Yes. High or low humidity, excessive heat, and rapid or wide swings in either condition can damage archival materials. Possible results include mold, faster chemical decay, warping, brittleness, wrinkled paper, and cracked photographic emulsion. Stable, monitored conditions reduce risk, but the right target depends on what your collection contains.
Temperature and relative humidity (RH) work together. Warm, damp air supplies moisture for chemical reactions and mold; warm, dry air can make paper and wood-based materials brittle. Materials that absorb and release moisture expand and contract as conditions change, creating mechanical stress. The National Park Service (NPS) summarizes the priority this way: “Pay special attention to fluctuations in temperature and relative humidity.”
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Why temperature and relative humidity matter together
Relative humidity is the amount of moisture in the air relative to the maximum that air can hold at its current temperature. Because warmer air can hold more water vapor, a room can change RH when its temperature changes even if no water is added. Archive materials respond to both variables, not to a humidity number in isolation.
Paper, board, cloth, wood, and many photographic components are hygroscopic: they exchange moisture with surrounding air. In a damp period they take up moisture and swell; in a dry period they release moisture and shrink. Repeated expansion and contraction can loosen structures, distort sheets, and stress coatings. The NPS identifies wrinkling paper and cracked photographic emulsion as visible examples. Its guidance also explains that high RH provides moisture for harmful chemical reactions and that warm, humid conditions can support mold and insects. NPS, “Mission Critical” Tips for Archives Management
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Temperature affects reaction rates as well. Heat generally accelerates chemical deterioration, especially when moisture is available. The Library of Congress reports that conservation scientists estimated—using accelerated-aging tests—that the useful life span of paper doubles for every 10°F drop in temperature. That is a paper-specific estimate, not a promise for every material or a prediction of an individual collection’s life. The Library of Congress page is Scrapbooks and Albums, indexed at approximately 1991; its publication year is not established here. Library of Congress, Scrapbooks and Albums
Cold is not automatically safe. Very dry conditions can embrittle materials, and moving an object rapidly between cold and warm environments can drive condensation or abrupt moisture exchange. The preservation goal is a cool, controlled, stable environment appropriate to the format—not the lowest possible temperature.
What different conditions do to common archive materials
Paper files, manuscripts, and books
Warm, humid storage can produce mold, musty odors, blocking (pages sticking together), distortion, and faster chemical decay. Dry storage can leave paper and bindings brittle and prone to cracking. Humidity cycling repeatedly changes the dimensions of pages, boards, and covering materials, which can cause cockling, warped covers, and stressed sewing or adhesives.
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For books, the Library of Congress warns that RH above 60% risks warping and mold, while RH below approximately 30% can also cause warping through contraction. Rapid humidity changes are an additional concern. Its current book-preservation guide states: “Most books are best stored at a stable, non-fluctuating relative humidity of no higher than 55%.” That statement applies to books; it is not a universal target for every archive format. Library of Congress, Preserving Your Books
Photographs and photographic negatives
Photographs have layered materials—support, image, and often a gelatin or other binder—that can react differently to moisture. The Library of Congress describes high humidity as softening gelatin binders and making them vulnerable to mechanical damage. Low humidity can shrink binders, cause cracking, and make supports curl. Repeated temperature and RH cycling promotes moisture movement and associated chemical and mechanical changes.
Photographic storage guidance therefore differs according to the photographic process and whether photographs are stored alone or alongside paper, parchment, or leather. Use the Library of Congress photographic leaflet for those format-specific decisions rather than applying a book or paper target to every image. Library of Congress, Care, Handling, and Storage of Photographs
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Mixed collections
A box containing books, photographs, leather, parchment, magnetic media, or other formats has competing needs. There is no single set point that is ideal for all mixed archival materials. Identify the most sensitive formats, separate materials where practical, and have a conservator or institutional preservation specialist establish targets and allowable ranges for the actual collection.
Useful target ranges—and why they are not universal
Published ranges are starting points tied to a material and a use context. Stability matters as much as the nominal number. The following values come directly from format-specific institutional guidance:
| Collection or guidance | Temperature guidance | Relative-humidity guidance | Important qualification |
|---|---|---|---|
| Paper archives (NPS general recommendation) | 60–70°F | 30–50% | General paper-archive range; stability is emphasized. |
| Most books (Library of Congress) | Cool conditions at or below 70°F | 30–55% | Keep RH stable and non-fluctuating, no higher than 55% for most books. |
These ranges should not be combined into a single “archive standard.” A room at 50% RH may be reasonable for paper but still inappropriate for a particular photographic process or a mixed collection. Institutions should document the selected range, the reason for it, and the alarm or response thresholds used by staff.
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How to monitor conditions effectively
Monitoring tells you what the collection is experiencing; it does not control the environment. NPS says temperature and RH in archive storage should be measured and recorded systematically. The Library of Congress notes that institutions can use continuous systems or regular human readings and logs to evaluate collection-space risks. Library of Congress, Collection Emergency Management
- Choose the storage space. Measure the room where materials actually sit, not only an adjacent office or hallway. Note locations near exterior walls, windows, doors, heating equipment, or plumbing, because conditions may differ within one room.
- Use a temperature-and-RH monitor or data logger. A logger records trends and reveals short spikes that a single daily reading can miss. It is an observation tool, not a dehumidifier, air conditioner, or other control system.
- Record readings consistently. Use continuous recording when available, or take regular readings at a defined time and keep a dated log. Record unusual events such as HVAC outages, storms, leaks, or room changes beside the measurements.
- Compare results with the format-specific target. Look for sustained high or low values, repeated excursions, and rapid swings. A stable room slightly outside a chosen range may pose a different risk from a room that repeatedly crosses the range.
- Investigate the cause before moving collections. Check building operation, airflow, water entry, occupancy patterns, and seasonal changes. Correcting the source is more reliable than repeatedly relocating boxes.
- Review the log and act on trends. Share records with the person responsible for facilities or preservation. A conservator can help set limits when valuable or mixed-format materials are involved.
Warm-humid, warm-dry, and fluctuating conditions compared
| Condition | Primary risks | What to watch for |
|---|---|---|
| Warm and humid | Mold, insect activity, faster chemical deterioration, softened photographic binders | Musty odors, surface growth, sticking pages, photographic emulsion damage |
| Warm and dry | Accelerated chemical aging combined with embrittlement of paper and wood-based materials | Cracking folds, brittle pages, friable boards or bindings |
| Rapid or wide temperature/RH swings | Repeated expansion and contraction; moisture movement; mechanical stress | Wrinkled paper, warped boards, curled supports, cracked photographic coatings |
| Cool and stable within a material-appropriate range | Lower environmental stress than extremes or cycling | Continue measuring; stability does not eliminate risks from water, light, pests, or poor handling |
The Northeast Document Conservation Center (NEDCC) likewise warns that warm, humid environments may lead to mold, warm, dry environments can make wood- and paper-based materials more brittle, and wide fluctuations in temperature and RH can damage collections. NEDCC, Preservation Leaflet 2.1
Why a basement, attic, or garage remains risky
The Library of Congress advises against storing valuable objects in basements, attics, or garages. These spaces commonly have poorer temperature and RH control and greater exposure to leaks, floods, roof failures, and outdoor weather. A plastic bag or bin does not remove those risks.
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Polyethylene or polypropylene bags and bins can provide secondary protection against brief water exposure, but they do not correct an unsuitable ambient environment. In high humidity, a closed plastic container can trap moisture and increase mold risk. Do not treat a container as a substitute for a suitable storage room, monitoring, or a response plan. Library of Congress, General Preservation FAQ
What to do after an environmental excursion
First, protect people and stop new exposure
If there is flooding, active leakage, or visible mold, address safety and the water source before handling materials. Isolate affected boxes from unaffected collections when that can be done without spreading contamination or causing further damage. Do not seal visibly damp materials in airtight plastic for long-term storage; trapped moisture can encourage mold.
Then document and triage
- Save the monitor’s readings or photograph the written log, including the time and duration of the excursion.
- Record which shelves, boxes, and formats were exposed, and note visible distortion, mold, sticking, odors, or photographic surface changes.
- Prioritize items that are wet, actively molding, structurally collapsing, or uniquely significant for advice from a conservator or emergency-response service.
- Keep affected formats separated when their environmental needs differ, and avoid making rapid temperature changes simply to return them to a preferred number.
Allow cold-stored photographs to acclimate
NPS gives a specific example for photographs removed from cold storage: place the photographs in a plastic bag at room temperature for at least 24 hours before examination. The gradual thaw reduces cracking risk. This is photograph-specific guidance, not a general instruction for books, paper files, or every archival material. NPS archival management guidance
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A practical policy for caretakers
A preservation policy turns readings into decisions. Start by listing the formats in each storage area and identifying photographs, parchment, leather, magnetic or optical media, and other materials that may require separate advice. Select a documented target for each area from the relevant institutional guidance, then define who reviews logs and who is called when conditions move outside the target.
- Keep boxes and shelving away from known leak paths and direct heat sources.
- Use stable, archival-quality enclosures sized to the material; do not rely on a sealed container to fix room conditions.
- Inspect for mold, distortion, brittleness, and photographic surface changes during scheduled collection checks.
- Retain monitoring records so seasonal patterns and equipment failures can be recognized.
- Ask a conservator to review mixed collections, unusual formats, or repeated excursions that building staff cannot resolve.
Temperature and RH management is one part of preservation. Even a stable room cannot prevent damage from light, pollutants, pests, water events, poor enclosures, or rough handling, so environmental records should support—not replace—a broader preservation program.
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