks, and wool-lined slipcases. Damage appears as irregular holes in leather and fabric.

Woodworm (Anobium punctatum and related species). The larvae of furniture beetles bore through paper, board, and wood, creating the characteristic round “wormholes” visible in antiquarian books. Woodworm is more common in older books stored in uncontrolled environments.

Cockroaches. Feed on paper, glue, and starch, leaving irregular damage and dark droppings that stain paper.

Rodents. Mice and rats gnaw on paper and board, typically at exposed edges and corners. They also use shredded paper for nesting material.

Protection

• Control humidity (pests thrive in damp conditions).
• Keep storage areas clean and free of food debris.
• Inspect new acquisitions before shelving them — incoming books can introduce pests.
• Use sealed bookcases in environments where pests are a concern.
• Place pest monitoring traps (sticky traps) near bookshelves to detect problems early.
• If you discover an active infestation, isolate the affected books immediately and consult a conservator. Do not use insecticide sprays, which can stain paper and damage bindings.
• For serious infestations, professional fumigation or controlled-atmosphere treatment (nitrogen or argon anoxia) may be necessary.

5. People

The most paradoxical enemy: the people who love books are also the ones who damage them. Human handling introduces oils, dirt, moisture, and physical stress that degrade books over time. The challenge is to enjoy and use books while minimising the cumulative damage of interaction.

How people damage books

Oils and perspiration. Human skin deposits oils and salts on everything it touches. Over time, these deposits darken paper, stain leather, and accelerate the breakdown of binding materials. The distinctive darkening along the fore-edge of a frequently handled book is the residue of decades of fingertip contact.

Improper handling. Pulling a book from the shelf by the headcap (the top of the spine) — the most natural and the most damaging gesture in book collecting — tears the spine and weakens the binding. Opening a book too far (beyond the natural range of its binding) cracks the spine and strains the hinges.

Careless shelving. Shelving books too tightly prevents air circulation and causes cover abrasion. Shelving books too loosely allows them to lean and cock (tilt the spine to one side). Shelving oversized books vertically when they should lie flat causes the text block to sag under its own weight.

Environmental carelessness. Eating and drinking near books, shelving books near windows or heat sources, and storing books in garages, attics, or basements without climate control.

Well-intentioned repairs. Amateur repairs — adhesive tape on torn dust jackets, rubber cement on loose pages, rubber bands around volumes — cause more damage than the original defect. Pressure-sensitive tapes stain paper permanently, rubber cement discolours, and rubber bands cut into bindings.

Protection

• Wash and dry hands before handling valuable books.
• Pull books from the shelf by gripping the middle of the spine, not the headcap.
• Support the book’s weight when reading — use a book cradle or rest the book on a table.
• Never force a book open beyond its natural range.
• Keep food and drink away from book storage areas.
• Do not attempt repairs unless you have been trained in conservation techniques.
• Educate everyone with access to your collection about proper handling.

The Interrelationship of the Five Enemies

These enemies do not act in isolation. Heat and humidity together create the ideal conditions for mould and pests. Light and heat together accelerate paper degradation faster than either alone. People, through careless environmental management, enable all the other enemies.

The good news is that addressing any one of the five enemies reduces the impact of the others. A climate-controlled room with consistent temperature and humidity automatically inhibits mould, discourages pests, and slows chemical degradation. Proper shelving and handling practices reduce physical stress and limit the opportunity for environmental damage. Awareness is the first and most effective form of preservation.